2.1.9 Investigating Resistance in Thermistors & LDRs

• Objective: To investigate how the resistance of a thermistor changes with temperature.
• Equipment Needed for Thermistor Investigation

  • Thermistor
  • Power supply
  • Ammeter
  • Voltmeter
  • Variable resistor (rheostat)
  • Water bath (with a thermometer)
  • Heating device
  • Insulated container (optional)
• Investigating Resistance in Thermistors

  1. Setup the Circuit

  2. Initial Measurements

  3. Temperature Variation

  4. Record Data

  5. Plot Results

  6. Analysis
• Setup the Circuit

  1. Connect the thermistor in series with a variable resistor, ammeter, and a power supply

  2. Connect the voltmeter in parallel across the thermistor
• Initial Measurements

  1. Set the power supply to a low voltage

  2. Measure and record the initial resistance using Ohm’s Law
• Temperature Variation

  1. Place the thermistor in a water bath or use a heating device

  2. Record the temperature at regular intervals
• Record Data

  1. Measure and record the voltage across the thermistor and the current through it

  2. Calculate the resistance at each temperature
• Plot Results


  Plot a graph of resistance (R) versus temperature (T)
• NTC Thermistor

  • Resistance decreases as temperature increases
• PTC Thermistor

  • Resistance increases with temperature
• For an NTC thermistor, resistance decreases as temperature increases.
• For a PTC thermistor, resistance will increase with temperature.
• Objective: To investigate how the resistance of an LDR changes with light intensity.
• Equipment Needed for LDR Investigation

  • LDR
  • Power supply
  • Ammeter
  • Voltmeter
  • Variable resistor (rheostat)
  • Light source
  • Light meter (optional)
• Investigating Resistance in LDRs

  1. Setup the Circuit

  2. Initial Measurements

  3. Light Variation

  4. Record Data

  5. Plot Results

  6. Analysis
• Setup the Circuit

  1. Connect the LDR in series with a variable resistor, ammeter, and a power supply

  2. Connect the voltmeter in parallel across the LDR
• Initial Measurements

  1. Set the power supply to a constant voltage

  2. Measure and record the initial resistance using Ohm’s Law
• Light Variation


  1. Adjust the light source to vary the light intensity

  2. Record the light intensity or describe the conditions
• Record Data

  1. Measure and record the voltage across the LDR and the current through it

  2. Calculate the resistance at each light intensity level
• Plot Results

  Plot a graph of resistance (R) versus light intensity
• The graph should show a negative correlation, with resistance decreasing as light intensity increases.
• The LDR’s resistance should decrease as light intensity increases, showing a clear inverse relationship.
• Key Points to Remember

  • NTC Thermistors: Resistance decreases with increasing temperature
  • PTC Thermistors: Resistance increases with increasing temperature
  • LDRs: Resistance decreases with increasing light intensity
• Plotting resistance against temperature or light intensity helps visualize the behavior of thermistors and LDRs.
• Ensure Safety: Be cautious when handling heating devices and electrical components.
• Accurate Measurements: Ensure accurate readings by calibrating your measuring instruments.
• Controlled Environment: For precise results, control the experimental environment.
• Conducting these investigations helps understand how thermistors and LDRs operate.
• Thermistors and LDRs can be used in various applications to measure and control temperature and light levels.