Required Practicals/Edexcel/Practical 8
8ASCP8

I-V characteristics of components (CP8)

Investigate the current-voltage characteristics of a resistor, filament bulb and diode; investigate how resistance varies with temperature for a thermistor.

Apparatus

  • Components: fixed resistor, filament lamp, diode, NTC thermistor
  • Variable power supply (0-6 V DC)
  • Ammeter and voltmeter (or multimeter)
  • Water bath and thermometer (for thermistor-temperature investigation)
  • Connecting leads

Safety

  • Do not exceed the rated voltage of the components; the filament bulb can become very hot.
  • Allow the bulb to cool between readings if investigating it at different voltages.

Method

  1. Connect each component in series with the ammeter; connect the voltmeter in parallel across it.
  2. Vary the supply voltage from 0 to 6 V in steps. Record I and V for each component.
  3. For the diode, also investigate negative bias (reverse the component or the supply polarity).
  4. For the thermistor: heat the water bath from ~10 degrees C to ~80 degrees C. Record resistance $R = V/I$ at each temperature.
  5. Plot I vs V for each component; plot R vs T for the thermistor.

Key Variables

Independent
Voltage V (I-V curves) or temperature T (thermistor)
Dependent
Current I (I-V curves) or resistance R (thermistor)
Controlled
Same component throughout; Temperature (for I-V curves - keep the bulb cool for short measurements)

Analysis and Results

  • Fixed resistor: straight line through origin (I proportional to V, Ohm's Law obeyed, R constant).
  • Filament bulb: curve showing decreasing gradient (R increases with temperature as filament heats).
  • Diode: negligible current until forward threshold (~0.6 V for silicon), then rapid rise; near-zero reverse current.
  • Thermistor: R decreases exponentially with T (NTC behaviour) due to more charge carriers being released at higher temperatures.

Common Errors

  • Leaving the bulb or thermistor energised at fixed voltage between readings (temperature changes affect the result).
  • Not applying reverse bias to the diode (misses the rectifying behaviour).
  • Confusing the threshold voltage of the diode with the breakdown voltage.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q13 marks
Sketch and explain the I-V characteristic of a filament lamp, explaining the shape of the graph.
Q23 marks
Explain, in terms of charge carriers, why the resistance of an NTC thermistor decreases as temperature increases.