The Thermodynamics of Cooking and How Different Cooking Methods Work The Thermodynamics of Cooking and How Different Cooking Methods Work "If you saw a heat wave, would you wave back?" Steven Wright By Meathead Goldwyn You may have thought you left physics and chemistry behind when you left school, but if you want to eat well, you need to understand that cooking is all about physics and chemistry, with a little magic mixed in. Here are some foundation concepts every outdoor cook needs to know. What is cooking? Foods are composed mainly of water, fat, protein, and carbohydrates. Cooking is the process of changing the chemistry of food usually by transferring energy in the form of heat to the food long enough so that it is safe and digestible, and achieves the desired flavor, texture, tenderness, juiciness, appearance, and nutrition. The three ways heat cooks food. Food gets hot when molecules vibrate so fast that their temperature rises. when cooking outdoors, heat is transferred to food by three methods. Which one you use is crucial. These processes have been described this way: Conduction is when your lover's body is pressed against yours. Convection is when your lover blows in your ear.

Radiation is when you feel the heat of your lover's body under the covers without touching. Let's be a bit more precise: 1) Conduction is when heat is transferred to the food by direct contact with the heat source. Cooking a hot dog in a pan is conduction. Heat from the burner is transferred to the pan whose molecules vibrate, and pass the vibe on to the wiener. As the surface of the meat gets hotter than the interior, the heat transfers to the center through the moisture and fats. That's also conduction. Grill marks are a good example of conduction. Heat is transferred to the grill grates and the hot metal brands the meat. 2) Convection is when heat is carried to the food by a fluid such as air, water, or oil. Cooking a hot dog in your kitchen oven where it is surrounded by hot air is convection cooking. So is boiling it in water, or deep frying it in oil (you really need to try a deep fried hot dog once). If you get one side of your grill hot and put the food on the other side, it is cooked by natural convection airflow. Most gas grills cook by convection.

A conventional indoor oven uses natural convection airflow to cook, but a "convection oven" has a fan and an extra heat source near the fan and uses forced airflow. A convection oven cooks 25% to 30% faster. It's like wind chill in reverse. The moving warm air transfers more energy that stagnant warm air. In either case, airflow only cooks the exterior of the meat, the interior of the meat is cooked by conduction as the heat travels through it. 3) Radiation is the transfer of heat by direct exposure to a source of light energy. Put a hot dog on a coat hangar and hold it to the side of a campfire and you are cooking by radiation. This is how most charcoal grills cook. To be complete, I need to mention that indoors there are two other methods heat is applied for food: 4) Excitation is how microwave ovens work. Microwaves are radio waves that penetrate the food and vibrate the molecules inside the hot dog until it gets hot without heating the air around it. Water in the food heats first in the microwave. Technically, this is a form of radiation. As yet, I haven't seen a grill that cooks with microwaves.

5) Induction is the latest technique for stove tops. A copper coil is placed under a smooth cooktop and an alternating current is sent through the coil creating a rapidly changing electromagnetic field. Electrons in steel or cast iron pots on the cooktop above the electromagnet are jostled by the rapidly changing magnetism, but they resist and the resistance heats the pot. The pot then transfers the heat to the food without the cooktop or the air around it getting hot. Induction is the most efficient way to convert energy to heat on a cooktop, and it very responsive to the control knob, but it does not work with aluminum, glass, or copper pots. The difference between heat and temperature Put an empty frying pan and pot of water in a 200°F oven for an hour. Open the door and stick your hand in the warm air of the oven and count how long you can hold your hand in there. Perhaps a minute? Then plunge one hand in the 200°F water and place the other hand on the frying pan. After you get back from the hospital, ponder the fact that not everything that is 200°F transfers energy at the same rate.

Cooking by convection air in a hot oven is a lot slower than by conduction from a pan because steel is 8,000 times more dense (it has more molecules in a cubic inch) than air which conducts heat at 1/50 the rate of steel. You can speed the process a bit by adding a fan to the oven and moving the air around as is done in a "convection oven", but all ovens are really convection ovens. Water is more dense than air and it holds more energy so it cooks faster than in the oven, and oil cooks faster still. According to the AmazingRibs.com science advisor, Dr. Greg Blonder, "Temperature measures the average energy of each atom. Heat is the total for all atoms. It's like money. If the average income in the US is $40,000 per family, the total wages are in the trillions." Also, radiant heat delivers more energy than convection heat. Let's say you have two gas grills. On one grill, the two burners on the right side are on full and two burners on the left are turned off.

The air temp on the left side, the indirect heat side, might be 325°F as convection flow of air from the right side circulates over the left side. Let's put a steak and a big honkin prime rib roast on the left, indirect side. On the second grill you have all four burners on medium and the air temp on the both sides is also 325°F. Let's put a steak and a roast on this grill too. The steak's bottom will brown better on the second grill because it is above direct radiant heat which is imparting more energy than the first grill where the heat is from convection air. But by the time the roast is done, it will be blacker than a mourning hat. On a charcoal grill, you can increase energy simply by increasing the number of coals. The distance from the energy source is also an important factor. Energy dissipates as you move away from the heat source.

In a $800 grill made of ceramics the heat source may be 2' from the cooking surface while on an $89 Weber Kettle the charcoal is only 4" away, and on a $30 Hibachi the coals may be as little as 1" away. The walls of a ceramic cooker are thick, they absorb huge amounts of heat, and then release it evenly making a very efficient oven with very steady cooking temps and very low fuel use. Perfect for roasts. But a steak on the cooking surface will not brown as well as on the Weber or even the Hibachi because the coals are so far away and because much of the heat comes from the ceramic side walls and dome, not glowing radiant hot coals right below the meat. When is food ready to eat? Food is ready to eat when it is safe and the target temperature is reached. A medium rare steak is 130 to 135°F in the center regardless of how thick, how much it weighs, or what temperature it is cooked at. The single best way to tell if it is time to eat is with an accurate digital thermometer.

You cannot tell by poking the meat unless you're a pro who has cooked the same steaks on the same grill for years. The single thing you can do to improve your cooking is to get a good digital instant read meat thermometer. The second most important thing you can do is get a good digital oven thermometer. OK, some foods are hard to read with thermometers. You don't need a thermometer to tell when asparagus is ready, and because of all the bones it is almost impossible to tell when ribs are ready. But for most meats, there is no substitute for knowing precisely what temp the food is. How heat moves within meat The most important thing to understand is that when you put food in an enclosed vessel like a grill, smoker, or an oven, the hot air, which has lots of vibrating molecules, transfers some of its energy to the exterior of the meat. The energy in the air excites the molecules on the surface of the meat and then they transfer heat to the molecules inside it and so on, slowly passing the energy towards the center of the meat like a wave of heat. It takes time because meat is about 75% water, and water is a good insulator, especially when trapped inside muscle.

