The Potential Divider
Often we want to be able vary the voltage across a component – to dim the lights or turn the sound up or down for example. We can do this by varying the resistance in a circuit. The output voltage can be varied with a potential divider shown below. Changing the value of the variable resistor R-2 changes the output voltage V-OUT .
The total resistance in the circuit isso if the battery voltage iswe can use Ohm's Law,to get the current:
Now apply Ohm's law to the output resistorto get the output voltage
If the output resistance R-2 is reduced to zero, the output voltage is zero. V-OUT can be increased to a maximum by increasing R-2 to its largest possible value.
In practice a slightly different arrangement is often used with a sliding contact on a wire.
Resistance is proportional to length so