4.5.3 Investigating Potential Divider Circuits

Investigating an LDR Circuit

Aims of the Experiment

• Independent variable = Light intensity (Lux)
• Dependent variable = Potential difference, V (V)
• Control variables:
  • E.m.f of the power supply

Equipment List

• Resolution of measuring equipment:
  • Voltmeter = 1 mV
  • Digital light sensor = 0.1 Lux

Method

LDR circuit apparatus

1. Set up the circuit with the LDR and resistor R in series
2. Record the value of the light intensity using the digital light sensor in a completely dark room
3. Record the voltage across the resistor for this light intensity from the voltmeter
4. Using the lamp with a dimmer switch, increase the light intensity slightly and record the new value recorded by the sensor
5. Record the voltage across the resistor again with this new light intensity
6. Repeat this process until the light intensity from the lamp cannot be increased any further

Analysing the Results

• Plot a graph of potential difference V across the resistor against the light intensity and draw a line of best fit
  • This can be used as a calibration curve

• Move the circuit to an area of unknown light intensity and record the voltage across the resistor
• Using the calibration curve, find the corresponding light intensity to a particular potential difference

LDR calibration curve

• The resistance of the LDR and thermistor decreases with the increase in light intensity and temperature respectively
• Ohm’s Law states that the current between two points is directly proportional to the potential difference across those points
• Since R = V / I, this means the resistance between those points is constant, and a non-ohmic device doesn’t obey ohm’s law
• The potential difference provided by the cell is shared between the resistor and the non-ohmic device (the LDR)
• Therefore, a greater proportion of the voltage is shared across the resistor as the resistance of the non-ohmic device decreases

Investigating a Thermistor Circuit

Aims of the Experiment

• Independent variable = Temperature, θ (°C)
• Dependent variable = Potential difference, V (V)
• Control variables:
  • E.m.f of the power supply

Equipment List

• Resolution of measuring equipment:
  • Voltmeter = 1 mV
  • Thermometer = 1 ºC

Method

Thermistor circuit apparatus

1. Replace the LDR in the circuit with a waterproof thermistor. Rather than light intensity being altered, it is now the temperature that must be altered
2. Begin by placing the thermistor in a beaker filled with boiling water from the kettle
3. Record the potential difference across the resistor as before, taking readings of the voltage for every 5°C decrease
4. As the temperature falls to room temperature, place ice in the beaker to further lower the temperature as much as possible

Analysing the Results

• Similar to the LDR, plot a graph of potential difference (V) against temperature (in C) and draw a line of best fit
• Using the calibration curve, find the corresponding temperature to a particular potential difference

Thermistor calibration curve

Evaluating the Experiment

Systematic Errors:

• Make sure the voltmeter and digital light sensor start at 0 to avoid a zero error

Random Errors:

• Repeating measurements for the potential difference will reduce random errors
• To increase the accuracy of the calibration curve, take as many readings as possible over a wide range so that there are as many data points as possible
• The temperature of the thermistor is assumed to be the same as the temperature of the water
  • Make sure the water is stirred thoroughly to make sure its temperature is even

Safety Considerations

• This is a very safe experiment, however, electrical components can get hot when used for a long period
• Switch off the power supply right away if burning is smelled
• Make sure there are no liquids close to the equipment, as this could damage the electrical equipment
• Be careful not to be scalded by boiling water by not letting it splash
• Since the lamp can become very bright, do not look directly into it as this will damage your vision
• Be careful not to trip when the room is dark. Keep all belongings under desks to avoid this