Resistance and Ohm's Law

When charge flows through a component, it encounters opposition to its flow. This opposition is called resistance, and understanding it is essential for analysing every circuit you will meet at A-Level.

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What Is Resistance?

Resistance is a measure of how much a component opposes the flow of current. It is defined as the ratio of the potential difference across a component to the current flowing through it:

R = V / I

where:

• R = resistance in ohms (Ω)
• V = potential difference in volts (V)
• I = current in amperes (A)

One ohm means that a potential difference of one volt drives a current of one ampere through the component.

This equation can be rearranged to give two other useful forms:

• V = IR (to find the p.d. across a component)
• I = V / R (to find the current through a component)

Worked Example 1

A current of 0.25 A flows through a 48 Ω resistor. What is the potential difference across it?

Solution:

• V = IR = 0.25 × 48 = 12 V

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Ohm's Law