Power and Work DoneAQA GCSE Physics: Revision notes
Section 1
How Is Energy Transferred by Work?
Energy can be transferred to or from a system by heating, by work done by a force, and by work done when a current flows. Whenever a force moves an object through a distance, or a current flows through a component, work is being done and energy is being transferred.
Section 2
Power: The Rate of Doing Work
Power is defined as the rate of energy transfer or the rate at which work is done: P = E/t and P = W/t (power in watts, W; energy/work in joules, J; time in seconds, s).
One watt is one joule of energy transferred per second.
Section 3
Comparing the Power of Two Machines
Power lets you compare how quickly different machines transfer the same amount of energy. For example, two electric motors lifting the same weight through the same height: the motor that does this in less time has a greater power, even though both transfer the same amount of energy overall.
A crane doing 6000 J of work in 3 s has a power of 6000/3 = 2000 W. A second crane doing the same 6000 J of work in 12 s has a power of only 500 W - it is far less powerful even though it does the same job.
Section 4
Work Done and Energy Stores
Work done against gravity to lift an object equals the gravitational potential energy gained: Ep = mgh. Work done to accelerate an object from rest equals the kinetic energy gained: Ek = 1/2 m v^2. In both cases, power = work done ÷ time taken, so the faster the work is done, the greater the power required.
If a question gives you a time as well as a force/distance or mass/speed, it is almost always asking you to find power, not just energy.
Must Know
- Work is done whenever a force moves an object or a current flows
- Power = rate of energy transfer = rate of work done: P = E/t = W/t
- Power is measured in watts (W); 1 W = 1 J/s
- Two systems can transfer the same energy but have very different powers if the time taken differs
- Ep = mgh and Ek = ½mv² can both be combined with time to calculate power
That's the notes covered.
Carry on to the next subtopic.