Energy, Work & 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; changes within a system involve energy transfer.
Energy stores include: kinetic, gravitational potential, elastic, magnetic, electrostatic, chemical, nuclear, and thermal.
Energy transfer pathways: mechanical (force), electrical (charge flow), heating (particle conduction), and radiation (electromagnetic waves).
Example: a battery-powered torch transfers energy electrically from the chemical store of the battery to the thermal store of the bulb.
Example: a falling object transfers energy mechanically from its gravitational potential store to its kinetic store.

Example of a system: an apple on a table

Kinetic Energy

Kinetic energy is the energy an object has due to its mass and speed: E_k = \frac{1}{2}mv².
Doubling mass doubles kinetic energy (directly proportional to mass).
Doubling speed quadruples kinetic energy (proportional to speed squared).
Energy is transferred to the kinetic store when an object speeds up, and away when it slows down.

A moving car has energy in its kinetic store

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.
Lifting an object transfers energy to its gravitational potential store; lowering transfers energy away.
Work is done against the weight force to lift an object.

Ball in freefall illustrating height and weight

Conservation of Energy

Principle of conservation of energy: energy cannot be created or destroyed, only transferred between stores.
For a closed system, total energy remains constant: total energy in = total energy out.
Energy can be dissipated (spread out) to the surroundings, often as wasted energy.
Example: a bat hitting a ball transfers kinetic energy to the ball (useful) and to thermal stores (wasted).
Energy flow diagrams show stores and transfers; Sankey diagrams show proportions of useful and wasted energy.

The principle of conservation of energy applied to a bat hitting a ball

Work Done

Work done occurs when a force moves an object over a distance: W = Fd.
Work done equals energy transferred: 1 N m = 1 J.
If no movement occurs, no work is done.
Example: a force of 500 N over 23 m does 11 500 J of work.

Work is done when a force is used to move an object over a distance

Power

Power is the rate of work done or energy transferred: P = \frac{W}{t} = \frac{\Delta E}{t}.
Unit of power is the watt (W): 1 W = 1 J/s.
A more powerful machine does the same work in less time.
Example: a 2000 W iron used for 5 minutes transfers 600 000 J of energy.

Two cars with different power ratings doing the same amount of work

Efficiency

Efficiency is the ratio of useful output to total input (energy or power): \text{efficiency} = \frac{\text{useful output}}{\text{total input}} \times 100\%.
High efficiency means most energy is used usefully; low efficiency means much is wasted.
Efficiency has no units; can be expressed as a decimal or percentage.
Example: a motor with 35% efficiency lifting a 7.2 kg load 5.0 m in 3 s has a power input of 336 W.

Sankey diagram: a modern energy-efficient light bulb

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Câu hỏi luyện tập

Xem trước miễn phí — 8 trên 55 câu hỏi. Đăng ký để xem tất cả.

1.What is the unit of energy?
Easy
- AJoule
- BNewton
- CWatt
- DMetre
2.Which of the following is an energy store?
Easy
- AElectrical
- BKinetic
- CMechanical
- DHeating
3.The principle of conservation of energy states that energy cannot be created or destroyed. It can only be:
Easy
- Atransferred from one store to another
- Bincreased or decreased
- Cchanged into mass
- Dstored as heat
4.Work done is equal to:
Easy
- Aenergy transferred
- Bpower
- Cforce
- Ddistance
5.A car of mass 1000 kg is moving at 20 m/s. What is its kinetic energy?
Medium
- A200 000 J
- B10 000 J
- C400 000 J
- D20 000 J
6.Which of the following correctly describes the energy transfer when a ball is thrown upwards?
Easy
- AKinetic to gravitational potential
- BGravitational potential to kinetic
- CChemical to kinetic
- DNuclear to thermal
7.A force of 50 N moves an object 2 m in the direction of the force. How much work is done?
Easy
- A100 J
- B25 J
- C52 J
- D48 J
8.Which of the following is an energy transfer pathway?
Easy
- AThermal
- BMechanical
- CKinetic
- DChemical

