Section 1 of 5Resistance Basics
Resistance
- The opposition a component gives to the flow of charge, measured in ohms, symbol Ω.
Ohm's law
- At constant temperature, potential difference equals current multiplied by resistance, so V = I × R.
Effect on current
- For a fixed potential difference, a greater resistance gives a smaller current in the component.
Resistors
- Resistors are used in circuits to control the amount of current flowing in a particular part of the circuit.
Section 2 of 5Factors Affecting Resistance
1
Electrons moving through a wire collide with the metal ions, transferring energy to them and resisting the flow of charge.
2
A longer wire gives each electron more collisions with ions, so its resistance increases.
3
A thicker wire gives more space for electrons to flow, so its resistance decreases.
4
A higher temperature makes the metal ions vibrate more, which increases the collisions and the resistance of a metal.
Section 3 of 5Ohm's Law and Graphs
Ohm's Law and Graphs
Gradient
- For a fixed resistor at constant temperature, the current against potential difference graph is a straight line through the origin with gradient 1 ÷ R.
Rearranging
- V = I × R can be rearranged to R = V ÷ I or I = V ÷ R using the formula triangle.
Non-ohmic components
- Filament lamps and diodes do not give straight-line I–V graphs, so their resistance changes with the current.
Measuring
- Resistance can be found by dividing the measured potential difference across a component by the measured current through it.
Section 4 of 5Worked Example
Question
- A resistor of resistance 12 Ω carries a current of 0.5 A.
- Calculate the potential difference across it.
- Step 1: List the known quantities: R = 12 Ω and I = 0.5 A.
- Step 2: Write the equation V = I × R.
- Step 3: Substitute the values: V = 0.5 × 12 = 6 V.
Answer The potential difference across the resistor is 6 V.
Section 5 of 5Exam Checklist & Tips
5.1.6 – Key Points for the Exam
- Resistance The opposition to the flow of charge in a component, equal to potential difference divided by current.
- Ohm The unit of resistance, symbol Ω, equal to 1 volt per ampere.
- Ohm's law The relationship V = I × R for a component at constant temperature.
- Ion An atom or group of atoms that has gained or lost electrons and so carries a charge.
- Resistor A component designed to have a fixed resistance in a circuit.
Common Mistakes
- Saying that a longer wire has less resistance;
- a longer wire has more resistance because electrons collide with more ions.
- Thinking that the resistance of a fixed resistor changes with current at constant temperature.
- Calculating V = I ÷ R instead of V = I × R.
- Forgetting to convert values such as milliamperes into amperes before substituting them into the equation.