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# Electricity Physics help??? Watch

1. Hi guys, so i've been reading through my physics notes for my end of unit test tomorrow, and came across this fact that just doesn't make sense for me;

according to the formula v=ir, if current increases, then resistance has to decrease. But if current increases, the temperature of the wire will also increase - does the temperature of the wire not = to an increase in resistance though? As the positive ions vibrate more, being more of an obstacle to the electrons passing through?

2. (Original post by grace_zzz)
Hi guys, so i've been reading through my physics notes for my end of unit test tomorrow, and came across this fact that just doesn't make sense for me;

according to the formula v=ir, if current increases, then resistance has to decrease. But if current increases, the temperature of the wire will also increase - does the temperature of the wire not = to an increase in resistance though? As the positive ions vibrate more, being more of an obstacle to the electrons passing through?

The thing about the equation V=IR, is that it doesn't really work unless the resistance is constant. You can only use it to describe how current varies with voltage (or vice versa) and not how resistance is affected by other things. To simplify, here's some things the equation does tell you:

• If you double the voltage, you will get twice as large a current
• If you halve the resistance you will get twice as large a current
• If you have double the current, then you may conclude that either the voltage has been doubled or the resistance has been halved (or some mixture of the two)

You cannot, however, say "because the current increases and voltage is constant, the resistance will decrease". There is no way to increase the current other than the two ways mentioned above. If you're thinking of a light bulb, what happens is: low initial resistance due to low temperature-->high starting current-->high resistance due to high temperature-->low operational current. The final current, resistance and temperature reaches an equilibrium value.

Basically, the issue with your sentence is that you cannot magically increase the current whilst keeping the voltage and material the same. The resistance is a property of the material, so you usually can't affect it.

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Updated: January 18, 2015
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