Energy Stores and Transfers
Understanding energy through studying changes in the way energy is stored when a system changes.
Introduction to Energy Systems
A 'system' is an object or a group of objects that interact. In physics, we can understand energy by studying changes in the way energy is stored when a system changes.
Energy Stores in Everyday Events
Throwing an Object Upwards
When you throw a ball upwards, just after the ball leaves your hand it has a store of kinetic energy. When the ball reaches its highest point, it has a store of gravitational potential energy. Just before you catch it again, it has a store of kinetic energy.
Energy Transfer: Kinetic → Gravitational Potential → Kinetic
Boiling Water in a Kettle
When you turn on your electric kettle, the water in the kettle gets hotter. There is now more internal (or thermal) energy stored in the hot water than there was in the cold water.
Energy Transfer: Electrical → Internal (Thermal) Energy
A Car Using Brakes to Slow Down
A moving car has a store of kinetic energy. When the car slows to a halt, it has lost this store of kinetic energy. The brakes exert a frictional force on the wheels, and the brakes get hot. The store of kinetic energy in the car has been transferred to a store of thermal energy in the brakes.
Energy Transfer: Kinetic → Thermal (Internal)
Holding Two Magnets with North Poles Facing
When you hold two magnets with like poles facing, you can feel a force which repels the magnets from each other. When the magnets are close together there is a store of magnetic potential energy. When you release the magnets, they move apart. The magnets' store of magnetic potential energy has reduced and their store of kinetic energy has increased.
Energy Transfer: Magnetic Potential → Kinetic
Types of Energy Stores
We use the following labels to describe the stores of energy you will meet:
Kinetic
Energy of motion
Chemical
Energy stored in chemical bonds
Internal
Thermal energy stored in objects
Gravitational Potential
Energy due to height in gravity field
Magnetic
Energy in magnetic fields
Elastic
Energy stored in stretched/compressed objects
Energy Transfer Methods
Light, sound and electricity are useful, but they are not stores of energy. They are ways of transferring energy from one store to a different energy store.
Example: A Torch
In a torch, the chemical energy stored in the battery causes an electric current (a flow of charge). The electric current causes the temperature of the bulb to increase so much that the bulb lights up. The light cannot be stored but it is useful. When the light strikes an object and is absorbed, the internal energy of the object increases.
Chemical Energy
Electrical Transfer
Light & Heat