The heat moves inward because physics dictates the the meat seeks equilibrium in an effort to make the temperature the same from edge to edge. So most of the meat is cooked by the meat, not by the air. Heat also cooks points and corners more because it can attack on multiple fronts. That's why the brownies and lasagna in the corners are crunchier. And bones heat at a different rate than muscle tissue because they are filled with air or fat, so in most cases they warm more slowly than the rest of the meat. Click here to read more about the effect bones have on cooking and have some myths debunked . When you remove the meat from the heat, it continues to seek equilibrium and continues to cook because the heat built up in the outer layers of the meat continues to be passed down towards the center while some is escaping into the air and cooling the exterior. This phenomenon is called carryover cooking . A thick piece of meat such as a turkey breast might rise as much as 10°F in about 15 minutes after removing it from the grill because of carryover. A thinner piece of meat such as a thick steak might only rise a couple of degrees, and chicken breast may not rise at all.

This is important to know this because 5 to 10°F can make the difference between moist turkey and cardboard, a medium rare steak and a drier medium steak. To compensate, use a good digital thermometer and remove thick cuts of meat about 5°F below your target temp. Thickness is more important than weight The temperature of your food moves slowly upward during cooking, but the thickness of the meat is a major factor in how long it takes to get the center to the desired temperature. A thin steak cooks faster than a thick steak. And a 5" thick prime rib that weighs 8 pounds will be done in the same time as a 5" thick prime rib that weighs 12 pounds. So any recipe that says "cook your steak for three minutes per side" without specifying the thickness of the steak, is seriously flawed. Likewise any recipe that says "cook your roast 30 minutes per pound" is suspect. About boiling temps and how they impact cooking Another important concept to know is that when liquid is heated, its temp will increase until it hits its boiling temp and not go any higher.

So no matter how high we turn the burners under a pot of water, the water will not get hotter than 212°F (at sea level). If we have a sauce made with water and olive oil it will heat to the boiling temp of water until all the water boils off, and then rise to the boiling temp of the oil, which is usually over 500°F depending on the type of oil. Usually, before oil boils, it starts to smoke, and then it bursts into flames. This phenomenon comes into play in many aspects of cooking. When you simmer or boil a home made barbecue sauce, because the water cannot exceed 212°F, you warm everything evenly without burning the ingredients (if you stir it so it doesn't stick to the bottom and dry out). If you simmer potatoes the starches on the inside get soft and the outside doesn't burn as it might on a grill. Steam can form at a lower temperature than 212°F as molecules of water get hot and escape the surface as the water warms. That's why we see water vapor escaping a pot of water before it boils.

When water boils, the steam can be hotter than 212°F if it comes into contact with the walls of a hot pot or if it is under pressure, but food submerged in boiling water will cook faster than food bathed in steam because under water the food's surface is completely surrounded by hot water molecules and in steam there is a lot of air mixed in and air does not conduct heat as well as water. Because meat is about 75% water, it is useful to think of it as if it is a saturated sponge. As it heats, water on the surface is energized and some of it escapes as steam. Even though the oven may be a lot hotter than 212°F, the surface will stick at close to 212°F as more and more water steams away. As the hotter molecules escape the cooler ones are left behind so the temperature of the meat is cools until water stops evaporating, usually because the surface dries out making a crust or bark .

At low cooking temperatures such as 225°F, the temperature I recommend for a lot of my recipes, the rate of evaporation can be so great that the surface and the interior of the meat can stick, usually in the 150 to 165°F range and remain pretty much unchanged for hours, totally freaking out the cook. This phenomenon, called the stall , does not happen if the oven temp is higher, say 325°F, another temperature I recommend frequently. The stall has four benefits: (1) It helps create the bark which can be very tasty, (2) it holds the meat at a temperature where fats and connective tissues can liquefy, significantly improving texture, juiciness, and flavor, (3) it gives enzymes a chance to tenderize the meat, and (4) it permits the meat to heat evenly so the center and exterior are similar in temperature. If you soak wood chips in water, as many books recommend, first of all they won't absorb much (that's why boats are built from wood), but when you toss them on the coals, the wood stays a cool 212°F until the water boils off.

It cannot smoke until it hits a much higher temp, so soaking wood chips so they will smolder better is just another old husband's tale. Click here to read more on soaking wood . One other thing about the boiling point: It changes as you move up or down from sea level. At sea level, a pot of water on a flame has a column of air sitting on top of it that weight 14.7 pounds per square inch (psi). At 5,000 feet it's only 12.3 psi, so water vapor has less resistance when it tries to escape. As a result, the boiling point is lower and leavening gases in breads and cakes expand more quickly. The impact on grilling or smoking is minimal, although there can be higher humidity in a smoker during a long cook and that can cool the meat slowing the process, but not a lot, and it can be offset because natural humidity can be lower at altitude. But altitude can have a profound effect on braising and other wet cooking methods. It can even impact slow cookers, microwaves, and frying. For more on the subject, read this by Colorado State University .

The magic of infrared Infrared radiation is the best method for delivering high heat to food on a grill. Charcoal grills produce a lot of direct scorching infrared radiation that is converted to heat when it strikes food. Gas and pellet grills produce mostly convection heat. If you remember your high school science, you know that infrared is a section of the wavelength continuum around us, just up the road from visible light and down the road from the radio in your car. Max Good , Director of Equipment Reviews at AmazingRibs.com elaborates: "The sun radiates intense infrared (IR) waves that travel great distances at the speed of light without dissipating until they contact solid matter, like your skin, which it warms. The sun also radiates other waves, like ultraviolet (UV), which cause sunburn as well as heat. So all IR heat is radiant, but not all radiant heat is IR." As another example Good says "Consider the IR heaters in warehouses or bus stops. They are often suspended from the ceiling, but one can easily feel the heat waves standing beneath them. If these same devices were simply conventional gas burners, most of the heat would rise upward.

Convection heat can be dissipated easily by air currents and distance while IR gets through." The AmazingRibs.com science advisor Dr. Greg Blonder adds "An oven set on broil is a radiant heater, with much of the heat is in the near IR, the almost visible section of IR. In principle, a pure IR heater would not be visible to the naked eye, since our eyes can't respond that far out in the light spectrum. In practice, an IR heater and charcoal glow a dull red." In the past few years a number of gas grills have been touting their superiority because they use infrared heat or they have an IR sear burner or sizzle zone designed to generate high heat for getting a dark sear on meat. This is an attempt by gas grill manufacturers to close the gap with charcoal grills. In an IR gas grill a plate of special glass, ceramics, or metal absorbs the heat and light from the flame and re-radiates it to the food in the form of IR. Dr. Blonder explains: "The atoms in the plate begin to oscillate. At low temps almost all of the energy is stored as vibrations in the plate. As the plate heats up, the rapidly vibrating atoms cause it to emit IR.

