Energy Work And Power

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Energy Stores & Transfers

Energy is a property stored or transferred, measured in joules (J).
A system is an object or group of objects; a change in a system involves energy transfer.
Energy stores include: kinetic, gravitational potential, elastic, magnetic, electrostatic, chemical, nuclear, thermal.
Energy transfer pathways: mechanical (force), electrical (charge flow), heating (particle collision), radiation (electromagnetic waves).
Example: a battery powering a torch transfers energy electrically from the chemical store of the battery to the thermal store of the bulb.

Energy transfer by heating from a hot coffee mug to cold hands

Kinetic Energy

Kinetic energy is the energy an object has due to its mass and speed: E_k = \frac{1}{2}mv².
Kinetic energy is directly proportional to mass (E_k \propto m) and to the square of speed (E_k \propto v²).
Doubling speed quadruples kinetic energy; doubling mass doubles kinetic energy.
Always square the speed when calculating E_k.

Kinetic energy of a moving car

Gravitational Potential Energy

Gravitational potential energy is the energy an object has due to its height in a gravitational field: \Delta E_p = mg\Delta h.
Work is done against weight to lift an object, transferring energy to its gravitational potential store.
Gravitational field strength on Earth is g = 9.8\,\text{N/kg}.
Round final answers to the lowest number of significant figures in the input values.

Gravitational potential energy of a lifted mass

Conservation of Energy

Energy cannot be created or destroyed, only transferred from one store to another.
In a closed system, total energy is constant: total energy in = total energy out.
Dissipated energy is spread out to the surroundings (often as thermal energy) and is usually wasted.
Energy flow diagrams show stores (labels) and transfers (arrows); the total energy is conserved.

Conservation of energy: a bat hitting a ball

Work Done

Work is done when a force moves an object over a distance in the direction of the force: W = Fd = \Delta E.
Work done and energy transferred are equivalent: 1 N m = 1 J.
No work is done if the object does not move (e.g., pushing against a wall).
Example: a bird flying does work against air resistance (drag).

Work done pushing a box

Power

Power is the rate of work done or energy transferred: P = \frac{W}{t} = \frac{\Delta E}{t}.
Power is measured in watts (W); 1 W = 1 J/s.
Common power ratings: torch 1 W, light bulb 100 W, large power station 10 GW.
A more powerful machine does the same work in less time.

Two motors lifting the same weight at different speeds

Efficiency

Efficiency is the ratio of useful energy (or power) output to total energy (or power) input.
Efficiency = (useful energy output / total energy input) × 100%.
Efficiency = (useful power output / total power input) × 100%.
Efficiency has no units; it can be a decimal (0–1) or percentage (0–100%).
Example: a typical thermal power station is about 30% efficient; 70% of energy is wasted.

Sankey diagram of a gas-fired power station

Energy Resources

Solar cells convert sunlight directly into electricity (photovoltaic effect); solar panels heat water using infrared radiation.
Wind turbines transfer kinetic energy of wind to electricity; efficiency ~50%.
Fossil fuels (coal, oil, gas) are non-renewable; burning releases CO₂ and SO₂.
Biofuels are renewable and carbon-neutral in principle, but have lower energy density.
Nuclear fission splits large nuclei to release energy; used in power stations.
Hydroelectric, wave, and tidal power use water movement to turn turbines.
Geothermal energy uses heat from Earth's core; renewable but location-dependent.

Wind turbines on a coastal wind farm

Nuclear Fission & Fusion

Nuclear fission: splitting a large nucleus into two smaller nuclei, releasing energy.
Nuclear fusion: joining two small nuclei to form a larger nucleus, releasing huge energy (occurs in stars).
Fission is used in nuclear power stations; fusion requires extremely high temperatures and pressures.

Nuclear fission of a target nucleus

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Practice questions

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1.Which of the following is a correct unit for energy?
Easy
- Ajoule
- Bwatt
- Cnewton
- Dmetre
2.A ball is dropped from a height. As it falls, what happens to its gravitational potential energy and kinetic energy?
Easy
- Agravitational potential energy increases, kinetic energy decreases
- Bgravitational potential energy decreases, kinetic energy increases
- Cboth gravitational potential energy and kinetic energy increase
- Dboth gravitational potential energy and kinetic energy decrease
3.State the principle of conservation of energy.
Easy
4.Complete the sentence about power.
Easy
Power is the rate at which ____ is done or energy is transferred.
5.Name one energy resource that does not come from the Sun.
Easy
6.An electric motor has an efficiency of 35%. It lifts a 7.2 kg load through 5 m in 3 s. What is the power input to the motor? (g = 9.8 N/kg)
Hard
- A117.6 W
- B336 W
- C352.8 W
- D100.8 W
7.Explain the difference between nuclear fission and nuclear fusion.
Medium
8.Which energy store is associated with a moving object?
Easy
- Akinetic
- Bgravitational potential
- Cchemical
- Delastic potential

