where: Ew is the work done on an object (J) F is the acting on an object (N) d is the distance or displacement of an object (m) What is by the term ‘work done’? Copy and complete this table: Work Done / J Force / N Distance / m (a) 100 30 (b) 25 6.2 (c) 300 000 150 (d) 40 2 (e) 1250 125 (f) 144 000 3200 What is the work done by a shopper pushing a shopping trolley with an average force of 480 N over a distance of 35 metres? What is the average force applied by a mother pushing a pram for a distance of 500 metres if her total work is 150 000 J? What is the distance that a boy pushes his bike if he does 240 000 J of work and applies a constant force of 6000 N? What is the work done by a truck if it drives 20 km with an average engine force of 1.5 kN?
Answers Weight (p4 – 5) Mass is a measure of how much of an object is there. It is measured in kg. Weight is a downward force caused by gravity. It is measured in N. (a) 30 N 2.25 N 30 kg 30 kg 8 N/kg 26 N/kg (a) 30 N 1 N 650 N 12 000 N The weight of the space shuttle decreases. This is because: There is less mass due to fuel being used up. The shuttle is getting further away from the surface of the Earth. 85 kg (a) 372 N 2418 N 372 N 0 N 75 kg Saturn (a) 13 N/kg Person pushes down on scales. Scales push back up on person. Work Done (p6 – 7) Work done is the energy transferred to an object. (a) 3000 J 155 J 2000 N 20 N 10 m 45 m 16 800 J 300 N 40 m 3 x 107 J 25 km 7890 N 6960 J 4.5 N
Efficiency (p14 – 15) (a) Electrical Light Electrical Heat Gravitational Potential Kinetic Light Electrical Electrical Sound (a) Chemical Heat Kinetic Electrical Water is stored in a high reservoir behind a dam. The water is released through the dam in to a low level reservoir. The gravitational potential energy of the water is turned in to kinetic energy, which turns turbines which generate electricity. Heat energy is lost to the surroundings in many places (boiler, etc.) (a) 25 kJ 16 250 J Missing energy is lost as heat and sound energy to the surroundings. (a) 60 % 5 % 2795 J 12 160 000 J 108 000 J 4 875 000 J 35 % 486 J (a) Electrical Sound 5488 J 6 240 000J 792 000 J Electrical Charge (p16 – 17) The image of the document is projected on to a positively charged copying plate. light is incident on the plate, charge leaks away. The negatively charged particles are attracted to the remaining positive areas. Paper is placed over the copying plate. The is transferred to the paper and sticks after heating. ‘Fluffing up’ a static duster causes it to become charged. This attracts charged particles to the duster. (a) The droplets of have the same charge so spread out more when sprayed. This keeps the coat of paint even. The car body has an opposite charge so paint drops that would miss are attracted to it. (a) When you unroll cling film, negative charges (electrons) from one layer get pulled across to the other. This creates patches of positive and negative charges. The plastic bowl is an electrical insulator. The negative charges can flow through the conducting metal bowl and away from the cling film. (a) Air and dust particles enter the precipitator through a negatively charged metal grid. The dust particles can be charged but the air particles can’t.
generated by a solar cell increases. A of 1/V2 against d will give a straight line. Capacitors (p30) A capacitor stores energy / charge. (a) 0 V 6 V 0V Increase the value of resistor, R, or increase the value of the capacitor, C. (a) Decrease the value of resistor, R, or decrease the value of capacitor, C. LEDs (p31) A is an electrical component that only allows current to flow in one direction through it. Electrical Light Advantages: Less current, Less heat given off. Disadvantage: Not as bright. (a) The resistor protects the LED from being damaged by a high current. 2800 Ω B only. Thermistors & LDRs (p32 – 33) As temperature increases, resistance decreases. As temperature increases, current increases. Any value greater than 5700 Ω.
