Year 8 · Science · Electricity and magnetism · Lesson 3
Resistance
What resistance is, calculating it with R = V ÷ I and rearranging, and what affects the resistance of a wire.
25-minute video lesson
Teaching, worked examples and 20 practice questions solved step by step.
Watch free for a monthWhat your child will learn
- Describe resistance and its unit
- Calculate resistance using R = V ÷ I
- Rearrange to find voltage or current
- Describe how length and thickness affect resistance
- Identify variables in a resistance investigation
The method
- Resistance opposes current and is measured in ohms (Ω).
- Resistance = voltage ÷ current (R = V ÷ I).
- Rearrange: V = I × R and I = V ÷ R.
- Longer, thinner or hotter wires have more resistance.
Key words: resistance, ohm, resistor, variable resistor, conductor, insulator, voltage, current.
Worked examples
The first three practice questions from the lesson, with every step.
Question 1
A lamp has a voltage of 10 V across it and a current of 2 A through it. What is its resistance?
- Step 1: R = V ÷ I
- Step 2: R = 10 ÷ 2
- Step 3: R = 5 Ω
Answer: 5 Ω
Question 2
A lamp has a voltage of 9 V across it and a current of 3 A through it. What is its resistance?
- Step 1: R = V ÷ I
- Step 2: R = 9 ÷ 3
- Step 3: R = 3 Ω
Answer: 3 Ω
Question 3
A lamp has a voltage of 4.5 V across it and a current of 1.5 A through it. What is its resistance?
- Step 1: R = V ÷ I
- Step 2: R = 4.5 ÷ 1.5
- Step 3: R = 3 Ω
Answer: 3 Ω