📚 Part 1: Concepts & Quick Checks

1. Which of the following is an example of an energy store?

Chemical store (e.g. fuel in a car)

Only motion (kinetic) energy

Only heat energy

None of these

2. The law of conservation of energy states:

Energy can be created or destroyed

Energy can be transferred and stored but the total energy is conserved

Energy always becomes light

3. Check all that apply: When a kettle boils water, which energy changes commonly occur?

Electrical → Thermal

Chemical → Thermal

Thermal → Kinetic (steam movement)

Electrical → Gravitational

4. Which formula gives efficiency (as a percentage)?

efficiency = (input energy / useful energy output) × 100

efficiency = (useful energy output / input energy) × 100

efficiency = input energy − useful output

🔗 Part 2: Practical Matching & Descriptions

Match each everyday scenario (left) to the best energy store on the right. Then describe the main energy transfer that occurs.

A. A drawn bow held ready to fire an arrow

B. A battery in a torch before it is switched on

C. A hot cup of tea cooling on a table

D. A moving bicycle that slows down when brakes are applied

1. Thermal store

2. Chemical store

3. Elastic (strain) store

4. Kinetic store

Write the letter-number matches (e.g. A–3) and then describe the energy transfer for each scenario.

A–___ Describe the energy transfer:

B–___ Describe the energy transfer:

C–___ Describe the energy transfer:

D–___ Describe the energy transfer:

🧮 Part 3: Calculations, Worked Examples & Practice

Worked example 1 — Efficiency

A 60 J electric lamp supplies 6 J of useful light. Calculate its efficiency.

Step 1: efficiency = (useful output / input) × 100 = (6 J / 60 J) × 100 = 10%.

Worked example 2 — Energy lost as heat

A motor input = 200 J, useful mechanical output = 160 J. Energy lost as heat = input − useful = 40 J.

1. A small heater takes in 5.0 kJ of electrical energy and delivers 4.0 kJ as useful heat. Calculate:

a) Efficiency (show working and final %)

b) Energy wasted (in J)

2. Rearrangement practice: If efficiency = (useful output / input) × 100, show the formula to find input when efficiency and useful output are known.

Use your rearranged formula to solve: A motor has useful output 2.4 kJ and efficiency 60%. Find:

a) Input energy (in kJ)

b) Energy lost as heat (in kJ)

3. Short explanation: Why is no real device 100% efficient? Give one brief reason.

Energy Concepts and Calculations

Energy Concepts and Calculations

📚 Part 1: Concepts & Quick Checks

🔗 Part 2: Practical Matching & Descriptions

🧮 Part 3: Calculations, Worked Examples & Practice

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