IGCSE CP13 – I-V Characteristics | Edexcel International GCSE Physics

Theory — I-V Characteristics

The relationship between current and voltage depends on which component is in the circuit.

Ohm's Law

For a metal conductor at constant temperature, current is directly proportional to voltage. This is called Ohm's Law:

V = I × R

V = voltage (V) · I = current (A) · R = resistance (Ω)

A component obeying Ohm's Law is called an ohmic conductor. Its I-V graph is a straight line through the origin.

The Three Components

Fixed resistor (carbon) — ohmic. Straight line on I-V graph. Resistance constant regardless of voltage.
Filament lamp — non-ohmic. As voltage rises, the filament gets hotter, increasing resistance. I-V graph curves and flattens.
Diode — non-ohmic. Conducts only in one direction (forward bias) above ~0.7V threshold. Blocks current in reverse bias. I-V graph is a hockey-stick shape.

The Circuit

A rheostat (variable resistor) is connected in series with the component. Adjusting the rheostat changes the voltage across the component. The ammeter (in series) measures current through the component. The voltmeter (in parallel) measures voltage across it.

R = V / I

Calculate resistance at any point on the I-V graph using this equation.

Procedure

Using a rheostat to vary voltage across each component and recording I-V data.

Equipment

Cells (1, 2 or 3 × 1.5V) · Fixed resistor (carbon, e.g. 10Ω) · Filament lamp · Diode · Rheostat (0–100Ω) · Ammeter (mA range) · Voltmeter · Switch · Connecting leads

1

Build the series circuit

Connect: cell → switch → rheostat → component → back to cell. Place the ammeter in series with the component. Connect the voltmeter in parallel across the component only.

💡 Ammeter in series measures current through. Voltmeter in parallel measures voltage across. Never swap these.

2

Set rheostat to maximum

Before switching on, set the rheostat to maximum resistance — this gives minimum voltage across the component at the start. Switch on.

💡 Always start from minimum voltage and increase gradually — prevents accidentally damaging the diode with too much forward current.

3

Decrease rheostat resistance gradually

Slowly reduce the rheostat resistance. As you do, the voltage across the component increases. At each setting, record V (voltmeter) and I (ammeter). Take at least 8 readings across the full range.

💡 For the filament lamp — let it settle for a few seconds at each voltage before reading, as the temperature takes a moment to stabilise.

4

Repeat for each component

Replace the component and repeat for all three: resistor, lamp, diode. For the diode also record readings in reverse bias by reversing the diode connections.

5

Plot I-V graphs

Plot I (y-axis, mA) against V (x-axis, V) for each component. Draw smooth curves. Describe and explain the shape of each graph.

💡 The shapes you get — straight line, flattening curve, and hockey stick — are the three characteristic shapes you must be able to sketch and explain in the exam.

      ⚡ Choose a component and cell number. Drag the rheostat slider to change voltage across the component — then Record each reading.
    

Component

Cell (EMF)

Rheostat

Resistance 50 Ω

0 Ω (max V)100 Ω (min V)

Meters

—

Voltage / V

—

Current / mA

EMF3.0 V

Rheostat R50 Ω

V across comp— V

I through comp— mA

R = V/I—

Readings recorded: 0

Data Table

Recorded readings for each component. Switch between components below.

#	V across component / V	I through component / mA	R = V/I / Ω
No readings yet. Use the Simulation tab.

I-V Characteristic Graphs

Current (y-axis) against voltage (x-axis). The shape tells you about the component.

Resistor

ShapeStraight line

Through origin?Yes

ResistanceConstant

Ohmic?Yes ✓

Gradient—

R from graph—

Exam answer

Straight line through origin → current directly proportional to voltage → obeys Ohm's Law → ohmic conductor → constant resistance.

Filament Lamp

ShapeCurves, flattens

Through origin?Yes

ResistanceIncreases with V

Ohmic?No ✗

Exam answer

Curved line, getting shallower → resistance increases as voltage increases → filament heats up → non-ohmic conductor.

Diode

Forward bias (>0.7V)Conducts

Threshold voltage~0.7 V

Reverse biasNo current

Ohmic?No ✗

Exam answer

No current below 0.7V threshold. Current rises sharply above 0.7V in forward bias. No current in reverse bias → conducts in one direction only → non-ohmic.

Comparison

ResistorStraight

LampCurves

DiodeHockey stick

Key point

Only the resistor is ohmic. For any point on any graph, R = V/I. For the resistor this gives the same answer everywhere. For the lamp and diode it changes.

Questions

Answer these questions. Reveal the answer when ready.

Question 1

A student records V = 3.0 V and I = 300 mA for a resistor. Calculate the resistance. What does the straight-line graph tell you about this resistor?

R = V/I = 3.0 / 0.300 = 10 Ω. The straight line through the origin shows current is directly proportional to voltage. Resistance stays the same for all values of voltage — the resistor is an ohmic conductor and obeys Ohm's Law.

Question 2

Explain why the I-V graph for a filament lamp curves and gets shallower as voltage increases, rather than being a straight line.

As voltage increases, more current flows and the filament heats up. The higher temperature causes the resistance of the metal filament to increase. Because resistance increases, each extra volt produces less extra current — so the graph curves and gets shallower. The lamp is non-ohmic: its resistance is not constant.

Question 3

Describe what happens when a diode is connected (a) in forward bias and (b) in reverse bias. Give one use of a diode.

(a) Forward bias: very little current flows until the voltage reaches about 0.7V (the threshold voltage). Above 0.7V, current increases sharply — the diode conducts with very low resistance. (b) Reverse bias: the diode blocks the current — almost no current flows regardless of how large the reverse voltage is. Use: a diode can be used to convert alternating current (AC) to direct current (DC) in a power supply (rectification). LEDs (light-emitting diodes) use the same principle to produce light.