Think of it (very loosely) as a teapot on a stove. At low temps most of the energy is confined the simmering liquid. At higher temps there is so much vibration that the water evaporates, forming steam and carrying away energy as a scalding hot jet." At the top of the page is an IR sizzle burner under cast iron grill grates on a Napoleon Gas Grill. Is IR heating better? Dr. Blonder says "IR energy is delivered faster than convection, but slower than conduction. So it can brown a little more effectively than a conventional grill but not as fast as a hot pan or grill grates. There are fewer hot spots from IR cookers. On the other hand, you can get thermal runaway with IR. If one section of the meat is dark, it absorbs more IR than the lighter sections. It browns, becomes darker still, and eventually burns." And real IR burners are not good at achieving low temperatures. I recommend gas grills with IR burner sections, especially them for getting dark sears on steaks and other meats when the browning of the surface creates thousands of new flavor compounds. How your grills work The illustration below shows how different outdoor cookers work.

1) Direct Heat Charcoal or Wood. Charcoal produces radiant heat in the bottom. The grates absorb heat and produce conduction heat on parts of the surface of the food making grill marks. The lid reflects mostly convection heat. The exterior of the food absorbs direct heat from below, converts it to conduction heat, and it moves to the center of the food. 2) Indirect Heat Charcoal or Wood. Charcoal off to the side produces both convection and radiant heat. The grates absorb heat and produce conduction heat on surface of the food making grill marks. The exterior of the food absorbs indirect convection heat from all sides, converts it to conduction heat, and it moves to the center of the food. 3) Lid Off Charcoal or Wood. Charcoal produces radiant heat. The grates absorb heat and produce conduction heat on surface of the food making grill marks. The exterior of the food absorbs radiant heat from below only. The bottom of the food converts the absorbed energy into conduction heat, and it moves to the center of the food. Without a lid, heat escapes from the food upward, cooling its top. 4) Flat Top. Gas or charcoal produce radiant heat in the fire box.

Solid griddle under the food absorbs heat and produces conduction heat on surface of the food. The bottom of the food absorbs conduction heat, browns where it is in contact with the surface, and conducts heat towards the top of the food. Without a lid, heat escapes from the food upward, cooling its top. 5) Direct Heat Gas. Burners produce radiant heat that heats metal drip protector bars, lava rocks, or ceramic briquets. They absorb heat and produce radiant and convection heat. The grates absorb heat and produce conduction heat on surface of the food. The exterior of the food converts all this incoming energy to conduction heat, and it moves to the center of the food. 6) Indirect Heat Gas. Burners off to the side produce radiant heat that heats metal drip protector bars, lava rocks, or ceramic briquets. They absorb heat and produce radiant and convection heat. The grates absorb heat and produce conduction heat on surface of the food. The exterior of the food absorbs indirect convection from all sides. The exterior of the food converts the energy to conduction heat and it moves to the center of the food. 7) Lid Off Gas.

Burners produce radiant heat that heats metal drip protector bars, lava rocks, or ceramic briquets. They absorb heat and produce radiant and convection heat. The grates absorb heat and produce conduction heat on surface of the food. The exterior of the food converts the energy to conduction heat and it moves towards the top of the food. Without a lid, radiant heat escapes from the food upward, cooling its top. 8) Infrared Gas. Burners produce radiant heat that in turn heats a special plate. It absorbs heat and emits it as radiant heat in the infrared part of the spectrum. Grates absorb heat and produce conduction heat where they contact the surface of the food. The bottom of the food absorbs the radiant heat and converts it to conduction heat and moves it to the center of the food. The top of the food is heated by indirect convection heat bounced from the lid. How a smoker works Smoke is the perfume, the sexy scent that makes us stand outdoors on cold rainy nights nurturing the rib cage of a hog as if we were delivering a child. Smoke is the indescribable difference between the crockpot pulled pork they serve at the ballpark and the hand crafted sandwich that gets you a standing ovation.

Smoke is what makes brisket simmered in a flavorful stew run and hide its head when it encounters the stuff that Texans drive 50 miles for, just for lunch. But not just any smoke will do. Nobody wants their turkey tasting like cigarettes. So there is an art to getting the right smoke . You can make superb smoked foods on just about any grill with a cover, even a gas grill. But adapting a grill for smoking is like adapting the family car for racing. The better solution is to buy a race car. With a high quality modern barbecue smoker , and a little practice, you can make food better than the ribs, brisket, and other faves holding in the warming ovens of restaurants. With a good modern smoker you no longer need to hover over your machine for five hours, constantly monitoring the temp, fiddling with the dampers, shoveling coal, adding wood, and spritzing your meat with a mist of secret moisturizer. With a good smoker you can get your food on the table on time and without fear that it is over-cooked (an expensive waste) or under-cooked (dangerous).

Most smokers have an indirect heat source with a deflector or a water pan that protects the food from searing infrared radiation and cooks with convection. They generate smoke by burning wood. A few use wood for both heat and smoke, but most use charcoal, gas, or electricity for heat, and wood for smoke. Master of the flame and keeper of time Good cooking is finding the proper combination of heat and time. The higher the heat, the less time needed. Lower the heat, you need more time. But you just can't set your cooker for Warp 10 on every food. Some demand low and slow. Read more about cooking temperatures here . Your indoor oven has three basic components to control heat and time: A heat source, a thermostat, and a timer. Your outdoor oven, and an oven is exactly what your grill or smoker is, has a heat source, but only a few have a thermostat or a timer. And that's why cooking outdoors is much more difficult than indoors. Tell that to your significant other (and sleep on the couch). To make good food outdoors you need to become master of the flame and keeper of time. Many grills come with a thermometer, but they are usually crap.

