Energy Changes in a System (AQA GCSE Physics Combined Science): Revision Notes
Energy stores and systems
What is a system?
A system can be a single object or a group of objects working together. When something happens to a system, the way energy is stored within it changes. For example, when you lift a book, you're changing the system made up of you, the book, and the Earth.
Understanding systems is crucial in physics because it helps us identify what we need to consider when analysing energy changes. The system boundary determines what we include in our analysis.
The eight energy stores
Energy can be stored in eight different ways. Each type of store has clear examples that you can see in everyday life:
Chemical energy store
- Found in: fuel, food, batteries
- This energy is released during chemical reactions
Kinetic energy store
- Found in: moving objects like cars, balls, people walking
- All moving things have kinetic energy
Gravitational potential energy store
- Found in: objects that have been raised up, like a book on a shelf
- The higher up something is, the more gravitational potential energy it has
Elastic energy store
- Found in: stretched or compressed objects like springs, elastic bands
- This energy is stored when objects are stretched or squashed
Thermal energy store
- Found in: hot objects like a cup of tea, radiators, your body
- All objects have some thermal energy
Magnetic energy store
- Found in: magnets when they interact with each other
- This happens when magnets attract or repel
Electrostatic energy store
- Found in: objects with electric charge, like when you rub a balloon on your hair
- This creates electrical attraction or repulsion
Nuclear energy store
- Found in: the nucleus of atoms, nuclear fuel
- This is released in nuclear reactions
Conservation of energy
One of the most important rules in physics is that energy cannot be created or destroyed. This means that in any process, the total amount of energy stays exactly the same.
The Law of Conservation of Energy is fundamental to understanding all energy transfers. No matter how complex the system, the total energy before any process equals the total energy after the process.
Energy can move from one store to another, and it can be transferred to useful purposes or wasted. When energy is wasted, it usually ends up as thermal energy in the surroundings.
Closed systems are special because no energy can flow in or out of them. In a closed system, you can easily track where all the energy goes.
How energy transfers
Energy moves between stores in three main ways:
By heating
- Energy flows from hot objects to cooler ones
- This happens through conduction, convection, or radiation
Through forces
- When forces do work on objects, energy transfers
- For example, when you push a trolley, you transfer energy to it
Electrically
- Energy travels through electric circuits
- This powers devices like lights, motors, and heaters
These three transfer methods cover all the ways energy can move between different stores. Understanding which method is operating helps predict how energy will behave in any situation.
Real examples of energy transfers
Here are some common situations where energy transfers happen:
Worked Example: Throwing a ball upwards
- Initial state: Ball has maximum kinetic energy in your hand
- During flight: Kinetic energy decreases as the ball slows down
- At highest point: Gravitational potential energy is at maximum, kinetic energy is zero
- Result: Energy transfers from kinetic → gravitational potential
Worked Example: Objects hitting obstacles
- Before impact: Moving objects have kinetic energy
- During collision: Objects lose kinetic energy when they crash
- After impact: This energy often becomes thermal energy due to heating from impact
- Result: Kinetic energy → thermal energy (plus sound energy)
Worked Example: Car brakes
- Initial: Moving car has kinetic energy
- Braking process: Kinetic energy of the moving car decreases
- Energy conversion: Friction in the brakes creates thermal energy, making them hot
- Result: Kinetic energy → thermal energy through friction forces
Worked Example: Boiling a kettle
- Energy input: Electrical energy from the mains transfers to the water
- Process: This increases the thermal energy store of the water
- Result: Electrical energy → thermal energy → steam (with some energy wasted to surroundings)
In all these examples, some energy always gets wasted as thermal energy that spreads to the surroundings.
Energy flow diagrams
Scientists use flow diagrams to show how energy moves through systems. These diagrams use boxes and arrows to show:
- Where energy starts (input)
- How it transfers
- Where it ends up (output)
Energy Flow Diagram Example: Battery-powered fan
Chemical energy (battery) → Electrical energy → Kinetic energy (fan blades) + Thermal energy (wasted to surroundings)
The arrows show the direction of energy transfer, and the boxes represent different energy stores.
Key Points to Remember:
- Energy cannot be created or destroyed - it can only be transferred between different stores
- There are eight main energy stores: chemical, kinetic, gravitational potential, elastic, thermal, magnetic, electrostatic, and nuclear
- Energy transfers in three ways: by heating, through forces, and electrically by electric current
- Energy is often wasted as thermal energy that spreads to the surroundings
- Flow diagrams help us track how energy moves through systems and where it goes