More questions

9.Work done is equal to energy transferred.
Easy
True or false?
10.Match each energy store to its description.
Medium
- Kinetic
- Gravitational potential
- Elastic potential
- Chemical
- Energy stored in stretched or compressed objects
- Energy stored in moving objects
- Energy stored in bonds between atoms
- Energy stored due to an object's height in a gravitational field
11.Arrange the following energy resources in order from the one that ultimately comes from the Sun to the one that does NOT come from the Sun. (Place the Sun-derived first.)
Medium
- Wind
- Geothermal
- Hydroelectric
- Tidal
12.The energy stored in a moving object is called gravitational potential energy.
Easy
True or false?
13.Select all the statements that correctly describe energy transfers in a coal-fired power station.
Hard
- AChemical energy in coal is converted to thermal energy by burning.
- BThermal energy is used to boil water, producing steam.
- CSteam directly turns the generator to produce electricity.
- DThe turbine converts kinetic energy into electrical energy.
- EEnergy is dissipated to the surroundings as waste heat.
14.A ball is dropped from a height. Ignoring air resistance, which energy change occurs as it falls?
Easy
- AKinetic energy decreases, gravitational potential energy increases.
- BKinetic energy increases, gravitational potential energy decreases.
- CKinetic energy remains constant, gravitational potential energy decreases.
- DKinetic energy increases, gravitational potential energy remains constant.
15.A motor lifts a load vertically at constant speed. The weight of the load is 200 N and it is lifted 3 m in 2 s. What is the power output of the motor?
Medium
- A100 W
- B300 W
- C600 W
- D1200 W
16.Which of the following is NOT a unit of power?
Easy
- AWatt
- BJoule per second
- CNewton metre per second
- DJoule
17.A solar cell has an efficiency of 20%. It receives 500 J of energy from sunlight. How much useful electrical energy does it output?
Medium
- A100 J
- B400 J
- C500 J
- D2500 J
18.A rollercoaster car of mass 500 kg is at the top of a hill 20 m high. It rolls down to the bottom. Ignoring friction, what is the kinetic energy of the car at the bottom of the hill? (g = 9.8 N/kg)
Medium
- A98 000 J
- B49 000 J
- C10 000 J
- D196 000 J
19.A student uses an electric motor to lift a weight. The motor does 200 J of work in 4 s. What is the power of the motor?
Easy
- A50 W
- B800 W
- C0.02 W
- D200 W
20.The efficiency of a device can be greater than 100%.
Easy
True or false?
21.Which of the following statements about energy resources are correct? (Select all that apply.)
Medium
- ASolar energy is a renewable resource.
- BFossil fuels are renewable because they can be regrown.
- CNuclear power uses fission of large nuclei.
- DWind turbines convert kinetic energy of wind into electrical energy.
22.Arrange the following energy resources in order from the one that ultimately comes from the Sun to the one that does NOT come from the Sun.
Medium
- Wind
- Tidal
- Geothermal
- Fossil fuels
23.A student investigates the energy changes in a bouncing ball. The ball is dropped from a height of 1.0 m. The ball bounces back to a height of 0.6 m. Which statement explains the decrease in height?
Medium
- AEnergy is created during the bounce.
- BEnergy is destroyed during the bounce.
- CSome energy is dissipated to the surroundings as heat and sound.
- DThe ball loses mass during the bounce.
24.Which of the following are examples of energy being transferred mechanically? (Select all that apply.)
Medium
- AA battery powering a torch
- BA person pushing a box across the floor
- CA kettle boiling water
- DA ball falling under gravity
- EA solar panel heating water
25.Match each energy resource to its description.
Easy
- Solar
- Wind
- Geothermal
- Tidal
- Energy from the Sun
- Energy from moving air
- Energy from hot rocks underground
- Energy from the Moon's gravity
26.A student uses an electric motor to lift a 2.0 kg mass through a height of 1.5 m. The motor is connected to a power supply that delivers 50 J of electrical energy. The mass is lifted at constant speed. What is the efficiency of the motor? (g = 9.8 N/kg)
Medium
- A29%
- B59%
- C41%
- D71%
27.The efficiency of a device can never be greater than 100%.
Easy
True or false?
28.Arrange the following energy resources in order from the one that ultimately comes from the Sun to the one that does NOT come from the Sun. Use the letters A, B, C, D.
Medium
- A. Wind
- B. Geothermal
- C. Fossil fuels
- D. Tidal
29.A student investigates the work done in lifting a weight. She uses a spring balance to measure the force needed to lift a 1.0 kg mass vertically at constant speed. Which additional piece of apparatus is needed to determine the work done?
