Electrical Resistance and Ohm’s Law
Electrical Resistance and Ohm’s Law
This GCSE physics lesson explores Electrical Resistance and Ohm’s Law. Students calculate resistance, use current-potential difference graphs and explain how component resistance changes with temperature, light or other conditions. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Lesson duration is: 55 minItems in this lesson
Electrical Resistance and Ohm’s Law
This GCSE physics lesson explores Electrical Resistance and Ohm’s Law. Students calculate resistance, use current-potential difference graphs and explain how component resistance changes with temperature, light or other conditions. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Resistance, KS4 Physics, GCSE Physics, Electrical Resistance, Ohm's Law, $V = IR$, Ohmic Conductors, Filament Lamp, Diode, LDR, Thermistor, Resistance in Wires, Temperature and Resistance, MCQs, Open Questions, Polls, Emoji Polls, Exit Tickets
Learning goal
I can explain electrical resistance and Ohm’s law, and use the relationship between voltage, current and resistance to calculate missing values in simple circuits.
This item has no instructions
Could you try to name as many words as possible that have some kind of link to electrical resistance?
Use this to surface prior knowledge and misconceptions such as confusing current with resistance, or voltage with current.
Electrical resistance
Resistance is the opposition to the flow of current.
It tells us how hard it is for charge to move through a component.
Resistance is measured in ohms (Ω).
Students often confuse resistance with current. Emphasise that resistance does not flow; it is a property of a component or material.
What affects resistance?
Resistance depends on the conductor itself:
Length of the conductor
Thickness of the conductor
Material it is made from
Temperature of the conductor
This item has no instructions
Length and thickness
A longer wire has more resistance.
A thicker wire has less resistance.
In a long wire, charge passes through more material.
In a thick wire, charge has more room to flow.
Link this to particle collisions in the conductor, but keep it simple at GCSE level: longer path means more opposition; wider path means easier flow.
Material and temperature
Different materials have different resistances.
Good conductors, such as metals, usually have lower resistance than poorer conductors.
In metals, resistance increases when temperature increases.
Hotter metal ions vibrate more, making it harder for charge to pass through.
You can mention that this temperature effect helps explain why some components are non-ohmic when they heat up.
Which change would increase the resistance of a metal wire?
Make it shorter
Make it thicker
Heat it up
Use more current
Correct answer: heat it up. Nearby slides teach that in metals, higher temperature increases resistance because ions vibrate more. 'Use more current' is a plausible misconception because students may confuse cause and effect.
Complete the summary
Electrical resistance is the opposition to the flow of current and is measured in ohms . A longer wire has more resistance, but a thicker wire has less. In metals, resistance increases when temperature increases.
Blanks target the key taught terms from slides 2 to 5.
Circuit quantities
Three linked circuit quantities are used in Ohm’s law:
Current (I): the flow of charge, measured in amperes (A)
Potential difference (V): energy transferred per unit charge, measured in volts (V)
Resistance (R): opposition to current, measured in ohms (Ω)
Potential difference may already be known as voltage; use both terms consistently: 'potential difference (voltage)' if needed.
Ohm’s law
For an ohmic conductor at constant temperature:
V = IR
This means:
potential difference = current × resistance
if resistance stays the same, a bigger voltage gives a bigger current
Stress the condition 'constant temperature'. Students often apply Ohm’s law too loosely to all components in all situations.
Rearranging the equation
The same relationship can be rearranged:
I = V / R
R = V / I
Choose the form that matches the value you need to find.
A quick reminder about units helps here: V in volts, I in amperes, R in ohms.
Using V = IR
Example calculations:
If V = 12 V and R = 4 Ω, then I = 12 / 4 = 3 A
If V = 9 V and I = 3 A, then R = 9 / 3 = 3 Ω
If I = 2 A and R = 5 Ω, then V = 2 × 5 = 10 V
Model careful substitution with units before calculating. Many students know the formula but lose marks through unit errors or rearrangement mistakes.
A 12 V battery is connected to a 4 Ω resistor. What is the current?
48 A
0.3 A
8 A
3 A
Students should use I = V / R, so 12 / 4 = 3 A. Distractors reflect common operation errors.
Complete the Ohm’s law paragraph
Ohm’s law for an ohmic conductor at constant temperature is V = IR . If you need to find current, use I = V / R . If you need to find resistance, use R = V / I . Potential difference is measured in volts .
Checks equation recall and rearrangement after the worked examples.
How do you choose which Ohm’s law equation to use?
Expected elements: students should explain that the formula triangle or rearranging V = IR helps them choose the correct form, then substitute values with units and calculate carefully.
Current–voltage graphs
A current–voltage graph shows how current changes when potential difference changes.
Different components give different graph shapes because they do not all behave in the same way.
This item has no instructions
Ohmic conductor
For a resistor at constant temperature:
current is directly proportional to potential difference
the graph is a straight line through the origin
resistance stays constant
This is called ohmic behaviour.
Make the link explicit: straight line through the origin means doubling voltage doubles current.
Filament lamp
A filament lamp is non-ohmic.
As current increases, the filament gets hotter.
Its resistance increases as temperature rises.
The graph curves because current does not increase evenly with voltage.
Students may expect every graph to be a straight line because of V = IR. Remind them that resistance is changing here.
Diode
A diode is also non-ohmic.
It allows current mainly in one direction.
In the reverse direction, current is almost zero.
In the forward direction, current rises sharply after a certain potential difference.
You do not need an advanced semiconductor explanation here; focus on the one-way behaviour shown by the I–V graph.
Why some components are non-ohmic
A component is non-ohmic when current is not directly proportional to potential difference.
This happens when:
resistance changes as conditions change
temperature changes during use
the component behaves differently in different directions
Tie this back to the examples: filament lamp changes resistance because it heats up; diode behaves differently depending on direction.
Which component has a straight-line I–V graph through the origin?
Diode
Filament lamp
Cell
Resistor
The correct answer is an ohmic resistor at constant temperature. The cell is a plausible distractor because students may confuse components with sources.
Complete the graph summary
An ohmic conductor has a straight line current–voltage graph through the origin because current is directly proportional to potential difference. A filament lamp is non-ohmic because its resistance changes as it gets hotter. A diode allows current mainly in one direction .
Targets the key comparisons across resistor, filament lamp and diode.
How do you feel about explaining what happens to current if resistance increases?
This item has no instructions
Glossary
resistance - opposition to the flow of current in a component or conductor
ohm (Ω) - the unit used to measure resistance
current - the flow of electric charge in a circuit
potential difference - energy transferred per unit charge between two points in a circuit
voltage - another name for potential difference
Ohm’s law - the relationship V = IR for an ohmic conductor at constant temperature
ohmic - describing a conductor that gives a straight-line current–voltage graph through the origin
This item has no instructions
Glossary
conductor - a material that allows electric charge to flow through it
filament lamp - a lamp with a metal filament that heats up and glows
diode - a component that allows current mainly in one direction
non-ohmic - describing a component whose current is not directly proportional to potential difference
constant temperature - a condition where the temperature does not change while measurements are taken
current–voltage graph - a graph showing how current changes with potential difference for a component
This item has no instructions
