Unizor - Physics4Teens - Energy - Heat - Molecular Movement
Channel: Zor Shekhtman on UNIZOR Education
Date: 15/04/2019

Unizor - Physics through a Looking Glass of Mathematician

Notes to a video lecture on http://www.unizor.com

Molecular Movement

Molecules

In order to understand the nature of heat and temperature, we have to go inside the objects we experiment with.
Tiniest amount of any substance, that retains the qualities of this substance, is called a molecule.
Any object or substance we are dealing with consists of certain number of molecules - the smallest particles that retains the qualities of this object or substance. 

States of Matter

There are three major states of matter: solid, liquid and gas.
When an object is solid or is in solid state, it means that it retains its shape and form regardless of surrounding environment, not intended to change its form. The molecules of this object are strongly connected to each other. Their movement relative to each other is rather restricted. This movement can be oscillating around some point, maintaining an orderly three-dimensional structure, for crystal (or crystalline type of) solids or just slow movement, changing their relative position, but not changing the overall form for amorphous (non-crystalline) type.
Examples of solid objects are ice (crystal), steel (crystal), plastic cup (amorphous).

When an object or substance is liquid or is in liquid state, it takes the shape of a vessel or reservoir it's in. The connection between the molecules in case of liquid is strong enough to hold the molecules together, but not strong enough to preserve the overall shape.
Examples of liquid substances are water, mercury ("quick silver"), oil.

When an object or substance is gas or is in gaseous state, it takes as much space as it is available. Connections between the molecules are weak and they fly in all directions in completely chaotic fashion. Examples of gases are air, helium, oxygen.

Nature of Heat

Now we are in position to talk about heat.
Heat is the energy of molecular motion inside any object or substance.
Heat is mechanical energy of molecules inside the object or internal energy of the object or substance.

As we know, mechanical energy can be transferred from one object to another, like during the collision of two billiard balls. Similarly, mechanical energy of one molecule can be transferred to its neighbors, from them - to their neighbors etc. This is a process of dissipation of heat. All what's necessary for this is the relative proximity of the molecules. This is exactly the way how heat is transferred from one body to another, from flame to pot, from pot to water, from water to vegetables in it, making soup.

We all know that mercury thermometer is working based on the property of mercury to expand as the temperature is rising.
This is a general property of most of the objects - to change physical dimensions with increase or decrease of amount of heat (internal energy) carried by their molecules during their constant motion.
This property is the principle, on which measuring of the intensity of molecular movement is based.

Heat and Temperature

Now let's address the issue of measuring the heat, that is amount of internal energy inside any object.
The term temperature is related to average intensity of the molecular movement inside an object or a substance. So, when we say that the temperature of an object has increased or decreased, we mean that average intensity of the molecular movement in it has increased or decreased correspondingly.