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
- Set up the circuit as shown with the fixed resistor
- 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
- For each voltage, record the value of the current from the ammeter 3 times andcalculate the average current
- Increase the voltage further in steps of 0.5 V and repeat steps 2 and 3
- Make sure to switch off the circuit in between readings to prevent heating of the component and wires
- Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)
- 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