Thêm câu hỏi

9.A 60 kg person climbs a 3 m ladder in 6 s. What is their power output? (Take g = 10 N/kg)
Medium
- A300 W
- B30 W
- C1800 W
- D1080 W
10.A motor has a power input of 500 W and a useful power output of 200 W. What is its efficiency?
Medium
- A40%
- B60%
- C250%
- D0.4%
11.Which of the following energy transfers occurs when a kettle boils water?
Easy
- AElectrical to thermal
- BChemical to kinetic
- CNuclear to electrical
- DThermal to gravitational
12.An object of mass 2 kg is dropped from a height of 5 m. Just before hitting the ground, what is its kinetic energy? (Ignore air resistance, g = 10 N/kg)
Medium
- A100 J
- B10 J
- C50 J
- D20 J
13.A wind turbine has blades that sweep out a circular area of 40 m². The density of air is 1.3 kg/m³ and the wind speed is 12 m/s. What is the mass of air passing through the swept area each second?
Medium
- A624 kg
- B48 kg
- C520 kg
- D31.2 kg
14.The kinetic energy of the air passing through the turbine blades each second is 240 J. The mass of air per second is 7.5 kg. What is the speed of the air?
Medium
- A8.0 m/s
- B4.0 m/s
- C16 m/s
- D32 m/s
15.A generator driven by a wind turbine produces a current of 2.0 A at a potential difference of 11 V. What is the output power of the generator?
Medium
- A22 W
- B5.5 W
- C0.18 W
- D44 W
16.A wind turbine has an input power of 240 W from the wind and an output electrical power of 22 W. What is the efficiency of the turbine?
Medium
- A9.2%
- B10.9%
- C91.7%
- D0.092%
17.A train engine has a power of 1500 kW and takes 30 minutes to climb a mountain. The train carries 80 passengers each of mass 65 kg through a vertical height of 1600 m. What is the efficiency of the engine? (Take g = 9.8 N/kg)
Hard
- A30.2%
- B3.0%
- C60.4%
- D15.1%
18.A bouncy ball of mass 0.07 kg is dropped from a height of 1.65 m. What is the gravitational potential energy of the ball just before it is dropped? (Take g = 9.8 N/kg)
Easy
- A1.13 J
- B0.11 J
- C11.3 J
- D0.69 J
19.Using the previous data (mass 0.07 kg, drop height 1.65 m, g = 9.8 N/kg), what is the speed of the ball just before it hits the floor? (Assume no air resistance)
Medium
- A5.69 m/s
- B32.3 m/s
- C1.13 m/s
- D0.57 m/s
20.A fork-lift truck lifts a crate of weight 450 N to a shelf 3.9 m high. What is the work done?
Easy
- A1755 J
- B115 J
- C1755 N
- D115 N
21.The fork-lift truck uses a total energy input of 2500 J to lift the crate. What is its efficiency?
Medium
- A70.2%
- B29.8%
- C1.42%
- D0.702%
22.A student of mass 54 kg climbs a ladder of vertical height 1.8 m in 15 s. What is the work done? (Take g = 9.8 N/kg)
Medium
- A952.6 J
- B97.2 J
- C972 J
- D529.2 J
23.What is the power output of the student in the previous question? (Work done = 952.6 J, time = 15 s)
Medium
- A63.5 W
- B14,289 W
- C0.0157 W
- D63.5 J
24.The student's body is only 17% efficient when climbing. What is the total power input?
Hard
- A373.5 W
- B10.8 W
- C63.5 W
- D108 W
25.In an experiment to measure power output when climbing stairs, a student of mass 60 kg climbs a vertical height of 2.5 m in 5.0 s. What is the power output? (Take g = 9.8 N/kg)
Medium
- A294 W
- B2940 W
- C30 W
- D735 W
26.A student investigates the efficiency of an electric motor. The motor lifts a 0.50 kg mass through a height of 1.2 m in 2.0 s. The electrical power input to the motor is 5.0 W. What is the efficiency of the motor? (Take g = 9.8 N/kg)
Medium
- A59%
- B29%
- C12%
- D88%
27.A force of 20 N is applied to push a box 3.0 m across a floor. The box moves at constant speed. What is the work done on the box?
Easy
- A60 J
- B6.7 J
- C0 J
- D23 J
28.Which of the following is a correct energy transfer pathway?
Easy
- AMechanical working
- BKinetic store
- CGravitational potential
- DChemical store
29.A car of mass 800 kg is moving at 30 m/s. What is its kinetic energy?
Easy
- A360 000 J
- B24 000 J
- C720 000 J
- D12 000 J
