Investigating $I$ - $V$ characteristics

In a nutshell

By using a circuit with a variable resistor, the $I$ - $V$ characteristics of a fixed resistor, a filament lamp and a diode will be investigated. This is done by varying the resistance of the variable resistor in the circuit and recording readings from an ammeter and voltmeter to test whether the component is an ohmic conductor.

Equations

Word equation	symbol equation
$potential \space difference = current \times resistance$	$V = I \times R$

Variable definitions

quantity name	Symbol	unit name	unit symbol
$potential \space difference$	$V$	$volt$	$V$
$current$	$I$	$ampere$	$I$
$resistance$	$R$	$ohm$	$\Omega$

Equipment list

The following equipment list can be used to conduct the experiment to investigate $I$ - $V$ characteristics.

Equipment	Use
Digital ammeter	To measure the current in the circuit.
Digital voltmeter	To measure the potential difference across components.
Element holder	To hold the test components in place.
Variable resistor	To adjust the potential difference across the test component.
Connecting leads	To connect each element of the circuit together.
Fixed resistor	This component is being tested.
Filament lamp	This component is being tested.
Diode	This component is being tested.
Battery or suitable power supply	To provide power to the circuit.

Experiment 1: The characteristics of a fixed resistor

Experimental variables

The independent variable is the variable you change. The dependent variable is the variable that changes depending on the other variables, and is the one you measure. The control variables are the variables that are kept constant.

Independent variable	$Potential\ difference$ , $V$
Dependent variable	$Current,$ $I$
Control variable	$Potential\ difference\ of\ power\ supply,\ use\ of\ the\ same\ equipment$

Safety precautions

When performing experiments, it is very important to consider safety precautions. This is so that no one gets hurt or injured during the experiment.

Hazard

risk

Safety measure

Hot components

The resistance can cause the components of the circuit to heat up. Components and wires can cause burns if touched.

Do not touch the components and wires directly if the circuit is switched on or has just recently been turned off. Switch off the power supply if burning can be smelled.

If burnt, run the burn under cold water for five minutes.

Liquids spilled onto circuit

Liquids can damage the electrical equipment and may cause a fire.

Make sure there are no liquids close to the electrical equipment.

Method

These are the instructions to complete the experiment. Record all measurements taken as you work through the experiment in a table.

Physics; Physics practicals; KS4 Year 10; Investigating I-V characteristics

1.	Power supply
2.	Variable resistor
3.	Test component (fixed resistor)
4.	Digital ammeter
5.	Digital voltmeter

1.	Use the circuit diagram shown above to set up your circuit.
2.	Switch the power supply on and record the readings on the ammeter and voltmeter. Make sure these readings are both positive numbers.
3.	Adjust the variable resistor and record the new readings on the ammeter and voltmeter. Tip: Switch off the power supply in between readings to prevent the circuit components from heating up.
4.	Repeat step 3 for a range of voltages (between $8-10$ different readings).
5.	Swap the connections on the battery or power supply. This means the ammeter should now be connected to the negative terminal and the variable resistor should be connected to the positive terminal. The readings on both the ammeter and voltmeter should be negative.
6.	Adjust the variable resistor and record between $8-10$ readings for the negative potential difference and current.

Analysis

This is how you will use the data recorded to form conclusions.

1.

Plot a graph of current (

y

-axis) against potential difference (

x

-axis). This is an

I

-

V

graph. Your graph should include all your positive and negative readings.

2.

Compare the graph to Ohm's law.

The equation that relates potential difference and current is Ohm's law:

V = I \times R

If the potential difference

V

, is proportional to the current

I

, then the resistance will be a constant. For the graph, the gradient gives the reciprocal of the resistance.

If the graph is a straight line (i.e. a constant gradient), the component obeys Ohm's law and it is known as an ohmic conductor. If the graph is curved (i.e. a changing gradient), it means that the component is a non-ohmic conductor and does not obey Ohm's law.

Conclusion

Physics; Physics practicals; KS4 Year 10; Investigating I-V characteristics

The $I$ - $V$ graph for the fixed resistor should look like this. This is a straight line graph and therefore shows that the fixed resistor is an ohmic conductor.

Evaluation

Once you have completed your experiment, it is important to consider how it could be improved. You should come up with ways to improve the accuracy of your values. If you got a result that seems unreasonable, think about why this was.

Experiment 2: The characteristics of a filament lamp

Method

Repeat the previous experiment's method, but replace the fixed resistor with a filament lamp.

Analysis

Repeat the same analysis as the previous experiment.

Conclusion

Physics; Physics practicals; KS4 Year 10; Investigating I-V characteristics

The $I$ - $V$ graph for the filament lamp should look like this. This is a curved line graph and therefore shows that the filament lamp is a non-ohmic conductor.

Evaluation

This is the same as for the previous experiment.

Experiment 3: The characteristics of a diode

Method

Repeat the previous experiment's method, but replace the fixed resistor with a diode.

Analysis

Repeat the same analysis as the previous experiment.

Conclusion

Physics; Physics practicals; KS4 Year 10; Investigating I-V characteristics

The $I$ - $V$ graph for the diode should look like this. This is a curved line graph and therefore shows that the diode is a non-ohmic conductor.

Evaluation

This is the same as for the previous experiment.