Energy Stores and Transfers

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CognitoEnergy stores and transfers

Key facts & equations

Energy stores

Kinetic: energy of moving objects. E_k = ½mv².
Gravitational potential: stored due to height. E_gpe = mgh.
Elastic potential: stretched/compressed elastic objects. E_e = ½ke².
Chemical: stored in bonds (food, fuel, batteries).
Thermal: internal energy of an object (particles moving).
Nuclear: energy stored in atomic nucleus.

Transferring energy

Energy is transferred by: heating, doing work (mechanical force), radiation (e.g. light, IR), electrical working.
Work done = force × distance. W = Fd. Unit: J (joule). 1 J = 1 N·m.
Work done = energy transferred.
E.g. lifting an object: work done against gravity = gravitational PE gained.
Conservation of energy: energy cannot be created or destroyed, only transferred between stores.

Key energy calculations

KE = ½mv². E.g. 2 kg at 10 m/s: KE = ½ × 2 × 100 = 100 J.
GPE = mgh. E.g. 5 kg lifted 4 m: GPE = 5 × 10 × 4 = 200 J.
Work done = Fd. E.g. 20 N over 5 m: W = 20 × 5 = 100 J.
Energy conserved: if ball dropped from height h, KE at bottom = GPE lost. ½mv² = mgh. Cancel m: v = √(2gh).
g = 9.8 m/s² (or 10 m/s² in estimates).

Energy dissipation

In real systems, some energy is always dissipated as thermal energy (due to friction, air resistance).
This does NOT violate conservation of energy — energy is still there, just less useful.
Wasted energy spreads out to the environment and is harder to use again.
Ways to reduce dissipation: lubrication, streamlining, insulation.

Exam questions — 5 questions · 11 marks · AQA 8463 style

3 marksGPE and KEAQA 8463 P1 style 🔢 Calculator

A 0.5 kg ball is dropped from a height of 8 m. Assuming no air resistance, calculate: (a) the GPE at the top, (b) the speed just before hitting the ground. (g = 10 m/s²) (3 marks)

Hint: (a) GPE = mgh. (b) GPE = KE at bottom → ½mv² = GPE → v = √(2×GPE/m).

+30 XP

3 marksWork doneAQA 8463 P1 style 🔢 Calculator

A person pushes a 30 kg box along a floor with a force of 80 N over 6 m. Friction force is 50 N. Calculate: (a) work done by the person, (b) work done against friction, (c) kinetic energy gained. (3 marks)

Hint: (a) W = F × d (full pushing force). (b) W = friction × d. (c) KE = net force × d (or a - b).

+30 XP

1 markConservation of energyAQA 8463 style

A ball is thrown upward. At the highest point:

A All energy has been destroyed

B All kinetic energy has been converted to gravitational potential energy (ignoring air resistance)

C KE and GPE are both at maximum

D The ball has more total energy than when it was thrown

1 markKinetic energy formulaAQA 8463 style

A car doubles its speed from 10 m/s to 20 m/s. By what factor does its kinetic energy increase?

A 2 (doubles)

B 4 (quadruples)

C 8

D 10

Energy Stores and Transfers

Energy stores

Transferring energy

Key energy calculations

Energy dissipation

Module complete! 🎉