# Ohm's Law and resistance

All show some opposition to electrical . This opposition to current is called . There are several factors that affect the resistance of a conductor;

• material, eg copper, has lower resistance than steel
• length - longer wires have greater resistance
• thickness - smaller diameter wires have greater resistance
• temperature - heating a wire increases its resistance

The two main ways of increasing the current in an electrical circuit are by increasing the voltage or by decreasing the resistance.

## Changing the voltage

If you increase the voltage across a component, there will be more current in the . Too high a voltage and the lamp will break.

Three series circuits with increasing voltage

## Changing the resistance

If you increase the number of lamps in a , there will be less current. The lamps resist current, so if you put more lamps into the circuit, there is more resistance.

Two series circuits, one with one lamp, the other with two

You could increase or decrease the resistance in a circuit by using a .

Three series circuits, all containing a cell, ammeter, lamp and variable resistor

The quantities voltage, current and resistance are linked by the :

$voltage = current \times resistance$

This relationship is called Law. We usually write Ohm's Law as;

$V = IR$

The symbol for resistance is R, it is measured in ohms $$(\Omega )$$.

The symbol for is V, it is measured in volts $$(V)$$.

The symbol for current is I, it is measured in amperes $$(A)$$.

Make sure that if there is more than one voltage or current in a problem, you use the voltage across the and the current through it, not just any values that you see in the question.