Physics · Both
Energy Stores & Transfers
Energy stores, pathways for transfer, conservation of energy and calculating efficiency.
Energy cannot be created or destroyed – only stored or transferred. Know the main stores, how energy moves, and how to calculate efficiency.
Energy stores
Main stores:
• Kinetic (moving objects)
• Gravitational potential (height in a field)
• Elastic potential (stretched/compressed)
• Chemical (fuels, food, batteries)
• Thermal (hot objects)
• Nuclear
• Electrostatic / magnetic
Energy transfers
Energy is transferred by:
• Mechanically (forces doing work)
• Electrically (current in a circuit)
• By heating
• By radiation (light, infrared, sound)
Example: falling object – gravitational store → kinetic store
Conservation and efficiency
**Conservation of energy:** total energy stays the same in a closed system.
**Efficiency** = useful energy output / total energy input
(Often × 100 for a percentage)
Energy not usefully transferred is usually dissipated as heat to the surroundings.
Worked example
A lamp uses 100 J of electrical energy and transfers 20 J as light. What is its efficiency?
Solution
Efficiency = 20 / 100 = 0.2 = **20%**
Practice questions
Try these without looking at the answers first.
Q1.Name the energy store of a moving car.
Show answer
Kinetic
Q2.A machine takes in 500 J and gives 150 J useful output. Efficiency?
Show answer
30%
Q3.What happens to wasted energy in most devices?
Show answer
Dissipated as heat (to surroundings)