4.2.3 Investigating Electrical Characteristics of Components

Aim of the Experiment

• The aim of the experiment is to investigate electrical characteristics for a range of ohmic and non-ohmic components
• These include a fixed resistor, lamp and diode

Variables:

• Independent variable = Potential difference, V
• Dependent variable = Current, I
• Control variables:
  • E.m.f of the power supply
  • Use of the same equipment eg. wires, diodes

Equipment List

• Resolution of measuring equipment:
  • Variable resistor = 0.005 Ω
  • Voltmeter = 0.1 V
  • Ammeter = 0.01 A

Method

Circuit diagram of the apparatus set up. The fixed resistor will be replaced by a filament lamp and diode

1. Set up the circuit as shown with the fixed resistor
2. Vary the voltage across the component by changing the resistance of the variable resistor, using a wide range of voltages (between 8-10 readings). Check the appropriate voltage reading on the voltmeter
3. For each voltage, record the value of the current from the ammeter 3 times and
   calculate the average current
4. Increase the voltage further in steps of 0.5 V and repeat steps 2 and 3
5. Make sure to switch off the circuit in between readings to prevent heating of the component and wires
6. Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)
7. Replace the fixed resistor with the filament lamp, then the diode, repeating the experiment for each

• An example of a suitable table might look like this:

Analysis of Results

• Plot a graph of average current against voltage (an I-V graph) for each component
• If the I-V graph is a straight line, it is an ohmic conductor. This is expected from the fixed resistor
  • This means it obeys Ohm's Law: V = IR

• If the I-V graph is a curve, it is a non-ohmic conductor. This is expected from the filament lamp and diode
• Compare the results from the graphs obtained to the known I-V graphs for the resistor, filament lamp and diode. These should look like:

The expected I-V graphs for the resistor, diode and filament lamp

Evaluating the Experiment

Systematic Errors:

• The voltmeter and ammeters should start from zero, to avoid zero error in the readings

Random Errors:

• In practice, the voltmeter and ammeter will still have some resistance, therefore the voltages and currents displayed may be slightly inaccurate
• The temperature of the equipment could affect its resistance. This must be controlled carefully
• Taking multiple readings of the current for each component will provide a more accurate result and reduce uncertainties

Safety Considerations

• When there is a high current, and a thin wire, the wire will become very hot. Make sure never to touch the wire directly when the circuit is switched on
• 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
• The components will get hot especially at higher voltages. Be careful when handling them - especially the filament lamp
• Disconnect the power supply in between readings to avoid the components heating up too much