Introduction
Energy is what makes everything happen. Different objects store energy in different ways. Energy can be transferred between these stores and different things happen depending on which stores it is moving between
The Essentials
You need to know the stores:
And the ways energy can be transferred between these stores:
Mechanically: when an object moves because a force acts on it, e.g. pushing, pulling, squashing, stretching.
Electrically: when a charge moves through a potential difference, e.g. electrons moving in wires in an electrical circuit.
By heating: energy being transferred from a hotter object to a colder object, e.g. warm cup of coffee to your hands
by radiation: where energy is transferred by waves, e.g. light and sound. Energy reaching earth from the sun by light, or the sound of a drum being hit reaching you ears due to sound waves.
You need to be able to remember and use the following equations to calculate energy in different stores:
Gravitational potential store of energy:
- Ep=mass x gravitational field strength x change in height
- Ep=m g Δh (remember Δ just means ‘change’)
Kinetic store of energy
- Ek= 1/2 x mass x (velocity)²
- Ek=1/2 m v²
Elastic Store of energy
- Ee=1/2 x spring constant x extension²
- Ee= 1/2 k e²
When a force acts on an object we know energy has been transferred, we call this ‘work done’.
- work done = Force x displacement (distance moved)
- W=F s
Power is how fast the energy is transferred between stores. The faster it is transferred the more powerful the object. a transfer of 1 joule (J) in 1 second (s) is a power of 1 watt 
Think about a kettle. A 2,000W kettle will transfer twice as much energy every second than a 1,000W kettle.
We can calculate power using these two equations:
- Power = Energy transferred/time taken
- P=E/t
- Power = work done/time taken
- P=W/t
Deeper Understanding
Energy can not be created or destroyed only transferred between stores.
When work is done or energy is transferred some of it will not be transferred usefully, for example when a kettle heats water the kettle itself will get hot so not all the energy has been transferred into the water. we see this as a waste energy transfer.
You need to able to identify the waste energy transfers in systems you are given (a system is just a fancy word for an object or group of objects you are interested in).
Looking at energy transfers can get very complicated so we can simplify it by looking at snapshots:
If we throw a ball into the air it goes up. We had to use a force to throw it so work has been done, energy has been transferred.
We use the chemical store of energy in our muscles to provide the force to throw the ball. It moves so it has kinetic store of energy. the energy was transferred between the stores mechanically.
As the ball goes up it slows down, so energy is being transferred out of its kinetic store. It is being transferred into the gravitational store of the ball mechanically (as it is being acted on by gravity)
As the ball reaches its maximum height all of the energy stored kinetically has been transferred as the ball stops moving. Most of it has been transferred into the gravitational store. However some will have been transferred to thermal stores within the surroundings and the ball itself. These are waste energy transfers.
Often when doing calculations we can ignore the energy wasted and just look at complete transfers, for example:
Energy lost from the Gravitational potential store = Energy gained by the Kinetic store
Other Links
Brian Cox explains Conservation of energy
Review and Rate your Understanding
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