Ohm's Law and resistance

The current through a certain wire depends on two things:
(a) the voltage (potential difference) between its ends
(b) the resistance of the wire
The way in which the current changes as the voltage is changed was discovered by Ohm. You can verify his results with the following experiment.




The experiment should show you that there is a steady rise in current for a steady rise in applied voltage. In other words the current through a conductor is directly proportional to the potential difference between its ends.
The symbol for a resistor in a circuit is:

OHM'S LAW

This result was first discovered by a man named Ohm and so it is called Ohm's Law.
Ohm's Law states that:


You can write this in an equation as:


Resistance is measured in units called Ohms (W). The resistance of a piece of wire is 1 ohm if a current of 1 A flows through it when a voltage of 1 V is applied between its ends.
If you plot a graph of current through a piece of wire against the voltage applied you should get a result like the one shown in the diagram. (Remember that the temperature of the wire must not change.



Ohm's Law is often written:

Using an ammeter and a voltmeter to measure resistance

To measure the resistance of say a piece of wire or a resistor we must find the voltage (potential difference) between its two ends and the current flowing through it. To do this we use the circuit shown in the diagram.

Remember: