Example 1: using V = I × R
A lamp has a p.d. of 6.0 V across it and a current of 0.30 A. Find its resistance.
- R = V ÷ I.
- R = 6.0 ÷ 0.30 = 20 Ω.
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IB DemystifiedMYP Sciences
Current, voltage and resistance
Why does a kettle element glow hot while the copper cable to it stays cool? Why do street lights switch on by themselves at dusk? The answers lie in three linked quantities (current, potential difference and resistance) and one of the most useful equations in physics: V = I × R.
Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D
On this page
Learning objectives
By the end of this topic you should be able to:
- describe current as a flow of charge and use Q = I × t
- use V = I × R to calculate potential difference, current and resistance
- interpret current–potential difference graphs for resistors and lamps
- explain the factors that affect the resistance of a wire
- analyse resistance data and design experiments
- discuss automatic street lights and house wiring
Before you start
You will use these skills. If any feel shaky, review them first.
- simple circuits, series and parallel (see Simple circuits)
- ammeters and voltmeters
- rearranging equations and drawing graphs
Key vocabulary
- Current
- The rate of flow of electric charge, in amperes (A).
- Charge
- Measured in coulombs (C); Q = I × t.
- Potential difference
- The energy transferred per unit charge, in volts (V).
- Resistance
- How much a component opposes current, in ohms (Ω): R = V ÷ I.
- Ohm's law
- For a conductor at constant temperature, current is directly proportional to potential difference.
- LDR and thermistor
- Resistors whose resistance changes with light and with temperature.
Understanding the ideas
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Current and potential difference
An electric current is a flow of charge, usually electrons in metal wires. Charge = current × time. The potential difference across a component tells us how much energy each coulomb of charge transfers to it.
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Resistance and Ohm's law
Resistance opposes current. The three quantities are linked by V = I × R. For a fixed resistor at constant temperature, current is directly proportional to potential difference (Ohm's law), giving a straight-line I–V graph through the origin.
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Changing resistance
Not every component behaves like this. A filament lamp gets hotter as more current flows, so its resistance rises and its I–V graph curves. The resistance of a wire increases if it is longer, thinner or hotter, and LDRs and thermistors change resistance with light and temperature.
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Why does it matter?
Resistance is used and controlled everywhere: heating elements need high resistance, cables need low resistance, dimmer switches use variable resistors, and sensors use LDRs and thermistors to control lights and heaters automatically.
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What does it connect to?
This topic builds on simple circuits and leads to electrical power, electricity in the home and electromagnetism.
Key ideas
- V = I × R, so R = V ÷ I and I = V ÷ R.
- Q = I × t (charge in coulombs, time in seconds).
- Series: total resistance = R₁ + R₂; parallel: total resistance is less than the smallest resistor.
- Resistance of a wire increases with length and temperature and decreases with thickness.
- I–V graphs: straight line = constant resistance; curve = changing resistance.
Resistance in the real world
Kettles, irons and heaters use high-resistance elements. Overhead power lines use thick aluminium cables to keep resistance low. Automatic street lights and phone screens use light sensors to adjust brightness.
Worked examples
Example 2: series resistors
A 5 Ω and a 15 Ω resistor are in series with a 10 V supply. Find the current.
- Total R = 5 + 15 = 20 Ω.
- I = 10 ÷ 20 = 0.50 A.
Assessment tips
Questions on this topic often combine V = I × R with circuits and graphs. Expect to:
- Calculate V, I, R and Q with correct units, including rearranging.
- Interpret I–V graphs and explain their shapes.
- Explain factors that affect resistance.
- Analyse data and design fair experiments.
Common mistakes: rearranging V = I × R wrongly; forgetting to convert time to seconds in Q = I × t; saying thicker wires have more resistance; and thinking every component obeys Ohm's law.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: light and an LDR
Partially guided investigation · about 40 minutes · pairs
- Research question
- How does the distance of a lamp from a light-dependent resistor (LDR) affect its resistance?
- Scientific background
- An LDR's resistance falls as the light shining on it gets brighter. Moving a lamp closer makes the light brighter.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- LDR, digital multimeter set to measure resistance (or an ammeter, voltmeter and battery), desk lamp, metre rule, a darkened room.
- Method
- Connect the LDR to the multimeter set to ohms (or build a circuit to measure V and I).
- Place the lamp 10 cm from the LDR, pointing straight at it, and record the resistance.
- Repeat at 20, 30, 40 and 50 cm, and in darkness.
- Repeat the whole set three times and find the mean resistance at each distance.
Safety. The lamp gets hot; do not touch the bulb. Keep cables tidy so no one trips in the darkened room.
Then evaluate: why must the room be dark, and how could you plot your results to show the pattern clearly?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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