9.1 What an I–V characteristic shows

An I–V characteristic shows how current through a component depends on potential difference across it. The shape reveals whether resistance is constant, increases or decreases. Measurements should include positive and negative voltages where safe because some components behave differently with reversed polarity.

If current is plotted vertically against voltage horizontally, the local gradient represents conductance. A steeper line means lower resistance. For a curved graph, resistance V/I at a point is not necessarily the reciprocal of the tangent gradient, so use the definition requested.

9.2 Fixed resistor

A fixed resistor at constant temperature gives a straight line through the origin. Reversing voltage reverses current, producing a symmetric line in the opposite quadrant. The constant ratio V/I shows that the component is ohmic.

At very high currents even a resistor may heat, causing its resistance to increase. In a school experiment, current is kept moderate and readings are taken promptly to maintain nearly constant temperature.

9.3 Filament lamp

A filament lamp has a curved characteristic. At low voltage the filament is relatively cool and has lower resistance, so the graph is steep. As current increases, the filament becomes much hotter, its resistance rises and the graph becomes less steep.

The curve is approximately symmetric because reversing polarity does not change the heating mechanism. The lamp is non-ohmic: current is not directly proportional to voltage because temperature is not constant.

9.4 Diode

A diode allows current mainly in one direction. In forward bias, little current flows until a characteristic turn-on region is reached, after which current rises rapidly. In reverse bias, current is extremely small for ordinary voltages.

A diode must be protected by a series resistor to limit current. An LED is a diode that emits light when forward-biased; it also needs correct polarity and current limiting.

9.5 Thermistor and LDR characteristics

An NTC thermistor has resistance that decreases as temperature increases. It is useful as a temperature sensor. An LDR has resistance that decreases when light intensity increases. Their full graphs need not be memorised in every detail, but the direction of change must be known.

Because these components respond to environmental conditions, a fair I–V measurement should keep temperature or illumination constant unless that variable is deliberately being investigated.

9.6 Experimental method

Use a variable d.c. supply or a variable resistor to change p.d. Connect an ammeter in series and voltmeter in parallel. Record readings, reverse the supply for negative values if the component permits, and avoid excessive current.

Plot a smooth best-fit curve rather than joining every point with jagged segments. Include the origin only if physically justified and measured behaviour supports it. Repeat suspicious readings and use fine meter ranges without exceeding their limits.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. Why does a filament lamp graph become less steep at large voltage?
2. Which component has an approximately straight I–V graph at constant temperature?
3. What safety component should be placed in series with an LED?