# 4.2.1 Resistance

### Resistance

• Resistance is defined as the opposition to current
• For a given potential difference: The higher the resistance the lower the current
• Wires are often made from copper because copper has a low electrical resistance
• Materials with low resistance are known as good conductors
• The resistance R of a conductor is defined as the ratio of the potential difference V across to the current I in it • Resistance is measured in Ohms (Ω)
• Ω is the Greek capital letter ‘Omega’
• An Ohm is defined as one volt per ampere (1 V A-1)
• The resistance controls the size of the current in a circuit
• A higher resistance means a smaller current
• A lower resistance means a larger current
• All electrical components, including wires, have some value of resistance

#### Worked Example

Calculate the potential difference through a resistor of resistance 10 Ω if there is a current of 0.3 A through it.

Step 1: List the known quantities

• Resistance, R = 10 Ω
• Current, I = 0.3 A

Step 2: Write the resistance equation Step 3: Rearrange for V

V = IR

Step 4: Substitute in the values

V = 0.3 × 10 = 3 V

#### Exam Tip

Although all electrical components have resistance, the resistance of wires is taken to be 0 in exam questions.

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