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.