Calculating Resistance: An Automobile Headlight

Problema e solução em inglês, como no livro original.

What is the resistance of an automobile headlight through which 2.50 A flows when 12.0 V is applied to it?

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Estratégia
We can rearrange Ohm’s law as stated by $I = V/R$ and use it to find the resistance.
Solução
Rearranging $I = V/R$ and substituting known values gives

$R = \frac{V}{I} = \frac{12 . 0 V}{2 . 50 A} = 4 . 80 Ω .$
20.16
Discussão
This is a relatively small resistance, but it is larger than the cold resistance of the headlight. As we shall see in Resistance and Resistivity, resistance usually increases with temperature, and so the bulb has a lower resistance when it is first switched on and will draw considerably more current during its brief warm-up period.

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