Energy Stores and Transfers
Key Facts
- Energy is measured in joules (J).
- Energy can be transferred between stores but cannot be created or destroyed.
- The total energy in a closed system always remains the same.
- No energy transfer is 100% efficient — some energy is always dissipated, usually as thermal energy.
The Eight Energy Stores
Energy can be stored in eight different ways. Kinetic energy is the energy an object has because it is moving — a running athlete, a moving car, air particles. Gravitational potential energy is stored in an object that has been raised above the ground — a book on a shelf, a roller coaster at the top of a hill. Elastic potential energy is stored in a stretched or compressed object — a drawn bow, a compressed spring, a stretched rubber band.
Thermal (internal) energy is related to the temperature of an object — the hotter something is, the more thermal energy it stores. Chemical energy is stored in the bonds between atoms — food, fuels and batteries all store chemical energy. Magnetic energy is stored when two magnets attract or repel each other. Electrostatic energy is stored when charges attract or repel. Nuclear energy is stored in the nucleus of an atom and is released during nuclear reactions such as fission and fusion.
How Energy Is Transferred
Energy moves between stores by four pathways. Mechanically — when a force moves an object (pushing, pulling, stretching). Electrically — when charges flow through a circuit. By heating — when energy flows from a hotter region to a cooler one (conduction, convection, radiation). By radiation — when energy is transferred as light, sound or other waves.
Conservation of Energy
The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another. When a ball is thrown upward, kinetic energy is transferred to gravitational potential energy. At the highest point, all the kinetic energy has been transferred to gravitational potential energy. As it falls back down, gravitational potential energy is transferred back to kinetic energy. In reality some energy is dissipated as thermal energy due to air resistance, but the total energy in the system remains constant.
Worked Example — Energy Transfers
Question: Describe the energy transfers when a battery-powered torch is switched on.
Step 1: The battery stores chemical energy.
Step 2: Chemical energy is transferred electrically through the circuit to the bulb.
Step 3: At the bulb, energy is transferred to the light radiation store (useful) and the thermal store (wasted).
Efficiency
Efficiency tells us what fraction of the total input energy is transferred to useful output energy. The equation is: efficiency = useful output energy / total input energy. Efficiency can be written as a decimal (0 to 1) or as a percentage (multiply by 100). No device is ever 100% efficient because some energy is always wasted, usually dissipated as thermal energy to the surroundings.
Worked Example — Efficiency Calculation
Question: A light bulb receives 100 J of electrical energy and converts 15 J into light. What is its efficiency?
Step 1: Useful output = 15 J. Total input = 100 J.
Step 2: Efficiency = 15 / 100 = 0.15.
Answer: Efficiency = 0.15 or 15%.
Practice Questions
- Name all eight energy stores.
- A kettle uses 200 000 J of electrical energy and transfers 170 000 J to the thermal store of the water. Calculate the efficiency of the kettle.
- Describe the energy transfers that occur when a stretched catapult fires a stone upward.
- Explain why a bouncing ball eventually stops bouncing, referring to energy stores and conservation of energy.
- State the law of conservation of energy in your own words.