Length of Wire - Required Practical

The resistance of a wire depends on the material it is made from, it's temperature and the length of the wire. The longer the wire, the more resistance it has.

Resistance is the opposition to the flow of charge. It opposes the movement of electrons. As electrons try to move through the wire, they collide with metal ions. This is what resists or slows down the movement of the electrons. The longer the wire, the more metal ions that electrons collide with. When electrons collide with metal ions, they transfer thermal energy to the metal ions. This causes them to heat up and vibrate more, which also increases the resistance of the metal.

We can prove this in an experiment -> how varying the length of the wire changes the resistance. We cannot directly measure resistance, so we need to include an ammeter and a voltmeter in our experiment.

Independent variable - length of wire, measured with a ruler.

Dependent variable - current in the circuit, measured with an ammeter.

Control variable - potential difference of the supply, measured with a voltmeter.

Method

• Set up the circuit as below. The wire should be as straight as possible, with no kinks which will affect our measurement of length.

• Use crocodile clips to change the length of the wire in the circuit.

• For each length, measure the current with the ammeter and potential difference with the voltmeter.

• Calculate resistance using:

• Repeat for several different lengths.

• Plot a graph of resistance vs length.

-> Note: When current flows through wires, they heat up. You can sometimes feel this when your phone has been on the charger for a long time. We don't want this to happen because the independent variable in this experiment is the length of the wire. We need everything else to be controlled. To prevent the wires from overheating, we should include a switch in our circuit to turn it off between readings, and we should use a low pd.

Wires heat up when current flows because electrons moving around the circuit collide with metal ions in the wire, transferring thermal energy to them. This will cause the metal ions to heat up, vibrate more and cause even more collisions, thus increasing the resistance. This would affect our results, which is why we need to minimise this.

The results you record will look a little like the ones below:

As you can see, we need to calculate the resistance of the wire. To do this, we can use the equation:

The resistances calculated would be:

We could then plot these on a graph of resistance of the wire/ (y-axis) against length of wire/ (x-axis).

The graph would look like this:

We can see that as the length of the wire increases, the resistance of the wire increases. They are directly proportional, because the line of best fit is straight and goes through the origin.

Rheostats are a specific type of variable resistor that use this relationship between resistance and length of wire. They consist of a long coil of wire with a slider in between the terminals - you can change the length of the wire, which changes the resistance of the wire. The longer the wire, the greater the resistance.