the gravity potential energy to electric potential energy analogy again!!!
Posted on 2010-08-16
As mentioned in previous posts electric potential energy (EPE) is always introduced by comparing it to gravitational potential energy (GPE). It goes a bit like this:
work is done on an object to move it against gravity. The work done on the object is equal to the gain in GPE of the object. When the object returns to its lower starting position the GPE is transferred to kinetic energy. Gravity has done negative work on the object (I'd never heard of negative work before today).
I'm trying to now compare this to the EPE example and I am concerned with getting concepts correct as well as the correct wording as I know this matters.
Lets imagine we have a copper wire with potential difference across it because there is a build up of electrons at one end. I don't know how to describe the force/work on the electrons. Is it correct to say the following:
a) the electrons move because there is a pushing force on them because of repulsion from the build up of electrons. So the electrons are moving in the direction of a force but there is no work done. But work is distance moved in the direction of a force. I can see why there is no work done but don't know how to word it. there is a distinction which i don't know how to describe between an existing force that makes something move (e.g. gravity pulls something down) and a force that has to be applied to make something move (i have to lift the box). In this case the electric potential already exists and is making something move. Granted we did work to make the potential difference but once that force is there things can move in the direction of that force without work being done.
b) is something doing negative work on the electrons? I would guess if graivty does negative work on an object then a voltage can do negative work on charge