Energy stores, transfers and the system boundary
Key idea: Energy cannot be created or destroyed. Choose a system and account for its energy stores and transfers, including energy passed to the surroundings.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Compare energy stores with ways of transferring energy.
- Choose the system before classifying its forces.
- Identify the forces between the blocks and the string.
- Choose both energy accounts without counting gravity twice.
- Balance the energy entering and leaving the system.
- Find the energy dissipated in kilojoules.
- Explain where the motor’s dissipated energy goes.
- Explain where the trolley’s kinetic energy goes.
- Find the increase in internal energy.
- State the system and write its signed energy balance.
- Follow the energy transfers and explain the crate’s kinetic-energy gain.
- Recall conservation, dissipation and how to choose the system.
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ContinueA car brakes to a stop, a motor lifts a load, a child slides down a slide. In each case energy is not used up: it moves from one store to another, or out of the body you are looking at into its surroundings. This lesson names the stores and the ways energy is transferred between them, and shows how to choose a system and balance its energy account.
Energy stores and transfers
A physical system can store energy. Common stores are kinetic, gravitational potential, elastic potential, electric potential, internal (the random kinetic and potential energy of the particles, which rises when a body warms up), chemical and nuclear.
Energy is transferred from one store to another, or from one body to another, in four ways:
- mechanically, when a force does work;
- electrically, when a current flows through a p.d.;
- by heating, because of a temperature difference;
- by radiation, such as light. Sound transfers energy mechanically through vibrations of a medium.
Friction and air resistance do not destroy energy. They transfer energy from a kinetic store to the internal energy of the surfaces and the air, which is why brakes get hot.
Choosing the system
Before you write an energy account, say what is inside your system. A force between two parts that are both inside is internal. A force from something outside is external, and it can transfer energy into or out of the system by doing work.
The same force can be internal for one choice of system and external for another. Friction from a track is external for the system “trolley”, but internal for “trolley and track”. Gravity works the same way:
- for the system ball and Earth, gravity is internal, and the system has a gravitational potential energy store;
- for the system ball only, the Earth is outside, so there is no gravitational potential energy store. Instead, the Earth’s gravitational force does work on the ball.
Worked example 1
Connected blocks
Problem
Two blocks joined by a light string are pulled across a horizontal surface by a rope attached to the front block. Take the system to be both blocks and the string. Which forces on the system are internal and which are external?
Show full solution
List what is inside
Method
Write down the bodies in the system.Reason
A force is internal only if both bodies that exert it on each other are inside.Working
Answer
Block A, the string and block B.
The string forces
Reason
Each block and the string are both inside the system.Working
Answer
The pulls between the string and each block are internal.
The other forces
Working
Answer
The rope’s pull, the friction and normal forces from the surface, and the weights from the Earth all come from outside, so they are external.
Quick check
For the system “block A only”, the string’s pull on block A would be external.
Check your understanding 1 One fall, two energy accounts
A ball falls with negligible air resistance. Choose the description for each system.
Show answer
For the ball alone, Earth’s force does positive external work and increases the ball’s kinetic energy. For ball and Earth, gravity is internal: gravitational potential energy decreases as kinetic energy increases. The energy accounts describe the same motion. Counting both gravity’s work and a potential-energy decrease in one account would count the same transfer twice.
Conservation of energy
Energy cannot be created or destroyed. It can only be transferred from one store to another.
So for any system, the change in the energy stored equals the energy transferred in minus the energy transferred out:
To use it: state the system, list every store that changes, list every transfer across the boundary, then write one balance. For a device, this is usually written as
The wasted energy is said to be dissipated: spread into the internal energy of the surroundings, from which little of it can be recovered. It has not been destroyed.
Guided practice 2
A motor lifting a load
Problem
A motor receives 5.0 kJ of electrical energy while it lifts a load from rest to rest. The load gains 3.6 kJ of gravitational potential energy. How much energy is dissipated, and where does it go?
