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