The thermometer on my gas grill is often off by 50°F! You absolutely positively must have a good oven thermometer. In addition to knowing what temp your cooker is, you need to know what temperature your meat is. Cooking without a meat thermometer is like driving without a speedometer. You might think you're under the limit, but try explaining that to the judge. You might think the meat is ready, but try explaining that to your guests when it is dry enough that they ask for a glass of saliva to help them swallow. Here is a Food Temperature Guide , and here's everything you need to know about thermometers with links to good ones you can buy. Some of the better digital thermometers have timers. If yours does not, get in the habit of wearing a watch when you cook and write down what time you put the meat into the heat in a cooking log . Some of the old timey barbecue artisans say you can learn to cook by sight, smell, and feel. That's just plain BS. Once you know how to cook you can rely more and more on your senses, but until you are ready to call yourself a top chef, get a thermometer, wear a watch, and keep a log. I do.

newsletter Please read this before posting a comment or question 1) Please use the table of contents or the search box at the top of every page before you ask for help, then please post your question on the appropriate page. 2) Please tell us everything we need to know to answer your question such as the type of cooker and thermometer you are using. Dial thermometers are often off by as much as 50°F so if you are not using a good digital thermometer we probably can't help you. Please read this article about thermometers . 3) If you post a photo, wait a minute for a thumbnail to appear. It will happen even if you don't see it happen. 4) Click here to learn more about our comment system and our privacy promise . Remember, your login info for comments is probably different from your Pitmaster Club login info if you are a member. Moderators: How Cooking Works: Convection and Conduction How Cooking Works: Convection and Conduction How Cooking Works: Convection and Conduction Lauri Patterson / Getty Images By Danilo Alfaro Updated November 20, 2016. How does food get hot? It's simple, right?

Just put it in the oven or heat it up on the stovetop. The way heat travels from something hot, like a flame or a pot of boiling water , to the food item we intend to cook, is a process called heat transfer and the different ways this can be accomplished determines how the food is cooked and what the end result will be. There are two main methods of heat transfer: conduction and convection. (A third method, radiation, is also recognized, but it's outside the scope of this discussion.) Conduction Conduction is probably the most basic and intuitive way of achieving heat transfer: Something hot touches something cool and the cool thing heats up. For instance, the water in a pot boils when the flame from the stovetop heats the pan, and the heat from the pan is transferred to the water via conduction. If you drop an egg into that boiling water, the heat from the water is then transferred to the egg. As the outer parts of the egg heat up, that heat is transferred inward, so that it is the hotter parts of the egg that end up cooking the cooler, interior parts of itself.

continue reading below our video Why Baking Pans Really Matter The transfer of heat from one part of an object to another part of the same object is also considered conduction. Whether heat is​ efficiently transferred in this way depends on the conductivity of the items involved. Copper is an extremely good conductor of heat, which means heat moves through copper cookware and is transferred to the food very quickly. By comparison, water or even stainless steel are relatively poor conductors of heat. For that matter, food itself is a poor conductor of heat, which is why we see that a roast will continue cooking for several minutes even after we take it out of the oven. It's also why roasting a turkey is so difficult . You're basically cooking the inside of the bird by getting the outside of it really hot. But by the time the inside (i.e. the thigh) is done, the outside (i.e. the breast) is overcooked. Convection Whereas conduction is a static process, convection is a more efficient method of heat transfer because it adds the element of motion. A convection oven heats food faster than an ordinary one because it has a fan that blows the hot air around.

Convection ovens can reduce cooking times by 25% or more compared with ordinary ovens. They also tend to increase the browning of food by concentrating more heat on the food's outer surface. The movement of steam or the motion of boiling water in a pot are also examples of convection. Stirring a pot of soup would be considered a form of convection, as it redistributes the heat from the bottom of a pot throughout the soup. Convection is also the reason that frozen items thaw more quickly under cold running water than if they are simply submerged in water. BBC - GCSE Bitesize: Heat transfer by conduction and convection Next Heat can be transferred from place to place by conduction [conduction: The transfer of heat energy through a material - without the material itself moving. ], convection [convection: The transfer of heat energy through a moving liquid or gas. ] and radiation [infrared radiation: Electromagnetic radiation emitted from a hot object. ]. Dark matt surfaces are better at absorbing heat energy than light shiny surfaces. Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.

There are several different types of energy, and these can be transferred from one type to another. Energy transfer diagrams show the energy transfers in a process. More efficient devices transfer the energy supplied to them into a greater proportion of useful energy than less efficient devices do. Heat transfer by conduction and convection Heat is thermal energy. It can be transferred from one place to another by conduction, convection and radiation. Conduction and convection involve particles, but radiation involves electromagnetic waves. Conduction Thermogram of a pan being heated on a stove Heat energy can move through a substance by conduction. Metals are good conductors of heat, but non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called insulators. Heat energy is conducted from the hot end of an object to the cold end. The electrons in piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now charged metal ions. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons.

These move through the structure of the metal, colliding with ions as they go. Heat transfer by conduction Next Convection Liquids and gases are fluids. The particles in these fluids can move from place to place. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection. Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size. The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up. Page: Conduction, Convection, and Radiation: An Introduction Conduction, Convection, and Radiation: An Introduction Curriculum Tie: Teacher demonstrations help students understand heat and the methods of transfer - conduction, convection and radiation.

Main Curriculum Tie: Investigate the movement of heat between objects by conduction, convection, and radiation. Materials: Heat is made up of molecules Hot and cold water Additional Resources Media The Convection of Heat, by Eureka! (Films for the Humanities and Sciences, PO Box 2053, Princeton, NJ 08543, Bill Nye the Science Guy: Heat (Disney Educational Products, (800) 295-5010, ; ISBN 1-932644-98-9 Attachments Paper_Spinner_Directions.pdf Background For Teachers: Heat is the transfer of thermal energy between substances that are at different temperatures. Energy is always transferred from the warmer object (which has a higher temperature) to the cooler one (which has a lower temperature). Similarly, molecules with a lot of energy move faster than molecules with a smaller amount of energy, thus causing the former to have more heat. Heat transfer will continue until both objects have reached the same temperature or the same speed. For example, an ice cube in a glass of water eventually melts.

This is because the heat from the water, which is warmer, flows to the ice cube until both are at the same temperature, and therefore no ice cube is left. There are three methods of heat transfer: conduction, convection, and radiation. Conduction occurs through direct contact. When two substances come into contact, their particles collide. The energy from the faster-moving substance is transferred to the slower-moving substance until they are moving at the same speed. At this point, their temperatures will be the same. An example of conduction is a spoon warming up when it is placed into a cup of hot cocoa. Convection is the transfer of heat in air or a fluid through currents. An example is a pot of water warming up on a hot stove. As it heats up, the particles spread out and become less dense. The warm water on the bottom of the pot rises and displaces the cold water. As this occurs, the cold water sinks. So hot air rises, cools, and falls. Radiation is the transfer of energy as electromagnetic waves. It does not need to directly touch anything or move particles as in conduction and convection.

