Required practical 16 · Physics · Physics Paper 1, both tiers

Current and potential difference characteristics

Use circuit diagrams to set up circuits and plot current against potential difference for a filament lamp, a diode and a resistor at constant temperature.

The exam trap. Only the resistor at constant temperature gives a straight line. The diode needs a protective resistor.

Variables

Independent (you change): Potential difference across the component
Dependent (you measure): Current through the component
Control (you keep the same): The component tested
Temperature of the resistor (small currents)
The same meters

Apparatus

Variable d.c. supply or variable resistor
Ammeter and voltmeter
Filament lamp, diode (with a protective resistor) and fixed resistor
Leads and switch

Method, and why each step matters

Set up the circuit with the component, an ammeter in series and a voltmeter in parallel.
Why This measures the current through and the potential difference across the component.
Increase the potential difference in steps, recording V and I each time.
Why A set of readings gives the I–V graph.
Reverse the supply connections and repeat for negative values.
Why The graph must show both directions, which is essential for the diode.
Use a protective resistor in series with the diode.
Why It stops too large a current damaging the diode.
Plot current against potential difference for each component.
Why The shape of the graph shows how resistance changes.
Switch off between readings for the resistor.
Why Keeping it cool keeps its resistance constant.

Risks

Hazard	How to reduce the risk
A filament lamp gets hot.	Do not touch it; switch off between readings.

A worked set of results

V in V	Lamp I in A	Resistor I in A
−4.0	−0.52	−0.40
−2.0	−0.34	−0.20
0	0	0
2.0	0.34	0.20
4.0	0.52	0.40
6.0	0.63	0.60

Resistor: a straight line through the origin, so its resistance is constant: 2.0 ÷ 0.20 = 10 Ω at every reading.
Lamp: a curve that flattens, because the filament heats up and its resistance increases (2.0 ÷ 0.34 = 5.9 Ω, then 6.0 ÷ 0.63 = 9.5 Ω).
Diode (in series with its protective resistor): no current in the reverse direction; forwards, almost none until about 0.7 V, then the current rises steeply.

Mistakes that cost marks

Not explaining that a hotter filament has a higher resistance.
Instead Hotter metal ions vibrate more, so electrons collide more: resistance rises.
Using the lamp's curve as if it were an ohmic resistor.
Instead Only the resistor at constant temperature gives a straight line.
Leaving out the negative potential differences.
Instead Reverse the connections to show the reverse direction.