I–V Characteristics | CAIE 9702 Paper 2 Notes

Chapters

Paper 2

Physics9702

I–V Characteristics

An I–V graph shows how current responds as the p.d. across a component changes.

Read the axis labels before using a gradient: this chapter plots current vertically and p.d. horizontally.

Recognise the three required shapes

Five hand-drawn electrical components: a resistance wire, filament lamp, semiconductor diode, bead thermistor and light-dependent resistor — From left: resistance wire, filament lamp, semiconductor diode, NTC thermistor and LDR. Their graphs describe how current or resistance responds to a changing condition.

Metal at constant temperature: a straight line through the origin because resistance remains constant.
Filament lamp: a symmetric curve that becomes less steep because the hot filament has greater resistance.
Semiconductor diode: almost no reverse current, followed by a steep forward rise after about 0.6 V. It conducts strongly in one direction only.

A metallic conductor at constant temperature: I is directly proportional to V, so the graph is a straight line through the origin. Same slope both ways round.

A filament lamp: near the origin the line is roughly straight, but at higher V the current falls short of the straight line. The filament gets hot and its resistance goes up.

A semiconductor diode: it only conducts one way, and even that way it stays off until about 0.6 V. Past the threshold the current climbs steeply.

Read resistance from a curved graph

At one point, read $V$ and $I$ , then use $R=V/I$ .
On an I-against-V straight line through the origin, the gradient is $I/V=1/R$ . On a curved graph, use the coordinates at the chosen point unless the question explicitly asks for a tangent.

voltage V2.00 V

metal / resistor (2.0 Ω)·I = 1.00 A·R = V/I = 2.00 Ω

Your turn9702/11/O/N/25 Q33

One I–V curve is symmetric and becomes less steep at larger currents. Another conducts strongly for only one polarity. Identify the two components. [2] (9702/11/O/N/25 Q33)

Show worked answer

The symmetric curved graph is a filament lamp. The one-direction graph is a semiconductor diode.