Radiation occurs through empty space, as in the sun heating the earth or feeling warm in front of a fire. Many demonstrations are needed for this experiment. Prior to beginning the lesson, prepare several cups of beans and BBs. Lay cloth and a piece of glass on the snow or colored paper and glass on the pavement at least an hour before the lesson. Plug in the hot plate. Heat a cup of water to almost boiling. Set up the candle spinner or create the paper spinner. Make a transparency of Hogle Zoo Heat! Make sure the radiation portion of this lesson is done on a sunny day. This lesson is divided into heat concepts and could be done over a two or three day period if desired. Intended Learning Outcomes: 2. Manifest science attitudes and interests 3. Understand science concepts and principles. Instructional Procedures: Invitation to Learn Turn a hot plate to high temperature. Obtain the metal ball and ring set and instruct students to obtain their science journals. Show how the ball easily passes through the ring. Now place the ball on top of the hot plate. Allow it to warm, then try to place the ball through the ring again. It will not work.

Ask the students to write the following in their journals: What happened when the ball was heated? Why do you think it won't it go through the ring now? What do you think will happen when both the ball and ring are heated? Set the ball and ring on the hot plate as the students write. When finished, again place the ball through the ring. This time, it will work. Were their predictions correct? Instructional Procedures Heat is made up of molecules After completing the invitation to learn, leave the hot plate on and begin heating up a cup of water. Ask if anyone has an idea of how or why heat affected the metal of the ball and ring. After listening to a few students, explain that metal expands when heated and contracts when cooled. Metal is a high conductor of heat. In other words, metal will quickly become hot when it is touched by heat. The handle of the ball and ring is made of wood because wood is not a high conductor of heat. Heat flows more quickly through metals than it does through woods. Write this definition of heat on the board: Heat is the transfer of energy between two objects at different temperatures. Instruct students to write the definition in their journals.

Explain that everything is made up of small particles called atoms, which have energy and are constantly in motion. Heat is directly associated with groups of atoms, called molecules. The hotter something is, the faster its molecules are moving. This means that molecules in cold objects move slower than the molecules in hot objects. For heat to transfer, one molecule has to bump into another molecule. Obtain the three clear glass cups (do not use plastic), the food coloring, and the measuring cup. Fill one glass with a cup of very cold water, one with a cup warm water, and one with the cup of almost boiling water. Ask the students to make a prediction in their journals: what will happen when I put one drop of food coloring in each glass? Allow time for writing then drop the food coloring in each. Watch the food coloring spread out, paying special attention to the streaks and whether it spreads out evenly within the water. Watch the clock and have the students write down observations after 0 seconds, 30 seconds, and 60 seconds. They will notice how quickly the coloring in the glass of boiling water spreads compared to the others. Ask what this proves about molecules and allow time to share.

Then explain that water and food coloring are both made up of molecules. The food coloring molecules in a glass of water are pushed around by the water molecules and eventually spread throughout the glass, even if you don't shake or stir the water. Since molecules in hotter objects move faster than molecules in slower objects, the food coloring spreads more quickly in the hottest water. Conduction Ask what situations the students can think of that involve heat. List the ideas on the board. These may include heating a home in the winter, sitting by a campfire, cooking or baking, getting into a hot tub, blowing on your hands on a cold day, hot car seats in the summer, sitting on a beach, touching a hot curling iron, etc. Think of as many ideas as possible. Leave the list on the board. Tell the students that there are three types of heat transfer which will be demonstrated today. To introduce the concept of conduction, bring out several cups of beans and BBs. Allow each student to put a finger into one cup, then the other. Which is colder? Ask students to write their observations in their journals. If you measure the temperature of the beans and BBs, you'll find they are about the same.

The students probably though the BBs were colder. Explain that this was caused by conduction. Explain that conduction occurs when objects touch. The heat from the warmer object is transferred to the cooler object, eventually evening out the temperatures. In other words, the faster molecules from the warmer object bump into the slower molecules of the cooler object until they are all traveling at the same speed. Think of �can touch� when you think of conduction. Have students write down their own definition of conduction in their journals. Remind the students about the invitation to learn. What was the ball and ring made of? (metal) What did we learn about metal? (conductor of heat) Brainstorm types of metals: steel, iron, gold, silver, copper, etc. Tell the students that BBs are also made of metal. Since metal is a conductor of heat, the BBs only feel colder than the beans because the metal is conducting heat away from your hand. You perceive the heat that is leaving your hand as cold. Remind your students about heating up the water for the food coloring on the hot plate. Since the saucepan touched the hot plate, heat was transferred from the hot plate to the pan via conduction.

Next, obtain the compound bar. Tell the students that this bar is made of two different metals. Allow the students to share their ideas about what they think will happen when you touch the bar to the hot plate. Turn the hot plate to high and place the bar on top. Watch it bend. Explain that the curving results from the two metals expanding unequally. Run the bar under cold water or place in a container of water. It will immediately resume its straight shape. Look at the list of ideas on the board and ask the students which of these have to do with conduction. If a student response is correct, have him/her explain why. Put a star next to the correct responses. Convection Looking at the list, ideas will remain that do not have to do with conduction. Introduce the second form of heat transfer. Tell the students that convection is the transfer of heat through air or fluid through currents. Ask if they have ever noticed the difference in temperature on the top floor of their house verses the basement. That is convection. Heat goes higher; cold creeps lower.

This is why heating vents in homes are located along the floor, while air-conditioning outlets are located near the ceiling (although in Utah the heating vents are sometimes located near the ceiling in basements so the builders do not have to put in another set of HVAC ducts). Again remind the students about heating the water for the food coloring demonstration. The hot plate caused the saucepan to get hot via conduction. But the water got hot due to convection. The hot water at the bottom of the pan expanded and became lighter than the colder water above it. So the heavier water sank to the bottom and the warmer, lighter water rose to the top. Eventually, after changing places several times, the water became hot enough to boil. Obtain a candle spinner or the pre-cut paper spiral attached to a string. Light the candles of the spinner and watch the vanes spin. If using the paper spinner, hold it above a hot plate. Be careful to hold it high enough that the paper will not burn. Watch it spin. Ask the students why they think this is happening. After listening to ideas, remind the students that hot air rises. The hot plate produces a current of hot air.

