Mini Physics
Part of Mini EducationSwitch subject
Search

Energy Stores, Transfers & Conservation

Key idea: Track energy stores and transfers, define a system boundary, and apply conservation of energy without treating dissipated energy as destroyed.

  • GCE A-Level H2 Physics 2027
On this page

Learning objectives

  • Track energy stores and transfers, then apply conservation of energy.

1. Definitions

An energy store describes where energy is accounted for in a system. Examples include kinetic, gravitational potential, electric potential, elastic potential, chemical and internal energy stores.

An energy transfer is a process that changes those stores. Energy may be transferred mechanically by work, electrically, by heating or by radiation.

A system is the object or collection of objects chosen for analysis. The surroundings are everything outside its boundary.

An isolated system exchanges neither energy nor matter with its surroundings. In most mechanics questions, “approximately isolated” means transfers across the chosen boundary are negligible during the interval considered.

2. Conservation of energy

Energy cannot be created or destroyed. For an isolated system:

E_(total, initial) = E_(total, final)

The total remains constant even though the amounts in individual stores may change.

Energy transferred between stores in an isolated systemA system begins with 100 joules in its kinetic store. After a resistive interaction, it has 18 joules in its kinetic store and 82 joules in internal energy stores, while the total remains 100 joules.Chosen system boundaryBeforeKinetic store100 Jresistive interactionAfterKinetic store: 18 JInternal stores: 82 JTotal = 100 JNo energy crosses the boundary: total energy remains constant.
Scroll diagram horizontally to read all labels.
Energy is not used up: within an isolated system, the total stays constant while the distribution among stores changes.
Mechanical energy is not total energy

Mechanical energy usually means kinetic plus potential energy. Friction or drag can reduce mechanical energy while total energy remains conserved because internal energy stores increase.

3. Building an energy balance

Use this method:

  1. State the system and the initial and final states.
  2. List the stores that change.
  3. Identify any energy transferred across the boundary.
  4. Write one balance with energy supplied equal to energy gained plus energy dissipated.

For a process with input energy:

Eᵢₙₚᵤₜ = Δ E_(useful stores) + E_dissipated

“Dissipated” does not mean destroyed. It means transferred to stores—often internal energy in the device and surroundings—from which recovery is difficult.

Choosing the boundary

For a falling object with negligible air resistance, choose object + Earth. Gravity is then an internal interaction and gravitational potential energy belongs to the system.

If the system is the object alone, Earth is outside the boundary. Gravity transfers energy mechanically into the object’s kinetic store by doing work.

4. Common mistakes

  • Writing “energy is lost” without naming the store or surroundings receiving it.
  • Calling a system isolated merely because its speed or kinetic energy is constant.
  • Omitting Earth from a system while including gravitational potential energy.
  • Conserving mechanical energy when resistive transfers are significant.
  • Mixing powers and energies in one balance without multiplying or dividing by time.

5. Exam Tips

Before finalising an energy equation, ask:

  • Does every potential-energy store include both interacting bodies in the system?
  • Have I accounted for transfers across the boundary?
  • Do the initial and final totals balance in joules?

6. Worked Examples

Modelled example 1

Falling object and the Earth

Core

Problem

A ball falls through a vertical distance with negligible air resistance. Describe the energy transfer for the system “ball + Earth”.
Study the worked solution

  1. Locate the interaction

    Method

    Gravity is internal to the chosen system.

    Reason

    Both interacting bodies—the ball and Earth—are inside the boundary.

    Working

    system = {ball, Earth}

  2. Track the changing stores

    Method

    Gravitational potential energy decreases while the ball’s kinetic energy increases.

    Reason

    With negligible air resistance, there is no significant energy transfer across the boundary.

    Working

    -Δ E_gpe = Δ Eₖ

  3. State conservation

    Method

    The system is approximately isolated and its total energy remains constant.

    Reason

    Energy moves between stores inside the boundary rather than entering or leaving.

    Working

    Δ Eₜₒₜₐₗ = 0

Common misconception 2

Energy dissipated on a rough track

Find and correct the mistake

Learner claim

A trolley begins with 36 J of kinetic energy and stops on a rough track. Treating trolley and track as an isolated system, a learner says friction destroys the 36 J. Diagnose the claim and state the final energy change.

Try this before viewing the solution

Destination of the 36 J

View solution step by step

  1. Track the kinetic store

    Method

    The trolley’s kinetic energy decreases by 36 J.

    Reason

    The trolley comes to rest.

    Working

    Δ Eₖ = -36 J

  2. Track the receiving stores

    Method

    The internal energy of the trolley and track increases by 36 J.

    Reason

    Friction transfers energy within the chosen isolated system.

    Working

    Δ Eᵢₙₜₑᵣₙₐₗ = +36 J

  3. Balance the system

    Method

    Total energy remains constant.

    Reason

    The decreases and increases cancel within the boundary.

    Working

    -36 + 36 = 0 J

Challenge 3

Motor energy balance

Minimal support

Independent transfer

A motor receives 5.0 kJ of electrical energy while a lifted load gains 3.6 kJ of gravitational potential energy. Find the energy dissipated and identify its likely destination.

Try this before viewing the solution

Unit: kJ

Hints

Hint 1: complete the energy balance
Use Eᵢₙₚᵤₜ = Δ E_useful + E_dissipated.
View solution step by step

  1. Write the balance

    Method

    Input equals useful gain plus dissipated energy.

    Reason

    All transfer pathways must appear in the conservation equation.

    Working

    5.0 = 3.6 + E_dissipated in kJ.

  2. Calculate the dissipated energy

    Method

    E_dissipated = 1.4 kJ.

    Reason

    Subtract the useful store increase from total input.

    Working

    E_dissipated = 5.0-3.6 = 1.4 kJ

  3. Name the receiving stores

    Method

    The energy mainly increases internal energy in the motor and surroundings.

    Reason

    Electrical resistance and friction heat the device and environment.

    Working

    1.4 kJ → internal-energy stores

7. Mind Stretchers

Mind stretcher 1: Falling ball as the only systemExtension

Reconsider a ball falling with negligible air resistance, but now choose the ball alone as the system. Is gravitational potential energy a store inside this system, and how does the ball gain kinetic energy?

Show answer

Earth lies outside the ball-only boundary, so the ball–Earth gravitational potential-energy store is not wholly inside the chosen system.

Gravity is an external interaction for this boundary. It does positive mechanical work on the falling ball, transferring energy across the boundary into the ball’s kinetic-energy store.

Continue with the next resource in this course.

Course and syllabus information
Course
GCE A-Level H2 Physics
Edition
GCE A-Level H2 Physics 2027