Resistance
In this topic you will:
- discover how resistance affects current in a circuit
- calculate resistance from voltage and current.
Key Words
filament, ohms, Ohm's law, resistor, resistance
Resistance
Resistance is a measure of how easy or difficult it is for electrons to move though a material.
Conductors, such as copper, have very low resistance.
Insulators, such as most plastics, have very high resistance.
Resistance is measured in units called ohms. Ohms have the symbol Ω, which is the Greek letter omega.
A 100 m length of copper wire, for example, can have a resistance of 0.5–1.0 Ω.
A 1 cm length of some plastics have a resistance of over a 1 000 000 000 000 Ω.
Resistance slows the flow of electrons, so lowers current.
Copper wire is designed to have very low resistance so the wires in a circuit affect the current as little as possible.
The resistance of insulators – such as the plastic around this copper wire – is so high that they do not allow current to flow at all under most circumstances.
Some conductors are designed to have a high resistance. Inside some lamps, there is a small wire called a filament. The filament has a high resistance. When current flows through the filament, thermal energy is transferred. There is so much thermal energy that the filament glows and emits light.
As resistance makes it difficult for current to flow:
the greater the resistance in a circuit, the smaller the current in the circuit.
Ohm’s law
Georg Simon Ohm was a scientist who studied resistance. Ohm discovered there was a link between voltage, current and resistance in wires. The link between voltage, current and resistance applies to many electrical components and not only to wires.
Ohm’s law states that:
voltage = current × resistance
Important Concept
Resistance controls how much current can flow in a circuit. Higher resistance means less current flows. Ohm’s law provides a simple formula to calculate this relationship using voltage and resistance.
These are often given letter symbols:
- V = voltage, in volts
- I = current, in amps
- R = resistance, in ohms.
The letter I is used for current because in the French language, current was originally called intensité du courant.
So, we can write Ohm’s law as:
V = I × R
or just V = IR
Worked Example
Question
A current of 2 A flows through a buzzer. The voltage across the buzzer is 10 V.
Calculate the resistance of the buzzer.
Answer
Using Ohm’s law, the equation can be rearranged to:
R = V ÷ I
V = 10 and I = 2, so
R = 10 ÷ 2
= 5 Ω
Resistors
A resistor is a type of electrical component designed to have a known resistance.
The picture shows some resistors.
Many resistors, such as those in the picture, have coloured bands. The colours form a code to show the resistance value in ohms.
This is the circuit symbol for a resistor:
The value of the resistor is usually written with the circuit symbol. When the value of the resistor is not important, or when you have to calculate the value, the letter R is usually written beside the symbol.
Quick Fact
Colour bands on resistors follow an international code. Each band represents a number that helps identify the resistance value in ohms.
Questions
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low, easy, high
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Ohms (Ω)
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V = I × R
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Current (in amps)
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R = V ÷ I
R = 6 ÷ 3
= 2 Ω
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R = V ÷ I
R = 9 ÷ 2
= 4.5 Ω
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R = V ÷ I
R = 1.5 ÷ 0.5
= 3 Ω
The current through the resistor is 2 A.
Calculate the voltage across this resistor. Show your working and give the unit with your answer.
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V = I × R
V = 2 × 6
= 12 V
The voltage across the resistor is 12 V.
Calculate the current through this resistor. Show your working and give the unit with your answer.
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I = V ÷ R
I = 12 ÷ 100
= 0.12 A
Think Like a Scientist
In this investigation, you will explore how the current through a resistor depends on the voltage applied across it.
You will need:
- minimum of four 1.5 V cells
- resistor of about 10 Ω
- ammeter
- voltmeter
- switch
- wires
- connectors
Steps
- Set up the circuit with one cell as shown in the diagram.
- Measure and record the current and voltage with one cell.
- Add another cell in series with the first.
- Measure and record the current and voltage with two cells.
- Repeat with three, then four cells.
Follow-Up Questions
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A table with columns for number of cells, voltage (V), and current (A), showing how values change as more cells are added.
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A straight line through the origin indicates a linear relationship between voltage and current (Ohm's law).
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As voltage increases, the current increases proportionally.
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According to Ohm’s law, current is directly proportional to voltage across a resistor (I = V ÷ R).
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The graph would still be a straight line, but with a shallower gradient (lower current for the same voltage).
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The graph would have a steeper gradient (higher current for the same voltage).
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Repeating the experiment with a different resistor confirms how resistance affects the current–voltage relationship.