Topic 8 - Energy - forces doing work Flashcards
What does work done equal?
Energy transferred = Work done
Explain that mechanical processes become wasteful whem they cause a rise in temperature.
In a mechanical system, energy is dissipated when two surfaces rub together. Work is done against friction which causes heating of the two surfaces – so the internal (thermal) energy store of the surfaces increases.
How can energy be transferred by electrical equipment?
Electrical devices work by transferring energy between different energy stores. For example, electric ion transfer energy electrically from the mains power supply to the thermal energy stores of their metal plates.
What happens when a system (an object or group of objects) change?
Energy is transferred. It can be transferred into or away from the system, between different objects in the system or between different types of energy stores. Some energy is always dissipated and stored in less useful ways.
What happens to the total energy in that system when there energy transfers in a closed system?
There is no net change to the total energy in that system.
How do you work out efficiency?
Useful energy transferred by device / Total energy supplied to device
How do you work out kinetic energy?
Kinetic energy (J) = 1/2 x mass (kg) x (speed) squared
How do you work out work done?
Work done (J) = force (N) x distance (m)
How do you work out power?
Power (W) = work done (J)/ time taken (s)
What is power?
The rate of the energy transfer.
How can energy be transferred by heating?
A pan of water is heated on a gas camping stove. When the system is the pan of water, energy is transferred into the system by heating to the thermal energy stores of the pan and the water, which increases their temperature. When the system is the camping stove, energy is transferred from the chemical energy store of the gas to the thermal energy stores of the pan and the water, increasing their temperature.
Revise Sankey and other energy diagrams
Revise Sankey and other energy diagrams
Explain when energy dissipation occurs and how
No system is perfect. Whenever there is a change in a system, energy is transferred and some of that energy is dissipated. No system is perfect. Whenever there is a change in a system, energy is transferred and some of that energy is dissipated.
Any energy that is not transferred to useful energy stores is said to be wasted because it is transferred to the surroundings.
Electrical cables warming up are a good example of this. It is not useful to have hot wires behind a television as energy is dissipated to the surrounding air.
How do you work out change in gravitational potential energy?
Gravitational potential energy = mass (kg) x gravitational field strength (N/kg) x Change in vertical height (m)
What is one watt equal to?
One watt = one joules per second