As the air rises, the convection produced causes the vanes or spinner to turn. Set up another example of convection by folding a piece of tissue paper in half widthwise, then in half widthwise again. Create a prism out of the paper. Stand it on top of the aluminum pie tin. Using safety precautions, use a lighter or match to light the top of each edge of the triangle formed. The flames will quickly travel down the prism and get trapped inside, then gently raise the tissue paper in the air. Eventually the tissue paper will float down. Catch it with the aluminum tin, let it cool, and then discard. Ask the students what is happening this time. Heat is again rising. When the heat escapes from the flames, it falls. Look at the remaining list of ideas on the board and ask the students which of these have to do with convection. Also ask the students if any of the starred ideas also have to do with convection. If a student response is correct, have him/her explain why. Put a smiley face next to the correct responses. Radiation Introduce radiation, the transfer of energy as electromagnetic waves. It does not need to directly touch anything or move particles as in conduction and convection.

Radiation occurs through empty space, as in the sun heating the earth or feeling warm in front of a fire. For the last time, remind the students about boiling water on the hot plate. Tell them that before the sauce pan was even placed on the hot plate, the heat could be felt without touching the burner. That is radiation. Tell the class that Ben Franklin, one of our founding fathers, experimented with radiation in the 1700s. One sunny winter day, he and a friend laid colored cloth patches and a pane of glass out on the snow and noted how deeply each eventually sank into the melting snow below it. Ask the students what they think the results might have been. After listening to the ideas, bring a pad of paper, a pencil, and a heat gun outside to the pre-set cloth patches (if it is a snowy day) or the paper (if there is no show) set out prior to beginning the lesson. Using the infrared temperature gun, test the temperature of each and record the data. The students will see that the black cloth/paper and the glass pane will be significantly warmer than the white cloth/paper. Dark colors will also be warmer compared to the lighter colors.

If you did use cloth, lift up each piece and note the indentation in the snow. The white will make little to no indentation; the black will be the deepest. Likewise, if using a light pane of glass, it too will sink as deep as the black paper. Return to the classroom and have the students note their observations in their journals. Explain that it took a long time for the results of Franklin�s experiment to make sense. We now know that black absorbs light and heat, while white reflects them. Similarly, since glass is clear, light travels right through it, but absorbs the heat radiated by the sun � the infrared � and therefore gets just as hot as the black cloth. Infrared radiation is reflected by the cloth patches because they are opaque. Look at the list of ideas on the board and ask the students if any of these have to do with radiation. Again, items involving conduction and convection may involve radiation, as well. If a student response is correct, have him/her explain why. Circle the correct responses. To wrap it up, tell the students that heat is usually transferred in all three ways, as in the boiling water discussed throughout the lesson. As another example, place the Hogle Zoo Heat!

transparency on an overhead projector. Cover the pictures. Take students through the situation presented, uncovering each picture as the time comes. You may also cover all of the words of the transparency and allow students to explain the type of heat transfer at each step to check for understanding. Pass out a copy of Test Yourself: Conduction, Convection, and Radiation to each student to assess their understanding of heat transfer. Have them write down any questions they have about these concepts in their journals. Extensions: Curriculum Extensions/Adaptations/ Integration Invite a health care professional to discuss how your body maintains temperature and why you might have a fever when you are sick. Show either or both of the DVDs The Convection of Heat or Bill Nye�s Heat to reinforce the concepts of energy, conduction, convection, and radiation. Show the seven-minute video from NASA entitled Infrared: More Than Your Eyes Can See to learn more about infrared radiation. Invite a firefighter to discuss fire prevention, how knowledge of heat transfer is important to get out alive, why a fire must be vented, and the difference in temperature between the top of a room and the floor. Learn about the importance of heat when blowing glass. If possible, attend a glass blowing demonstration.

The Bill Nye video contains a small section about glass blowing. Family Connections Cook a meal using a Dutch or solar oven. Discuss how conduction, convection, and radiation help in the cooking process. Carefully place a paper cup full of water in a fire and watch it boil. Make sure the cup is surrounded by glowing embers. Discuss how conduction, convection, and radiation prevent the cup from burning. Assessment Plan: Ask students to try any or all of the questions on the Test Yourself: Conduction, Convection, and Radiation sheet. Over a three-day period, have students write down three encounters with heat per day in their journals. How did these experiences have to do with conduction, convection, and radiation? After the three days, collect the journals to check and assess. Bibliography: Research Basis Loucks-Horsley, S., & Olson, S. (Eds.). Inquiry and the National Science Education Standards: A Guide for Teaching and Learning. Washington, D.C: National Academies Press, 2000. Word Smith as you Choose discusses engaging students in exploring and communicating their thinking through writing, which has shown tremendous benefits in science and mathematics.

Writing explorations help students in understanding concepts more deeply, improving problem solving strategies, assessing their personal thinking skills, learning to consider themselves as deep thinkers, and overall becoming better learners in and out of the classroom. Journals are concrete and visible evidence of student thinking, effort, and progress. They should show if students have or have not achieved the desired learning, if they have misconceptions, their reflections, their journey towards understanding a concept, and/or if more instruction is needed. Kruger, A., & Sutton, J. (Eds.). (2001). EdThoughts: What we know about science teaching and learning. Colorado: Mid-continent Research for Education and Learning. This book supports standards-based reform of science education. Research and best practices are provided, as well as ways to improve classroom instruction. A list of additional resources is also available for those desiring deeper understanding of certain concepts. All articles rely on the national standards for best practices. A common theme is the importance of quality science education for all students.

Author: Mechanisms of Heat Transfer: Conduction, Convection & Radiation - Video & Lesson Transcript | Study.com Mechanisms of Heat Transfer: Conduction, Convection & Radiation Watch short & fun videos Start Your Free Trial Today An error occurred trying to load this video. Try refreshing the page, or contact customer support. You must create an account to continue watching Register for a free trial Are you a student or a teacher? I am a student Start Your Free Trial To Continue Watching As a member, you'll also get unlimited access to over lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed. Coming up next: What is Humidity? - Definition, Measurements & Effects You're on a roll. Keep up the good work! Your next lesson will play in 10 seconds 0:09 Heat Is Mobile Add to Add to Add to Want to watch this again later? Log in or sign up to add this lesson to a Custom Course. Custom Courses are courses that you create from Study.com lessons. Use them just like other courses to track progress, access quizzes and exams, and share content. Teachers Organize and share selected lessons with your class.

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Lesson Transcript Instructor: Sarah Friedl Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology. There are three main ways that heat is transferred between substances or objects. In this video lesson you'll learn about each one, and identify the differences between them. Heat Is Mobile Have you ever touched a hot pan and burned yourself, how about warmed yourself in front of a nice campfire on a cold night? Ever noticed that, most of the time, air vents in houses in the North are on the floor while in the South they're on the ceiling? These are all related to the movement of heat and how it gets transferred between substances and locations. Heat can move between objects or spaces, and there are three main ways it does this: conduction, convection and radiation. Let's look at how each one works. Conduction If someone pours you a hot cup of coffee, you're likely going to pick it up from the handle instead of around the main part of the mug. This is because the coffee mug will conduct heat from the hot liquid inside to your hand. This direct transfer of heat from one object to another is called conduction.

