Ohm’s law relates the resistance of a component to its voltage and current. Applying circuit rules for current and voltage with Ohm’s Law allows us to formulate rules to determine total resistance.

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All conductors show some opposition to electrical current. This opposition to current is called resistance. 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.

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

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

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

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

\[voltage = current \times resistance\]

This relationship is called Ohm's 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 voltage 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 resistor 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 power 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.

- Question
A torch lamp takes a current of 0.3 amperes from a 3 volt battery. Calculate its resistance.

\[V = IR\]

\[3 = 0.3 \times R\]

\[R = \frac{3}{{0.3}}\]

\[R = 10\Omega\]

\[R=10 ohms\]

- Question
Calculate the reading on the ammeter in the circuit shown

\[I =\frac{V}{R}\]

\[=\frac{12}{2.7 \times 10^{3}}\]

\[= 4.4 \times 10^{-3}A\]

\[I = 4.4 \times 10^{-3}A\]

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.

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*.