Resistance
Section: Physics | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Resistance?
- Resistance is the opposition to the flow of electric current in a circuit, measured in ohms (Ω)
- Think of resistance like obstacles in a pipe that water flows through:
- more obstacles = harder for water to flow = less water gets through
- more resistance = harder for current to flow = less current gets through
- resistance opposes or resists the flow of electric charge
- Resistance is measured in ohms (Ω), symbol R – all components in a circuit have resistance
- higher resistance = lower current (if voltage stays the same)
- lower resistance = higher current (if voltage stays the same)
What Affects Resistance?
| Factor | Effect on Resistance |
|---|---|
| Type of material | Different materials have different resistances |
| Length | Longer wire = more resistance |
| Thickness (area) | Thicker wire = less resistance |
| Temperature | Higher temperature = more resistance (in most conductors) |
Conductors and Insulators
- Conductors are materials with low resistance that allow current to flow easily
- examples: copper, aluminium, silver, gold, iron
- metals are good conductors
- used for wires and cables
- allow electrons to move freely
- Insulators are materials with high resistance that do not allow current to flow easily
- examples: plastic, rubber, wood, glass, air
- non-metals are usually insulators
- used to cover wires for safety
- prevent electrons from moving
- Why this matters:
- wires are made of copper (good conductor) – low resistance, current flows easily
- wires are covered in plastic (good insulator) – prevents current escaping, keeps us safe
- resistors are designed to have specific resistance – control current in circuits
How Resistance Affects Current
The higher the resistance in a circuit, the lower the current that flows through it (if voltage stays constant).
- High resistance → low current
- Low resistance → high current
- Resistance and current are inversely proportional
- Double the resistance = half the current
- Triple the resistance = one-third the current
Exam Tip
- If you have a 12V battery and increase the resistance: low resistance (2Ω) gives high current (6A); medium resistance (4Ω) gives medium current (3A); high resistance (12Ω) gives low current (1A). More resistance means more opposition to current, so less current flows
Ohm's Law
- The current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance, provided the temperature remains constant
- Formula: V = I × R
- V = voltage (potential difference) in volts (V)
- I = current in amperes (A)
- R = resistance in ohms (Ω)
Rearranging Ohm's Law
You can rearrange the formula to find any of the three quantities:
| To Find | Formula | When to Use |
|---|---|---|
| Voltage (V) | V = I × R | When you know current and resistance |
| Current (I) | I = V ÷ R | When you know voltage and resistance |
| Resistance (R) | R = V ÷ I | When you know voltage and current |
Cover the quantity you want to find, and the triangle shows how to calculate it from the other two
- To find V: cover V → multiply what's left: V = I × R
- To find I: cover I → divide what's left: I = V ÷ R
- To find R: cover R → divide what's left: R = V ÷ I
Calculating with Ohm's Law
Worked Example: Finding Voltage
Museli has a circuit with a current of 2A and resistance of 5Ω, and wants to find the voltage.
- Step 1: write down the formula. V = I × R
- Step 2: substitute the values. V = 2 × 5
- Step 3: calculate the answer. V = 10V
Worked Example: Finding Current
Muumbe connects a 12V battery to a 4Ω resistor, and wants to find the current.
- Step 1: write down the formula. I = V ÷ R
- Step 2: substitute the values. I = 12 ÷ 4
- Step 3: calculate the answer. I = 3A
Worked Example: Finding Resistance
Muchindu has a lamp with a voltage of 6V across it and a current of 0.5A flowing through it, and wants to find its resistance.
- Step 1: write down the formula. R = V ÷ I
- Step 2: substitute the values. R = 6 ÷ 0.5
- Step 3: calculate the answer. R = 12Ω
Worked Example: More Complex Problem
Museli has a circuit with a 9V battery and three identical resistors in series, with a total current of 1.5A, and wants to find the resistance of one resistor.
- Step 1: find the total resistance. Rtotal = V ÷ I = 9 ÷ 1.5 = 6Ω
- Step 2: write down the series rule. Rtotal = R1 + R2 + R3
- Step 3: substitute, since resistors are identical. 6Ω = R + R + R = 3R
- Step 4: calculate the answer. R = 6 ÷ 3 = 2Ω per resistor
Resistance in Series and Parallel
Resistance in Series Circuits
- Rule: resistances in series add up. Rtotal = R1 + R2 + R3 + ...
- adding resistors in series increases total resistance
- current has to flow through all resistors
- more obstacles = more total resistance
- Example: three resistors in series, 2Ω, 3Ω, and 5Ω. Rtotal = 2 + 3 + 5 = 10Ω
Resistance in Parallel Circuits
- Rule: total resistance in parallel is less than the smallest resistor. 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...
- adding resistors in parallel decreases total resistance
- current has multiple paths to flow through
- more paths = less total resistance
- For identical resistors in parallel, there's a shortcut: Rtotal = R ÷ n (where n = number of resistors)
- two 6Ω resistors in parallel: Rtotal = 6 ÷ 2 = 3Ω
- three 9Ω resistors in parallel: Rtotal = 9 ÷ 3 = 3Ω
Variable Resistors
- A variable resistor is a resistor whose resistance can be changed/adjusted, also called a rheostat or potentiometer
- has an adjustable slider or knob
- can increase or decrease resistance
- used to control current in a circuit
- How variable resistors work:
- increase resistance → current decreases → bulbs get dimmer, motors slow down
- decrease resistance → current increases → bulbs get brighter, motors speed up
Real-Life Uses
Interactive revision notes, videos and practice questions load below.