Investigating Potential Divider Circuits
Investigating an LDR Circuit
Aims of the Experiment
The overall aim of the experiment is to calibrate a potential divider circuit for use as a light meter using an LDRVariables- 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
- Set up the circuit with the LDR and resistor R in series
- Record the value of the light intensity using the digital light sensor in a completely dark room
- Record the voltage across the resistor for this light intensity from the voltmeter
- Using the lamp with a dimmer switch, increase the light intensity slightly and record the new value recorded by the sensor
- Record the voltage across the resistor again with this new light intensity
- 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
The overall aim of the experiment is to calibrate a potential divider circuit using a thermistor to find an unknown temperatureVariables- 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
- 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
- Begin by placing the thermistor in a beaker filled with boiling water from the kettle
- Record the potential difference across the resistor as before, taking readings of the voltage for every 5°C decrease
- 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