1.1 Changes in Energy Stores

A free GCSE Physics lesson for AQA (8463) from OBA School: energy stores, the four ways energy is transferred, systems and energy flow diagrams. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the eight energy stores and give an example of each
• Describe the four ways energy can be transferred: by forces (work), by electric current, by heating and by waves
• Use the idea of a system and draw energy flow diagrams for everyday devices
• Describe the changes in energy stores when an object is thrown up, falls and hits the ground

Key words

• Energy store: A way in which energy is held by an object or system, e.g. kinetic, thermal or chemical.
• Energy transfer: Energy moving from one store to another, by a force doing work, an electric current, heating or waves.
• System: The object, or group of objects, that you choose to study.
• Kinetic energy store: Energy an object has because it is moving. Faster or heavier objects have more.
• Gravitational potential energy store: Energy an object has because of its height above the ground (its position in a gravitational field).
• Elastic potential energy store: Energy stored in an object that has been stretched, squashed or bent.
• Thermal energy store: Energy a substance has because of its temperature (also called the internal energy store).
• Chemical energy store: Energy held in fuels, food and batteries, released by chemical reactions.

Energy stores

Energy is never seen directly, but we can keep track of it by saying which store it is in. Physicists use eight stores:

• Kinetic; Energy is stored because… the object is moving; Example a rolling skateboard
• Gravitational potential; Energy is stored because… the object is raised above the ground; Example a flowerpot on a windowsill
• Elastic potential; Energy is stored because… the object is stretched, squashed or bent; Example a drawn bow, a squashed spring
• Thermal (internal); Energy is stored because… of the temperature of the substance; Example a bowl of hot soup
• Chemical; Energy is stored because… of the chemical bonds in a substance; Example petrol, porridge, a phone battery
• Magnetic; Energy is stored because… two magnets attract or repel; Example two fridge magnets held apart
• Electrostatic; Energy is stored because… two charges attract or repel; Example a balloon rubbed on a jumper
• Nuclear; Energy is stored because… of the particles in the nucleus of an atom; Example uranium fuel in a reactor

Exam tip: Use the proper names. Say “kinetic energy store”, not “movement energy”; say “thermal energy store”, not “heat energy”.

Four ways to transfer energy

Energy is useful because it can move from one store to another. There are four transfer pathways:

• Mechanically – a force moves an object. We say the force does work. Example: your leg kicking a ball.
• Electrically – an electric current carries energy around a circuit. Example: a charger filling a phone battery.
• By heating – energy moves from a hotter object to a cooler one. Example: a hob warming a pan.
• By waves (radiation) – light, infrared and sound waves carry energy away. Example: the Sun warming your face.

Key fact: Electricity is not an energy store. An electric current is a flow of charge that transfers energy, for example from the chemical store of a battery to the kinetic store of a motor.

Systems and energy flow diagrams

A system is the object, or group of objects, you are looking at. When a system changes, energy moves between stores. A flow diagram shows the stores in boxes and the transfer pathways on the arrows.

Worked example: a camping stove heats a pan of water

Starting store: the chemical store of the gas fuel (and oxygen).
Pathway: the burning gas transfers energy by heating (and some by light).
Stores that increase: the thermal store of the water (wanted) and the thermal stores of the pan and the surrounding air (not wanted).

Answer: chemical store of fuel → heating → thermal store of water, pan and air

Objects that go up, fall and land

• Thrown up: the ball slows down. Gravity does work on it, transferring energy from its kinetic store to its gravitational potential (GPE) store. At the very top (thrown straight up) its kinetic store is zero.
• Falling: the ball speeds up. Energy is transferred from its GPE store to its kinetic store.
• Landing with a thud: the ball stops, so its kinetic store falls to zero. The energy is transferred by heating to the thermal stores of the ball, the floor and the air, and a small amount is carried away by sound waves (which also end up warming the surroundings).

Worked example: a dropped pencil case

A pencil case falls off a desk. Its GPE store goes down by 3.0 J. Air resistance transfers 0.1 J to the thermal store of the air.
Energy in its kinetic store just before landing = 3.0 J − 0.1 J = 2.9 J.
When it lands and stops, all 2.9 J is transferred to the thermal stores of the case, the floor and the air (mostly by heating, a little by sound).

Answer: 2.9 J in the kinetic store just before landing

Key fact: Much less energy is carried away by sound than by heating. In an impact, say the energy goes mostly to the thermal stores of the objects and the surroundings.

Common mistakes

• Calling a store “movement energy”, “motion energy” or “heat energy”. Use kinetic and thermal.
• Saying a battery stores “electrical energy”. It has a chemical store; the current is the transfer pathway.
• Saying energy is “used up” or “lost”. It is transferred to other stores, usually the thermal store of the surroundings.
• Forgetting that when an object lands and stops, its kinetic store goes to the thermal stores of the object and its surroundings.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these is a way of transferring energy, not an energy store? (Choose from: kinetic, electric current, chemical, thermal.)

Answer: electric current

Working: An electric current is a flow of charge that transfers energy between stores. Kinetic, chemical and thermal are all stores.

Question 2. True or false? When a ball falls freely, its gravitational potential energy store decreases and its kinetic energy store increases.

Answer: True

Working: True. It loses height (GPE store decreases) and speeds up (kinetic store increases).

Question 3. A mug of hot chocolate is left on a table and cools down. Which energy store of the hot chocolate decreases?

Answer: thermal

Working: Its thermal (internal) energy store decreases as energy is transferred by heating to the cooler surroundings.

Related lessons

• Next lesson: 1.2 Conservation of Energy
• In this chapter: 1.3 Energy and Work
• In this chapter: 1.4 Gravitational Potential Energy Stores
• In this chapter: 1.5 Kinetic Energy and Elastic Energy Stores

Continue in the full course

This lesson is section 1.1 (AGCPH0101) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.