Re: Voltage, what exactly is it?

Hi Kaare,
This is a tricky one! It's not easy to explain electricity because 
there's nothing quite like it in everyday life. The closest analogy I 
have found is a bicycle (Yes, that sounds odd but please bear with me!):

The chain of a bike transfers energy from the pedals to the wheel.
An electrical current transfers energy from the battery to a lightbulb 
(or whatever).
Think of the current as equivalent to how fast the chain goes round.
Interestingly, if you cut a chain or a circuit at any point then it stops 
transfering energy.

The rate at which energy is transfered depends on two things: how fast 
the chain travels (the current) and how hard the pedals are pushed (the 
voltage [potential difference] - which you call U and I call V).
You could measure the force applied to the chain by comparing how tense 
the chain is before and after it has gone round the wheel's gears.

Obviously, the harder you push the pedals, the faster the chain goes 
round. The relationship depends on the resistance of the wheel. Sometimes 
you have to push very hard and do a lot of work to move the chain quite 
slowly (riding uphill - high resistance) but sometimes you can pedal very 
fast and still the chain is pretty slack, and you put little energy into 
the wheel (riding downhill - low resistance).

I found this analogy very helpful for series circuits. For parallel 
circuits it doesn't really work! Sometimes people use water in pipes as 
an analogy, with rate of flow as current, pressure as voltage, pumps as 
batteries and turbines as lightbulbs (or whatever energy output you 
prefer). This is fine but I have a better feel for riding a bike than I 
have for plumbing!

I hope this analogy helps you as much as it helped me.

Ewen