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10.1Energy and Its Conservation

Energy Stores, Transfers and Work — American System Physics Revision Notes & Topical Questions

10 min read 7 parts Lesson 3 of 5 1 quiz

📚 American System Physics Hub / Topical Revision
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Lesson summary

Name the energy stores and give an example of each.Work done = force × distance moved in the direction of the force: W = F d (W in J, F in N, d in m).What the symbols mean: W = work done = energy transferred (J) · F = force (N) · d, s or x = distance moved in the direction of the…

  • Energy is a scalar
  • Energy moves between stores by four pathways
  • Example of a full description
  • Work done is the same as energy transferred
  • Mechanical working
  • Electrical working

Part 1 of 7Key ideas

1W = F d
Work done = force × distance moved in the direction of the force: W = F d (W in J, F in N, d in m).
What the symbols mean: W = work done = energy transferred (J) · F = force (N) · d, s or x = distance moved in the direction of the force (m)
2Work done
Work done energy transferred
W = ΔE; both measured in joules (J).
3
1 kJ = 1000 J; 1 MJ = 1 000 000 J.
4
If the force is perpendicular to the motion, no work is done by that force (e.g. the weight of a box carried horizontally).
5
If the force is at angle θ to the motion, only the component along the motion does work: W = F d cos θ (extension).
6Energy is a scalar
Energy is a scalar it has size but no direction.
7
Energy can be transferred between stores but the total energy of an isolated system stays constant.
8
Most energy transfers eventually increase the internal (thermal) store of the surroundings, which becomes spread out (dissipated).

Part 2 of 7Learning objectives

1
Name the energy stores and give an example of each.
2
Describe energy transfers by mechanical working, electrical working, heating and waves.
3
Define work done and use W = F d = ΔE.
4
Describe energy flow in everyday events using the store-and-transfer model.

Part 3 of 7Essential knowledge

1
10.1 Energy Stores, Transfers and Work concept map: Work done = energy transferred, Energy is a scalar, Energy moves between stores b, Example of a full description, Work done is the same as ener
Visual guide to the key ideas explained in this lesson.
2
Energy is never used up; it is transferred from one store to another. When we say we have 'used' energy, we mean it has been moved into less useful stores, usually the internal store of the surroundings.
3
The main stores are kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal). Each store is linked to a physical quantity you can measure, such as speed for kinetic or height for gravitational potential.
4
Energy moves between stores by four pathways
Energy moves between stores by four pathways a force doing work (mechanical working), a current doing work (electrical working), heating (because of a temperature difference) and waves such as light and sound.
5
Example of a full description
Example of a full description a battery-powered fan. Energy transfers from the chemical store of the battery by electrical working to the kinetic store of the motor and blades; some is transferred by heating to the internal store of the motor and surroundings, and some by sound waves.
6
Work is done when a force makes an object move. The amount of work is the force multiplied by the distance moved in the direction of the force. Pushing on a wall that does not move does no work on the wall, even though you get tired (your muscles transfer energy internally).
7
Work done is the same as energy transferred
Work done is the same as energy transferred if you do 50 J of work lifting a bag, 50 J goes into the gravitational potential store of the bag (ignoring losses). This is why work and energy share the unit joule.
8
Friction and air resistance do work against the motion, transferring energy from the kinetic store to the internal store of the surfaces and surrounding air, which warm slightly.
9
When a force acts at right angles to the direction of motion (e.g. the tension in a string whirling a ball in a circle, or gravity on a horizontally carried box) it does no work, because there is no displacement along the force.
10
Energy transfers can be shown with a Sankey diagram (described in words here): the width of each arrow is proportional to the amount of energy, the input equals the sum of all outputs, and useful output goes straight on while wasted output branches off.
11
In a US physics course, the work–energy theorem states that the net work done on an object equals its change in kinetic energy: Wnet = ΔKE. This links forces (Newton's laws) to energy.

