Section 1 of 5Resistance Basics
Resistance measures how strongly a component opposes the flow of current.
Resistance
- The ratio of potential difference across a component to the current through it, measured in ohms.
Ohm
- One ohm is the resistance of a component that allows a current of one amp when a potential difference of one volt is applied.
Ohm's law
- For a metallic conductor at constant temperature, the current is directly proportional to the potential difference.
Equation
- V = I × R, where V is in volts, I in amps and R in ohms.
Longer wire
- A longer wire of the same material has a greater resistance, because charge meets more opposition.
Section 2 of 5Using V = IR
Writing the known values first makes the correct rearrangement clear.
1
Write down the potential difference V in volts and the current I in amps.
2
Use R = V ÷ I to calculate the resistance in ohms.
3
Rearrange V = I × R to find the voltage or current when they are asked for.
4
Check the units are volts, amps and ohms before giving the answer.
Section 3 of 5Component Behaviour
Component Behaviour
Fixed resistor
- A resistor at constant temperature has a constant resistance and a straight line on a graph of V against I.
Filament lamp
- As the filament heats up, its resistance increases, so the current rises more slowly than the voltage.
Thermistor
- The resistance of a thermistor falls as temperature rises, which is used in temperature sensing.
Heating effect
- Current through a resistance transfers energy as thermal energy to the surroundings.
Section 4 of 5Worked Example
Question
- A resistor has a potential difference of 4.5 V across it and a current of 0.30 A through it.
- Calculate its resistance.
- Then calculate the voltage needed to drive 0.50 A through a 12 Ω resistor.
- Resistance R = V ÷ I = 4.5 V ÷ 0.30 A = 15 Ω.
- Voltage V = I × R = 0.50 A × 12 Ω.
- V = 6.0 V.
Answer The first resistor has a resistance of 15 Ω, and 6.0 V is needed to drive 0.50 A through the 12 Ω resistor.
Section 5 of 5Exam Checklist & Tips
2.1.3 – Key Points for the Exam
- Resistance The opposition to current in a component, equal to voltage divided by current.
- Ohm The unit of resistance, written as Ω.
- Ohm's law The relationship V = IR for a conductor at constant temperature.
- Ohmic conductor A conductor whose resistance stays constant when the current or voltage changes.
- Filament lamp A lamp whose resistance increases as its temperature rises.
Common Mistakes
- Writing R = V × I; the correct form is R = V ÷ I.
- Leaving out units or mixing up milliamps and amps in the calculation.
- Stating that resistance changes the voltage of a cell;
- the cell sets the voltage and the resistance limits the current.
- Assuming that all components obey Ohm's law; filament lamps and thermistors do not have constant resistance.