S-Cool Revision Summary

S-Cool Revision Summary

Define work

Energy can be quite difficult to understand, but can be thought of as 'the capacity of a body to do work'.

The Principle of Conservation of Energy

Nothing to do with turning off your stereo when you leave a room. This fundamental piece of science says that you cannot destroy or create energy. (If you find a way - keep it quiet until you have taken out a patent - and you'll be a rich person!)

People often say that you use up energy. That is misleading because it suggests that once it is used you can't get it back. In fact, when you use energy, you are simply converting it into other forms of energy - so it is still out there it is just no longer in the nice, useful form that you had it in a moment before.

Quite often if you can't quite put your finger on where the energy has gone (i.e. what form you have turned it into) you have turned it into heat, which is lost to the surroundings.

Efficiency

A light bulb is designed to turn electrical energy into light energy. But most bulbs produce a lot of heat energy too. That energy has not been lost but it has been wasted -you don't say "It's cold in here. Turn on the light!" do you.

To measure the efficiency of a device, calculate what percentage of the total energy put in, became useful output energy.

The Work Equation

At the heart of energy is the equation:

Work = force x distance moved in the direction of the force

Or

W = F s

The point that causes the most confusion here is that the distance moved is actually a vector because its direction is important.

Kinetic Energy

K.E. = ½ mv2

Units: J, joules

Kinetic energy stored in an object that is moving equals the amount of work done accelerating it to that speed.

Gravitational Potential Energy

Ep = P.E. = mgh

Units: J, joules

Potential Energy is the energy stored in objects which have been lifted against a gravitational field (or in fact a magnetic or electric field - see later).

Equations and Definitions

W = F s

Work = force x distance moved in the direction of the force Ep = mgh U = Ek + Ep

Symbols

W = work done, J

F = force, N

s = displacement, m

t = time, s

v = velocity, ms-1

K.E. or Ek = kinetic energy, J

Ep = potential energy, J 