Part 4 of 7Process & sequence

1
Identify the system (the objects you are interested in) and its starting state.
2
List which energy stores hold energy at the start (e.g. chemical store of a battery).
3
Decide how the energy is transferred: mechanical working, electrical working, heating or waves.
4
List the stores that hold the energy at the end (e.g. kinetic store of a motor, internal store of the surroundings).
5
Check that total energy at the end equals total energy at the start (energy is conserved).
6
Identify which final stores are useful and which are wasted (usually internal energy of the surroundings).

Tracing energy in a battery-powered torchwrite these steps in this order; each step is a marking point.

1
Start: energy is held in the chemical store of the battery.
2
Transfer: electrical working as current flows through the circuit.
3
The bulb/LED filament: energy transferred to the internal store of the bulb.
4
Useful output: energy carried away by light waves.
5
Wasted output: energy transferred by heating to the internal store of the surroundings (dissipated).

Part 5 of 7Worked examples

A student pushes a box with a horizontal force of 45 N across a floor for 3.2 m. How much work does the student do?
  • W = F d = 45 N × 3.2 m = 144 J ≈ 1.4 × 102 J (2 s.f.).
    • This is the energy transferred from the student's chemical store to the box.
      • (kinetic store and, through friction, internal store)
A crane does 2.4 × 105 J of work lifting a steel beam through a height of 30 m at constant speed. What upward force does the crane exert?
  • W = F d, so F = W ÷ d = 2.4 × 105 J ÷ 30 m = 8000 N = 8.0 × 103 N.
    • At constant speed this force equals the beam's weight.
0 J
A waiter carries a 12 N tray horizontally for 8.0 m at constant height. How much work does the waiter's upward force do on the tray?
  • The upward force (12 N) is perpendicular to the horizontal motion, so the distance moved in the direction of the force is 0 m.
    • W = 12 N × 0 m = 0 J.

Part 6 of 7Summary

1
Energy is held in stores and moved by transfers.
2
The four transfer pathways are mechanical working, electrical working, heating and waves.
3
Work done = force × distance moved in the direction of the force.
4
Work done equals energy transferred, measured in joules.
5
Wasted energy usually ends up dissipated as internal energy of the surroundings.

Part 7 of 7Compare: Compare

Energy stores and everyday examples
StoreWhere it is foundTypical change that empties/fills it
KineticAny moving objectA car braking (store empties)
Gravitational potentialObject raised in a gravitational fieldA book lifted onto a shelf (store fills)
ChemicalFuels, food, batteriesPetrol burning in an engine
ElasticStretched/compressed springs, rubber bandsA catapult being released
NuclearNuclei of atoms (uranium, hydrogen in stars)Fission in a power station
ElectrostaticSeparated chargesA lightning strike
Internal (thermal)All matter; greater when hotterA cup of tea cooling

Light and sound are not stores; they are ways of transferring energy (by waves).

The four ways energy is transferred
Transfer pathwayWhat drives itExample
Mechanical workingA force moving through a distancePushing a trolley
Electrical workingAn electric currentKettle element heated by current
HeatingA temperature differenceHot plate warming a pan
Waves (radiation, sound)Oscillations carrying energySunlight warming the Earth

Key terms

Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Energy · Joule (J) · Energy store · Energy transfer · Work done · Mechanical working · Electrical working · Heating · Chemical energy store · Elastic (strain) energy store · Nuclear energy store · Internal (thermal) energy store · Newton metre (N m)
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary, never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

Words to avoid

1
energy is used upenergy is transferred to other stores, often dissipated to the surroundings.
2
light energy store / sound energy storeenergy transferred by light waves / by sound waves.
3
work = force × distancework done = force × distance moved in the direction of the force.
4
energy is lostenergy is wasted, transferred to the internal (thermal) store of the surroundings.
5
heat energythermal (internal) energy, transferred by heating.

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