The coffee mug transfers heat well, as do many other substances, including metal. You certainly wouldn't touch a hot metal pan on the stove or hold a nail over a flame without some sort of protective glove, because you know that the heat from the metal will quickly transfer to your hand and burn you. It makes sense then that these substances that conduct heat well are called conductors. Some substances are very bad at transferring heat, and these are called insulators. Wood is a very good insulator (therefore a very poor conductor), which is why your metal pot on the stove may have a wooden handle. You can do a little experiment to see this in action. If you have an area in your house where wood floor meets some tile, put a bare foot on each one. You'll find that the tile floor feels cooler, but both floors are actually the same temperature! The tile is a much better conductor of heat than the wood, so it transfers the heat from the bottom of your feet better than the wood does, making your foot feel cold. Air is also a very poor conductor, which is why you can reach your hand in the oven to grab a dish without burning yourself (unless you forget your oven mitts and grab the dish itself!).

Your house is insulated with something like rock wool or fiberglass because these substances are poor conductors of heat. Imagine how cold your house would be if there was no protection from the cold outside during winter, or how hot it would be if you weren't protected from the heat outside in the summer! Interestingly, though, insulators don't actually stop the flow of heat, just slow the transfer considerably. That's why you still need to run your heat in the winter and A/C in the summer - just not as much as if you had no insulation at all. Convection Remember how I said houses in the North commonly have their air vents on the floor while houses in the South have theirs on the ceiling? This is due to the way heat flows in fluids, and both gases (like air) and liquids (like water) are considered fluids because of the way they move. When heat is transferred by currents in a fluid this is called convection. Convection happens in all fluids like the air in your home, water in a lake and magma underground, just to name a few. Here's how it works: imagine a room full of children who have just had a bunch of cookies. They become quite energetic and start running around the room.

If you open the door and let them outside, they burst out and start running around the larger space, spreading out in the yard. The same thing happens when a fluid is heated. The molecules at the bottom gain energy and begin to move faster. As they move faster, they begin to spread apart, which makes the fluid less dense. But instead of being confined to a yard, the fluid's warmer molecules begin to move upward, while the cooler, denser fluid begins to sink (just like the children start settling down as their cookie rush subsides). Once the cooler molecules reach the bottom where the heat is, it's like they ate another cookie and get an increase in energy again. They rise back up to the top, while the cooler molecules sink back to the bottom. The cycle continues as long as there is a heat source at the bottom. What does this have to do with air vents in a house? Well, if you live in a cold climate, you probably use your heat more often than your A/C. And if you live in a warmer climate, you probably use your A/C more than your heat.

So, if hot air rises, and you use your heat more often, you'll want your vents on the floor so that hot air travels upward to fill the room as it comes out of the vents. The opposite is true for air conditioning: if cool air sinks, you wouldn't want it to start out on the floor because it wouldn't cool the room as effectively as if it starts from the ceiling, where it can cool the room as it falls. Radiation When you sit in front of a campfire, you are warmed by its heat. Hover your hand over a hot light bulb, and you'll feel the heat coming off it, even without touching it. Stand out in the sunshine, and you'll quickly get warmed by its rays. These examples illustrate our final mechanism of heat transfer, radiation. Radiation is when heat is transferred by electromagnetic waves, like radio waves, infrared waves, X-rays and even visible light! All objects both emit and absorb radiant energy - even you! Though, like everything else, some objects and substances are much better at this than others. Those that are good at emitting radiant energy, like the sun, are also very good at absorbing it.

A radio antenna is designed to be a good absorber of radio waves, but that also means it's very good at emitting them. When it comes to radiant heat, much of what is produced on Earth comes from the earth itself. It absorbs radiant energy from the sun and then emits it back toward space. This radiant energy emitted by the earth is called terrestrial radiation, and it is what makes life possible. Like a big blanket, gases in the atmosphere trap terrestrial radiation on Earth and keep our planet nice and warm. × Cooking For Engineers Cooking For Engineers Heat Transfer and Browning Foods by Aaron Chan Normal view Yesterday, in one of my lazy moods, I succumbed to the microwave pizza dinner. Opening the box, I noticed the instructions mentioned placing the frozen pizza on a shiny, metallic disk but only if I used the microwave and not the oven. This got me thinking about the effect of the disk on the pizza and so I did a little research on the browning of foods. But first, a look at cooking methods and an examination of how each method affects browning. Cooking, at its simplest definition, is preparing food through the use of heat. The different methods of cooking or heat transfer are broken down into conduction, convection, and radiation.

Every method of cooking involves one or more of these heat transfer methods. Conduction This is the exchange of thermal energy through direct contact between a heating element and the food. Different materials result in different heating times and temperatures. Please refer to the article Equipment and Gear: Common Materials of Cookware for a complete breakdown of materials that directly affect conduction. Pan-frying or sautéing are common forms of conduction. The pan heats up and, through direct contact with the food, cooks the food. Fat or oil used in the frying provides uniform contact with heat, lubrication to prevent sticking, and some flavor of its own. Oddly enough, cooking in oil is considered a dry technique because the oil acts more like a cooking material than anything else. The moisture in the food will still be contained because it will not mix with the oil surrounding it. Convection Whereas in conduction heat is transferred through direct contact, in convection, heat is transferred by the movement of molecules in either gas or liquid. The fast moving molecules of the convection medium collide with the slower molecules in the food and heat them up. Baking and roasting are common forms of convection cooking.

The heating elements within the oven heat the air and that comes in contact with the food. Boiling and steaming are also forms of convection with water or steam acting as the convection fluid. In deep-frying, the oil envelops the food, like a fluid pan that completely encases the food and heats the surface evenly. Convection relies much on the density of the fluid. Liquid convection, either through boiling, steaming, or deep frying, is a much more effective transfer of heat than gas convection. This is why boiling a potato is much faster than baking. The denser the fluid, the more often the molecules collide with the food and the fast the food heats up. Therefore in convection methods involving air such as baking, the temperatures must be much higher than in liquid convection. This is why you can stick your hand into a 500°F oven without burning yourself but you cannot stick your hand into a pot of boiling water at only 212°F. Radiation While conduction and convection are heating methods through molecule to molecule contact, radiation is the transfer of heat through waves of pure energy. Most of the heating energy comes from the infrared radiation below visible light.

