AQA Physics (8463) 4.2.1.4 · Required practical
Resistors and I–V characteristics
Double the potential difference across a component. Does the current double? For a resistor, a lamp and a diode, find out, and learn the method AQA will ask you to describe.
Triple Science · Higher tier
Start here
A bulb takes a big gulp of current, then settles.
Switch on an old filament bulb and, for the first fraction of a second, it draws several times more current than it does once it is glowing. Filament bulbs usually blow at the moment they are switched on, not while they are lit.
Commit first
What must be different about the cold filament?
Required practical · the AQA method
Build the circuit. Change the p.d. Measure the current.
- Connect the component in series with an ammeter, a variable resistor and the battery. Connect a voltmeter in parallel across the component.
- Use the variable resistor to set the p.d. Record the p.d. and the current.
- Repeat for a range of p.d. values, switching off between readings so that the resistor stays at a constant temperature.
- Reverse the connections to the battery and repeat, to get negative values.
- Plot current (y-axis) against p.d. (x-axis) for each component.
Risks. A filament lamp gets hot enough to burn: do not touch it while it is on, and let it cool. A resistor carrying a large current also warms up, which changes its resistance: keep currents low and switch off between readings.
Run the practical
Resistor · call the graph first
0 of 3 components
Commit first. Which graph will the resistor give?
Think again · thermistors
“Things resist more when they get hot, so a thermistor's resistance goes up as it warms.”
Your call
PredictThe thermistor in this circuit is warmed with a hairdryer. What happens to the ammeter reading?
Variables
Independent, dependent or control?
For the filament-lamp investigation.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Equation · processing the data
V = I R
R = V ÷ I
p.d. in volts (V), current in amperes (A), resistance in ohms (Ω).
Work out R from one reading at a time. On a curved I–V graph, R is different at every point: never use the gradient to find it.
Command words in this lesson
- Describe a method
- Circuit, what you change, what you measure, how many readings, how you stay safe.
- Sketch
- The right shape and labelled axes. No exact values needed.
- Calculate
- R = V ÷ I from one reading, with the current in amperes.
Key fact
A resistor at constant temperature is ohmic: current is directly proportional to p.d. A filament lamp's resistance rises as it heats, and a diode lets current flow in one direction only.
Examiner tip
Key note · AQA 4.2.1.4 (8463) · RP
Resistors and I–V characteristics
- Required practical: component in series with an ammeter and variable resistor; voltmeter in parallel across it.
- Change the p.d. with the variable resistor; reverse the connections for negative values; plot I against V.
- Resistor at constant temperature: straight line through the origin. Ohmic: I is directly proportional to V.
- Filament lamp: curve. Resistance increases as the filament gets hotter.
- Diode: current flows in one direction only; very high resistance in reverse.
- Thermistor: R falls as temperature rises. LDR: R falls as light intensity rises. R = V ÷ I, current in amperes.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask Mr Badmus AI
Muddled about which graph belongs to which component?
The simulation uses model components: a 10 Ω resistor held at constant temperature, a 6 V filament lamp, and a silicon diode that starts conducting at about 0.6 V. Each reading carries a small random scatter, as real meters do, and one lamp reading is misread on purpose. A protective resistor in series with the diode is assumed and not drawn.