 not just described, Explained

# Different models of electrical resistance

## The analogy depends on what you want to explain

It's very difficult to come up with a universal analogy for resistance.  There are essentially two sorts of things you want to explain

1. The effects of resistance on a circuit
2. The causes of resistance at a sub-atomic level

For example the constriction in a water pipe, uneven ground, band sawrope circuit, ball circuit, hot water system and horse and sugar lump analogies all try and describe the effects of resistance rather than explaining its causes.

The bus route, crowded room and rough sea analogies try to account for resistance at a sub-atomic level.

## The effects of resistance on a circuit

We want to be able to explain

1. Why big resistance means small current
2. How resistance accounts for energy transfer (i.e. power and voltage)

(Below when we talk about 'current' we mean it as a short-hand for 'the relationship between resistance and current' rather than current per se.)

The ball circuit analogy is really only good at explaining current.

The constriction in a water pipe analogy is meant to explain current and energy but most students only understand the current part.

The uneven ground analogy is okay at explaining current and can be drafted into explaining energy if you're really pushed.

The band saw and rope circuit analogies are pretty good at explaining both current and energy.

The hot water system, horse and sugar lump and gravitational analogies only really deal with energy well.

It's more difficult to find analogies for more subtle ideas with resistance.  For example

• why resistances in series add, rather than depending only on the largest resistance
• how voltage is shared among different resistances in a series circuit
• why energy is transferred fastest where the resistance is largest, even though the bigger the resistance the slower energy is transferred overall

All of these ideas can be explained.  It's just sometimes difficult to make one of the existing analogies do the job.

## The causes of resistance at a sub-atomic level

The essential point about conductors is that there are charges, like electrons or ions, that are free to move around.  Something won't be such a good conductor (i.e. will have a bigger resistance) if

1. there are fewer free charges around
2. they're not so free to move

Most of the analogies that can deal with current have as an explicit assumption that there's something exterior to the current that restricts its motion, normally some sort of friction.

The rough sea analogy and the crowded room analogy explain how charges are restricted in their motion by something that happens at a sub-atomic level.  They can both explain why increased temperature causes increased resistance.

The bus route analogy can explain why metals are good conductors but semiconductors aren't.

back to Lesson 7: Resistance and Ohm's Law