As brightness of light increases, the resistance decreases. As brightness of light increases, the current increases. Any resistance greater than 3.3 kΩ. Resistors in Series (p34) As more resistors are connected in series, the total resistance increases. (a) 245 Ω 950 Ω 830 Ω 625 Ω (a) 550 Ω 6390 Ω Resistors in Parallel (p35) As more resistors are connected in parallel, the total resistance decreases. (a) 20 Ω 30 Ω 600 Ω 150 Ω (a) 40 Ω 300 Ω Resistors in Mixed Circuits (p36 – 38) (a) 44 Ω 30 Ω 75 Ω 600 Ω 45 Ω (a) 108 Ω 12 Ω 54 Ω 24 Ω (b) Is the lowest resistance 400 Ω Voltage Dividers (p39 – 41) As its resistance increases, voltage across a resistor also increases. As resistance increases, electrons will lose more energy as they pass through the resistor. Therefore, the voltage across the resistor will increase. (a) 125 Ω 225 Ω 1.2 kΩ (a) 250 Ω 300 Ω
40 Ω (a) 3 V 3V 0.5 V (a) 3.8 V 1.0 V 0.6 V (a) V1 = 6 V, V2 = 6 V V1 = 4 V, V2 = 10 V V1 = 7 V, V2 = 3 V V1 = 16 V, V2 = 6 V V1 = 4.5 V, V2 = 13.5 V V1 = 5.5 V, V2 = 3 V Switching Circuits (p42 – 47) (a) 0V 15 V 15 V 0V (a) 4V 8 V 0 V 12 V (a) 700 Ω 3.5 V 2.5 V 3 V 100 °C (a) 6 V 2.5 V 0.016 A (a) (NPN) Transistor As the temperature increases, the resistance of the thermistor decreases. The voltage across the thermistor decreases and the voltage across R1 increases. When this voltage reaches 0.7 V, the transistor switches on and the LED lights. Swap the positions of the thermistor and R1. (a) MOSFET As brightness of light decreases, the resistance of the LDR increases. The voltage across the LDR increases. When this reaches a high enough voltage, the MOSFET switches on and the lamp lights. The brightness at which the lamp turns on can be altered. (a) 13.3 V 1900 Ω 150 Ω This circuit could turn on a fan when a computer overheats. As the temperature increases, the resistance of the thermistor decreases. The voltage across the thermistor decreases and the voltage across R1 increases. When this voltage reaches 0.7 V, the transistor switches on and the LED lights. Electrical (p48 – 50) The light bulb uses up 60 J of electrical energy every second. (a) 80 W 85 W 45 000 J 1 305 000 J 250 s 140 s 90 W 62 640 000 J 600 s
P = I V P = (V/R) x V P = V2 / R (a) 20 W 0.5 W 15 V 6 V 20 Ω 132.25 Ω 100 W 7 V 66 Ω (a) 407 Ω 0.57 Ω 11 232 000 J (a) Electrical Heat S1 and S2 4.6 A 1058 W 37.5 Ω 1411 W 63 480 + 677 280 = 740 760 J High current in transmission lines results in high energy loss. Using high voltages reduces the current in transmission lines and therefore reduces energy loss. (a) 0.36 W 10.8 W 900 W Specific Heat Capacity (p58 – 59) Heat is a form of energy. Temperature is a measure of how hot or cold an object is. 4180 J of heat energy is needed to change the temperature of 1 kg of water by 1 °C. (a) 47 000 J 112 750 J 4100 J / kg °C 270 J / kg °C 0.4 kg 5.5 kg 7.5 °C 4 °C 752 400 J 380 J / kg °C 2.5 °C 12 kg (a) 618 J / kg °C The experimental value is larger. This is because not all heat energy is absorbed by the copper block. Heat Transfer (p60) (a) Heat energy is transferred by vibrations of particles that are in contact with each other. Particles that are heated rise while particles that have cooled fall. There must be space between particles for this to happen. Heat energy travels in straight lines in the form of electromagnetic radiation. No particles are required for heat to be transferred by radiation. Despite having a lower temperature, a cup of coffee has more heat energy than a single spark from a sparkler. The tomato has a higher specific heat capacity. More heat energy is required to change its temperature. Fire fighting clothing is made of a substance that is fire resistant and a poor conductor of heat. The clothing is insulated using trapped air which prevents heat transfer from surroundings. Reflective panels are used for added visibility but also reflect heat radiation. Conduction: a fabric insulating cover on the outside of the boiler prevents heat loss. Convection: The top of the boiler is air-tight to prevent heat loss. Radiation: The interior (or exterior) of the boiler can be coated with a reflective material (such as foil) to prevent heat loss. Pressure (p61 – 62) Pressure is the force applied to an area of a surface.
(a) 75 Pa 8000 Pa 220 000 N 720 N 60 m2 3.5 m2 149 Pa 0.08 m2 (a) 4032 Pa 260 Pa (a) Answer depends on student. Answer depends on student. A person is more likely to fall through an icy lake on tip-toes. The area of contact with the ice is low, so the pressure on the ice will be high. 0.0005 m2 Gas Laws (p63 – 67) When a particle is heated, it vibrates. If it is heated enough, the particle will vibrate enough to break free from surrounding particles. This turns a substance from a liquid to a gas. The particles in the air are moving around all the time. When the air particles collide with the inside surface of the tyre, they impart a force and, therefore, a pressure is created. -273 °C is the temperature at which particles stop vibrating and so, as a result, it’s the coldest possible temperature. (a) 273 K 293 K 0 K 373 K (a) and (b) Pressure is directly proportional to temperature (in Kelvin). As temperature increases, the particles in the gas gain kinetic energy. Therefore, they collide with the sides of the container more often and with more force. This results in a pressure increase. As the tyre heats up during travel, the air particles inside will collide with the inside surface of the tyre more often and with more force. If the pressure gets too high, the tyre burst. 129 kPa 30 °C (a) Pressure is inversely proportional to volume. As the volume of a gas increases, the particles will hit the sides of a container less often so the pressure decreases. If a balloon is squeezed, the volume of the air inside decreases. Therefore, the pressure of the gas increases and the balloon could burst. 1.68 x 105 Pa 899 Pa (a)