Medium
- AA stopwatch
- BA metre rule
- CA thermometer
- DA newton meter
30.A ball is dropped from rest and falls freely under gravity. Air resistance is negligible. Which graph shows how the kinetic energy of the ball varies with the distance fallen?
Medium
- AA straight line through the origin with positive slope
- BA curve that increases and then levels off
- CA curve that increases with decreasing gradient
- DA horizontal line
31.Energy is measured in units of watts.
Easy
True or false?
32.Which of the following is an example of energy in the kinetic store?
Easy
- AA ball sitting on a shelf
- BA moving car
- CA stretched spring
- DA hot cup of coffee
33.Which energy transfer pathway involves a force acting on an object?
Easy
- AElectrical working
- BHeating by particles
- CMechanical working
- DRadiation
34.Which of the following is a correct unit for power?
Easy
- AJoule
- BWatt
- CNewton
- DMetre
35.Which of the following are energy resources that do NOT come from the Sun? (Select all that apply.)
Medium
- AGeothermal
- BWind
- CNuclear
- DTidal
- ESolar
36.Energy can be created or destroyed.
Easy
True or false?
37.A student drops a ball from a height of 2.0 m. The ball bounces back to a height of 1.2 m. Which statement best explains the energy change?
Medium
- ASome energy is transferred to the kinetic store of the ball during the bounce.
- BSome energy is dissipated to the thermal store of the surroundings during the bounce.
- CThe gravitational potential energy of the ball is conserved during the bounce.
- DThe kinetic energy of the ball is converted entirely into gravitational potential energy.
38.A wind turbine has a power output of 1.5 MW when the wind speed is 12 m/s. The efficiency of the turbine is 40%. What is the total power in the wind incident on the turbine blades?
Hard
- A0.6 MW
- B2.5 MW
- C3.75 MW
- D6.0 MW
39.The unit of power is the watt (W), which is equivalent to one joule per second.
Easy
True or false?
40.Calculate the kinetic energy of a 1200 kg car moving at 27 m/s. Give your answer in joules, to 2 significant figures.
Medium
41.A 72 kg person climbs stairs of height 3.0 m. Gravitational field strength is 9.8 N/kg. Calculate the change in gravitational potential energy. Give your answer in joules, to 2 significant figures.
Medium
42.A force of 500 N is applied to bring a car to rest over a distance of 23 m. Calculate the work done by the brakes. Give your answer in joules, to 1 significant figure.
Medium
43.An electric iron has a power rating of 2000 W and is used for 5 minutes. Calculate the energy transferred. Give your answer in joules.
Medium
44.Complete the sentence: Power is the rate of transfer of ______.
Easy
Power is the rate of transfer of ____.
45.Calculate the work done when a force of 250 N moves an object 12 m in the direction of the force.
Easy
46.Complete the sentence about energy transfer.
Easy
When work is done, energy is ____ from one store to another.
47.A force of 50 N is applied to push a box 6 m across a floor. Calculate the work done.
Easy
48.Explain why a rollercoaster at the top of a hill has gravitational potential energy, and describe the energy changes as it rolls down to the bottom.
Medium
49.Explain why the efficiency of a coal-fired power station is less than 100%. Include two reasons in your answer.
Medium
50.A student investigates the efficiency of an electric motor. The motor lifts a 0.50 kg mass through a height of 1.2 m. The electrical energy supplied to the motor is 8.0 J. Calculate the efficiency of the motor. (g = 9.8 N/kg)
Hard
51.Calculate the kinetic energy of a 1200 kg car moving at 27 m/s. Give your answer in joules, to 2 significant figures.
Medium
- A396000 J
- B484000 J
- C440000 J
- D528000 J
52.A 72 kg person climbs stairs of height 3.0 m. Gravitational field strength is 9.8 N/kg. Calculate the change in gravitational potential energy. Give your answer in joules, to 2 significant figures.
Medium
- A1890 J
- B2520 J
- C2310 J
- D2100 J
53.A force of 500 N is applied to bring a car to rest over a distance of 23 m. Calculate the work done by the brakes. Give your answer in joules, to 1 significant figure.
Medium
- A12000 J
- B10000 J
- C9000 J
- D11000 J
54.An electric iron has a power rating of 2000 W and is used for 5 minutes. Calculate the energy transferred. Give your answer in joules.
Medium
- A540000 J
- B600000 J
- C720000 J
- D660000 J
55.Calculate the work done when a force of 250 N moves an object 12 m in the direction of the force.
Easy
- A3600 J
- B3300 J
- C3000 J
- D2700 J
56.A force of 50 N is applied to push a box 6 m across a floor. Calculate the work done.
Easy
- A360 J
- B270 J
- C330 J
- D300 J

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