When you hold your hand near glowing coals or a stovetop burner, the heat you feel is infrared. Technically, everything emits thermal radiation including you and me, and so every cooking method has an element of radiation. Grilling and broiling, the former with heat below the food, and the latter with heat above, are two methods of radiation cooking. Of course there is convection from the air in between the heat source and the food and conduction from the grate, but the heat is primarily radiated. Microwaves are below infrared waves on the spectrum and so carry much less energy. Infrared waves have enough energy to heat up almost all types of molecules, but microwaves tend to only heat up polar molecules such as water, sugar, and fats. Foods containing water are heated by these microwaves which penetrate about an inch into the food's surface. The interior of the food is still heating by conduction of the heat from the surface into the interior. Cooking Method No The Browning Reactions: Caramelization and the Maillard Reaction Heating foods intensifies flavors already latent within the foods; however, browning creates new flavors that are intrinsic to the cooking process.

This is why a poached salmon and a grilled salmon both tastes identifiably like salmon, but you can also easily distinguish one food as poached and the other as grilled. There is flavor within the cooking method itself created by caramelization and the Maillard Reaction. Caramelization We have all had caramel candies before, but how many of us realize that those sugary delights are not much more than sugar itself. The caramelization of sugar is the simplest browning reaction happening at around 330°F/165°C. Plain table sugar melts into a thick syrup, then gradually darkens into a light yellow and eventually a dark brown. The flavor begins sweet and clean, but develops acidity, bitterness, and a rich aroma. The chemical process itself is complicated, but the reaction products include organic acids, sweet and bitter derivatives, fragrant molecules, and brown polymers. The Maillard Reaction Named for Louis Camille Maillard, the French physician who documented these complex reactions around 1910, Mailliard Reactions are responsible for bread crusts, chocolate, coffee, dark beers, and roasted meats.

The sequence begins at about 220°F/115°C when a carbohydrate molecule and an amino acid bind together in an unstable structure, producing flavorful by-products. The involvement of amino acids brings nitrogen and sulfur creating meaty and earthy flavors. These reactions create that crust on seared foods and the brown coloring of a good roast as well as multitudes of other browned foods. Both caramelization and the Maillard Reaction require relatively high temperatures beginning above the boiling point of water 212°F/100°C. As a result, wet processes such as boiling and steaming will never be able to brown foods because the temperature of the food will only get as high as the 212°F with slight adjustment due to elevation and atmospheric conditions. Dry methods are able to reach much higher temperatures allowing the browning reactions to occur. This is why braised foods are usually seared first to create those flavors and colors that otherwise won't occur in a wet, low temperature setting. There are notable exceptions to browning above the boiling point. Basic solutions, concentrated mixtures of carbohydrates and amino acids, and long cooking times can create the same reaction. Examples include reductions of stock to create demiglace and brewing beer.

Back to my microwave pizza. Metal placed in microwaves usually creates dangerous sparking through the buildup of electric fields. Very small amounts of metal however can be heated without creating a danger and when this metal is heated, it reaches temperatures far beyond the boiling point of water. This is the function of the metallic disk with my pizza. The disk is placed underneath the crust and so when it is heated by the microwaves, it subsequently heats the crust through conduction at temperatures high enough for Maillard Reactions to occur. This is how microwave pizza makers brown the crusts. The effect can also be seen in Hot Pocket brand stuffed sandwiches which utilize a microwave sleeve slipped around the Hot Pocket with similar metallic coating. The microwave sleeve heats up hot enough to brown the crust of the Hot Pocket. Aaron Chan believes that since everyone needs to eat to live, food should be everyone's top priority. Written by Aaron Chan HEAT TRANSFER HEAT TRANSFER FIND OUT MORE Heat energy can be transferred from one place to another by three main processes. In CONVECTION , heat energy is carried by the movement of particles of matter. In CONDUCTION , heat is transferred by particles vibrating. In RADIATION , heat is carried directly by electromagnetic waves.

When a hot object touches a cool object, heat moves from the hot one to the cool one. When objects transfer heat, they cool down to a lower temperature unless the heat energy they lose is constantly replaced. EVAPORATION Another process by which heat is transferred is called evaporation. When a dog sticks out its tongue and breathes hard (pants), the moisture on the tongue turns into water vapour – it evaporates. Heat energy is needed to turn a liquid into a gas, so heat is removed from the dog’s tongue in the process. This helps to cool the dog down. People cool themselves down by sweating through pores (tiny holes) in their skin, which removes heat from their bodies in the same way. CONVECTION Convection is like an invisible conveyor belt that can transfer heat through fluids (liquids and gases). When part of a fluid is heated up, it expands. This makes it lighter and less dense than the fluid around it, so it rises upwards. As it rises, it moves away from the source of heat. Then it starts to cool and move downwards, before starting the cycle over again. HOT AIR BALLOON A burner at the base of the balloon warms the air inside.

As the air warms up, it moves upwards, cools, and moves round in a circular pattern known as a convection current. When the balloon is full of hot air, it lifts off the ground because the hot air inside it is less dense and lighter than the cold air outside it. SEA BREEZES During the day, sunlight warms the land more quickly than the sea. Warm air rises from the land by convection, moves out to sea, and cools, creating a circular current. This is why, at ground level, sea breezes blow from sea to land during the day. At night, the land cools more quickly than the sea. Warm air rises from the sea, the convection current reverses, and the breezes blow from land to sea. BIOGRAPHY: ALEXANDER VON HUMBOLDT German, 1769-1859 Explorer Alexander von Humboldt explained how the oceans circulate using convection. Water warms and rises at the Equator, where Earth is hottest, then flows along the surface before cooling and sinking at the poles. He gave his name to the Humboldt Current, which travels up the South American coast.

CONDUCTION Heat travels through solids by conduction. If one end of a metal bar is heated, heat energy moves rapidly along the bar. The hot particles do not move along the bar, but vibrate and pass energy to their neighbours. Materials that conduct electricity are also good conductors of heat. Metals conduct heat well, but wood, plastics, and glass conduct heat only poorly. ALUMINIUM POT Cooking pots are made from metal, often aluminium. This metal is a good conductor of heat, so it rapidly transfers heat energy from the stove to the food. The handles of cooking pots are often made of wood or plastic. These materials do not conduct heat very well and are called insulators. RADIATION All the light and heat energy we receive on Earth comes from the Sun, and travels through space in invisible electromagnetic waves known as radiation. Space is vast and empty, so heat energy cannot travel from the Sun by conduction or convection. Hot objects on Earth, such as fires and radiators, also radiate heat. FIND OUT MORE

Question: When baking, by what primary means does the heat reach the food? Convection, conduction, or radiation?
Answer: Convection