30.A roller coaster car of mass 500 kg starts from rest at a height of 40 m above the ground. It descends to a height of 10 m. Assuming no energy losses, what is its speed at the lower height? (Take g = 9.8 N/kg)
Hard
- A24.2 m/s
- B14.0 m/s
- C28.0 m/s
- D19.8 m/s
31.In an experiment to measure the efficiency of a motor, a student measures the electrical energy input and the gravitational potential energy gained by a load. Which additional measurement is needed to calculate efficiency?
Medium
- ATime taken
- BMass of the load
- CHeight lifted
- DNo additional measurement needed
32.Which of the following is a unit of power?
Easy
- AWatt
- BJoule
- CNewton
- DNewton-metre
33.A wind turbine has blades that sweep out an area of 50 m². The density of air is 1.2 kg/m³ and the wind speed is 10 m/s. What is the mass of air passing through the blades each second?
Medium
- A600 kg
- B60 kg
- C6000 kg
- D500 kg
34.A student uses a force meter to pull a wooden block across a bench at constant speed. The force meter reads 2.5 N. The student pulls the block 0.80 m. What is the work done on the block?
Easy
- A2.0 J
- B3.1 J
- C0.32 J
- D1.7 J
35.A 100 W electric motor lifts a weight of 50 N at a constant speed. How high does it lift the weight in 4.0 s?
Medium
- A8.0 m
- B2.0 m
- C0.50 m
- D20 m
36.Which of the following is a correct energy store?
Easy
- AKinetic
- BMechanical
- CElectrical
- DHeating
37.Energy is measured in which unit?
Easy
- AJoule (J)
- BNewton (N)
- CWatt (W)
- DKilogram (kg)
38.Which energy transfer pathway involves a force acting on an object?
Easy
- AMechanical working
- BElectrical working
- CHeating
- DRadiation
39.A bouncy ball is dropped from a height of 1.65 m. Just before it is dropped, which energy store is full?
Easy
- AGravitational potential store
- BKinetic store
- CElastic potential store
- DThermal store
40.A motor has an efficiency of 0.8. If the useful power output is 160 W, what is the total power input?
Medium
- A128 W
- B200 W
- C320 W
- D800 W
41.When a system is in equilibrium, energy is transferred.
Easy
True or false?
42.The principle of conservation of energy states that energy can be created but not destroyed.
Easy
True or false?
43.What is the unit of power?
Easy
- AJoule
- BNewton
- CWatt
- DKilogram
44.Efficiency is defined as the ratio of useful energy output to total energy input.
Easy
True or false?
45.When work is done, what is transferred?
Easy
- AForce
- BEnergy
- CPower
- DMomentum
46.Match each energy store to its correct description.
Medium
- Kinetic
- Gravitational potential
- Elastic potential
- Chemical
- Energy stored due to height in a gravitational field
- Energy stored in stretched or compressed objects
- Energy stored due to motion
- Energy stored in fuels and batteries
47.Which of the following are energy transfer pathways? (Select all that apply)
Medium
- AElectrical working
- BMechanical working
- CNuclear store
- DRadiation
- EHeating by particles
48.Arrange the following steps in the correct order to show the energy transfers when a hydroelectric power station generates electricity.
Medium
- Kinetic energy of turbine
- Gravitational potential energy of water
- Electrical energy from generator
- Kinetic energy of water
49.A motor has an efficiency of 0.4. If the useful power output is 80 W, what is the total power input?
Medium
- A32 W
- B200 W
- C120 W
- D50 W
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 in 2.0 s. The electrical power input to the motor is 6.0 W. What is the efficiency of the motor? (Take g = 9.8 N/kg)
Medium
- A49%
- B98%
- C20%
- D10%
51.Calculate the kinetic energy of a 500 kg car travelling at 20 m/s.
Easy
52.Complete the sentence.
Easy
The unit of energy is the ____.
53.State the equation that links work done, force, and distance moved in the direction of the force.
Easy
54.State the principle of conservation of energy.
Easy
55.Calculate the kinetic energy of a 500 kg car travelling at 20 m/s.
Easy
- A90000 J
- B100000 J
- C120000 J
- D110000 J

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