# 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 :

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

The symbol for resistance is R, it is measured in ohms .

The symbol for is V, it is measured in volts .

The symbol for current is I, it is measured in amperes .

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.

When a resistor is kept at a constant temperature, its resistance will remain unchanged. We can confirm this experimentally by connecting a resistor to a supply and measuring the current in the resistor as the supply voltage is increased.

Plotting voltage (potential difference) against current for the resistor will produce a straight-line graph that passes through the origin.

Graph of potential difference (V) against current (A)
Question

A torch lamp takes a current of 0.3 from a 3 volt . Calculate its resistance.

Question

Calculate the reading on the ammeter in the circuit shown

A change in temperature can cause a change in resistance for some materials. These materials are known as non-ohmic conductors.

For example, a thermistor’s resistance depends on its temperature.

A voltage-current graph for a thermistor is not a straight line.

voltage - current graph for a thermistor

This means that the resistance of the thermistor is not constant for different values of current. When the current decreases, the resistance of the thermistor increases.