A single object (e.g. the air in a piston) or group of objects (e.g. two colliding vehicles) that you're interested in
Closed systems
Systems where neither matter nor energy can enter or leave
The net change in the total energy of a closed system is always zero
Energy transfer when a system changes
1. Energy can be transferred into or away from the system
2. Between different objects in the system
3. Between different types of energy stores
Energy transfer by heating
1. Energy is transferred to the water's thermal energy store (causing the temperature of the water to rise)
2. Energy is transferred electrically to the thermal energy store of the kettle's heating element, which transfers energy by heating to the water's thermal energy store
Work done
Energy transferred
Energy transfer by doing work
1. Work done when current flows (work is done against resistance in a circuit)
2. Work done by a force moving an object
3. Work done by the gravitational force
4. Work done by the frictional force
5. Work done by the normal contact force in a collision
Energy stores pop up everywhere in physics
Work done = force x distance moved in same direction
Energy can be transferred between objects by work done, heating or cooling, and electrical current.
The SI unit of energy is the joule (J).
The greater the force applied, the more work done.
The further the object moves, the more work done.
If the forces are at right angles to each other, no work is done.
