Energy Stores, Transfers & Conservation

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:

Eₜₒₜₐₗ, ᵢₙᵢₜᵢₐₗ = Eₜₒₜₐₗ, fᵢₙₐₗ

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

Scroll diagram horizontally to read all labels.

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 Eᵤₛₑfᵤₗ ₛₜₒᵣₑₛ + Edᵢₛₛᵢₚₐₜₑd

“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. Worked examples

Example 1: Falling object and the EarthCore

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

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The system is approximately isolated. Its gravitational potential energy decreases while the ball’s kinetic energy increases:

-Δ Egₚₑ = Δ Eₖ

Gravity is an internal interaction because both the ball and Earth are inside the boundary.

Example 2: Energy dissipated on a rough trackCore

A trolley begins with 36 J of kinetic energy and stops on a rough track. The trolley and track are treated as an isolated system. State the final energy change.

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The kinetic store decreases by 36 J and the internal energy stores of the trolley and track increase by 36 J. Total energy remains constant.

Example 3: Motor energy balanceCore

A motor receives 5.0 kJ of electrical energy. The lifted load gains 3.6 kJ of gravitational potential energy. Find the energy dissipated.

Show answer

Edᵢₛₛᵢₚₐₜₑd = 5.0 kJ-3.6 kJ = 1.4 kJ

This energy is transferred mainly to internal energy stores in the motor and surroundings; it is not destroyed.

6. Exam check

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?

8. Practice, Quiz and Next Step

Close your notes and use Energy Stores, Transfers & Conservation in the supplied context below. This requires a constructed explanation or working, not recognition of an option.

Fresh context: An unfamiliar data set or physical system requires you to apply Energy Stores, Transfers & Conservation while stating the model, regime and assumptions.

1. Retrieve: define energy stores, transfers & conservation in your own words, including units, sign or conditions where relevant.
2. Represent: Choose and label an appropriate diagram, graph, table or symbolic model; derive or justify the relationship used.
3. Apply: Reach a conclusion, then evaluate it using units, uncertainty, a limiting case and one practical or modelling limitation.

Check the response before looking back

• The model, regime, coordinates and assumptions are explicit.
• The derivation or multi-step reasoning is visible rather than implied.
• The conclusion is tested against units, data quality and a limiting case.
• A practical control, uncertainty or model limitation is evaluated where applicable.

If one check fails, name that exact gap, revisit the matching explanation or worked example, and redo the task with different values or a different situation. Then use theA-Level Physics course hub orpractice browser for an independent re-test.