Find the energy dissipated
Hints
Hint 1
the balance
Which energy store changes when the load starts and ends at rest?
Show full solution
Balance
Working
Answer
5.0-3.6 = 1.4 kJ is dissipated. It goes mainly to the internal energy of the motor’s wires (resistance) and moving parts (friction), and then to the surroundings.
Check your understanding 2 Where the dissipated energy goes
The lifting motor dissipates 1.4 kJ. Explain where that energy goes.
Show answer
The wires gain internal energy through electrical resistance, and moving parts gain internal energy through friction. Energy then leaves the warm motor for the surroundings. Compare your answer for both a destination and a transfer mechanism.
Spot the mistake 3
Does friction destroy energy?
Problem
A trolley with 36 J of kinetic energy comes to rest on a rough, level track. A learner says friction has destroyed the 36 J. Taking the trolley and track as the system, where has the energy gone? Neglect energy transfer to distant surroundings during stopping.
Choose where the energy goes
Show solution step by step
Changes in the stores
Reason
In a system with no transfers across its boundary, every decrease is matched by an increase.Working
Answer
Δ Eₖ = -36 J and Δ Eᵢₙₜₑᵣₙₐₗ = +36 J
Total
Working
Answer
-36 + 36 = 0: the total energy is unchanged. The trolley and track are slightly warmer.
Common mistake
Energy that is “lost” or “dissipated” has moved to a less useful store. It still exists.
Try it yourself 4
A child on a rough slide
Problem
A child slides from rest down a rough slide. The gravitational potential energy decreases by 490 J and the child’s kinetic energy increases by 200 J. Neglect energy transfer to distant surroundings during the slide. Choose a system with no energy crossing its boundary, and find the increase in internal energy.
Find the increase
Hints
Hint 1
the system
Which interacting bodies must be inside a system with no energy leaving it?
Show solution step by step
System
Reason
With everything that interacts inside, no energy crosses the boundary.Working
Answer
Child, Earth, slide and nearby air.
Balance
Working
Answer
-490 + 200 + Δ Eᵢₙₜₑᵣₙₐₗ = 0, so the internal energy of the child, slide and air increases by 290 J.
Check your understanding 3 The slide’s system and balance
For the child on the rough slide, state your system and write the signed energy balance. You may work on paper.
Show answer
Include the child, Earth, slide and nearby air, and neglect transfers beyond this boundary during the slide. Gravity and friction are internal. -490 + 200 + Δ Eᵢₙₜₑᵣₙₐₗ = 0, so the increase is 290 J. Compare your boundary and all three signed store changes.
Check your understanding 4 A motor pulling a crate
A battery-powered motor pulls a crate across a rough floor at increasing speed. Describe the energy stores and transfers, and say which part of the motor’s work increases the crate’s kinetic energy.
Show answer
The battery’s chemical store decreases. Energy is transferred electrically to the motor, then mechanically to the crate by the pull of the motor. The crate’s kinetic store increases, and friction increases the internal energy of the crate, floor and air. Only the work done by the resultant force (the pull minus friction) increases the crate’s kinetic energy.
Check the starting store, both transfer mechanisms, the kinetic gain and the increase in internal energy.
Common mistakes
- Saying friction or an inefficient device destroys energy; the energy goes to internal energy of the surroundings.
- Writing an energy account without first saying what the system is.
- Counting a gravitational potential energy store and the work done by gravity in the same account.
- Using “heat” as a store; heating is a transfer, and the store is internal energy.
Before you move on
Check your understanding 5
Without looking back: state the principle of conservation of energy. What does “dissipated energy” mean? What must you say before deciding whether a force is internal or external?
Show answer
Energy cannot be created or destroyed, only transferred from one store to another. Dissipated energy has been spread into the internal energy of the surroundings, from which it is hard to recover. You must state what is in the system.
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Syllabus and review details
- GCE A-Level H2 Physics 2027 · 2027
Content Overview, PDF pages 9–10; Subject Content, PDF pages 11–30
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