Investigating the resistance of a length of wire

In a nutshell

By changing the length of a piece of wire, the resistance of electrical circuits will be investigated. This is done by measuring the potential difference and current across a circuit with a varying length of thin resistance wire at a constant temperature and finding the resistance using Ohm's law.

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 resistance.

Equipment	Use
Digital ammeter	To measure the current in the circuit.
Digital voltmeter	To measure the potential difference across components.
Thin resistance wire	To measure the resistance of.
Crocodile clips	To attach to the length of wire.
Connecting leads	To connect each element of the circuit together.
Battery or suitable power supply	To provide power to the circuit.
Metre ruler	To measure the length of the resistance wire.

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	$Length\ of\ the\ wire,\ L$
Dependent variable	$Resistance,\ R$
Control variable	$Potential\ difference\ of\ power\ supply,\ temperature\ of\ the\ wire$

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 wires

The resistance can cause the wires used in the circuit to heat up. These can cause burns if touched.

Do not touch the 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 the resistance of a length of wire

A.	Ammeter
B.	Power supply
C.	Voltmeter
D.	Crocodile clips
E.	Thin resistance wire
F.	Ruler

1.	Set up the apparatus as shown in the diagram. Attach one crocodile clip to the start of the ruler and the other at the $10\,cm$ mark. Set the power supply to $1.5\,V$ . Tip: Make sure that the first crocodile clip is attached exactly at $0\,cm$ to prevent a zero error (systematic error from not starting exactly at zero).
2.	Record the readings from the voltmeter and ammeter.
3.	Move the second crocodile clip $10\,cm$ further away and take new readings of the voltmeter and ammeter. Note: The power supply should be turned off between readings and whilst moving the crocodile clip.
4.	Repeat step 3 for more measurements until the second crocodile clip reaches the end of the ruler.

Analysis

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

1.	Calculate the value of resistance, $R$ , for each length of wire using a rearranged form of the Ohm's law equation: $V = I \times R \to R = \dfrac{V}{I}$
2.	Plot a graph of resistance, $R$ ( $y$ -axis) against the length of wire, $L$ ( $x$ -axis). Draw a line of best fit.

Conclusion

Physics; Physics practicals; KS4 Year 10; Investigating the resistance of a length of wire

The graph should look like this and have a positive correlation with the line of best fit going through the origin. The results of this graph conclude that the longer the length of wire, the higher the resistance across it. Therefore, resistance is directly proportional to the length of the wire.

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.