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Investigate how length affects the resistance in a piece of constantan wire.

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Investigation into the factors affecting resistance

Aim: I am going to investigate how length affects the resistance in a piece of constantan wire.

First, we need to know what resistance is.

“An electric current flows when charged particles called electrons move through a conductor. The moving electrons can collide with the atoms of the conductor. This makes it more difficult for the current to flow, and causes resistance.” BBC Bitesize

There are 4 factors that affect resistance: Length of wire, the cross-sectional area of the wire, the material that the wire is made out of, and the temperature of the wire.

Cross-sectional area

The cross-sectional area affects the resistance, as it changes how tightly the electrons are packed, therefore changing the amount of collisions, which produces resistance.

        The diagram shows this. In the first diagram, the cross-sectional area is higher, so the electrons are less densely packed, resulting in less collisions, and a lower resistance. In the second diagram, the cross-sectional area is lower, so the electrons are more densely packed, resulting in more collisions, and a higher resistance.



Substance of wire

If you change the substance of the wire, you change the arrangement of the atoms. This may make it easier for the electrons to pass through, or harder, resulting in more collisions.


...read more.





During the experiment, we kept the wire submerged in a water bath, to prevent melting. However, we had difficulty fitting the larger lengths of wire into the bath. Because of this, during the experiment we will wrap the wire around a pencil, forming a coil.

        Our results are slightly anomalous. This could be because the water was also acting as a short circuit, allowing electricity to bypass most of the wire. Due to this, we will use de-ionised water in the bath, which does not conduct electricity.

        We also had slight problems with the wire overheating, even with the water bath. Because of this, we will reduce the voltage to 6 volts.

Main Experiment

Circuit Diagram


Equipment required

Power pack set to 6 volts

5 insulated copper wires

2 crocodile clips

An Ammeter

A Voltmeter

5cm, 10cm, 15cm, 20cm, 25cm, 30cm, 35cm and 40cm of constantan wire (non insulated)

A pencil

2000cm³ water bath, containing 1500cm³ of de-ionised water.


  1. Complete the circuit as shown in the diagram, using 5cm of constantan wire as the test wire.
  2. Wrap the 5cm of wire around a pencil, forming a coil.
  3. Place the coil into the de-ionised water bath
  4. Check that the power pack is set to 6 volts, and turn on.
  5. Record the voltage and current, taking the readings off the voltmeter and ammeter respectively. Write this down in a table.
  6. Turn off the power pack, and remove the constantan wire. Replace it with the next wire, and repeat the above steps.
  7. When finished, repeat the entire method twice
  8. When you have done all the tests, disconnect the equipment, making sure iot is dry, and pack away.

Controlling variables

There are several variables that need to be kept constant, for this to be a fair test.


Why it needs to be controlled

What effect it will have if not controlled

Substance of wire

If you change the metal that the wire is composed of, the atomic structure of the metal will change, resulting in a different resistivity, and an anomalous result

The resistance will change due to the resistivity of the metal, not due to the length

Cross-sectional area of wire

The cross-sectional area of the wire affects how much room the electrons are given to avoid the atoms, and pass through, therefore reducing or increasing the chance of collisions

If the cross-sectional area increases, resistance will decrease. If the area decreases, resistance will increase


Temperature gives the atoms more kinetic energy, resulting in more collisions, and more resistance

If the temperature increases, so will resistance. If the temperature decreases, so will the resistance.

Voltage of the power pack

The voltage of the power pack affects how much charge the electrons are carrying, therefore affecting the resistance

If the voltage increases, the resistance will increase

Current produced by the power pack

The current affects the amount of electrons, therefore affecting resistance

If the current increases, the resistance will decrease.

...read more.


        However, we have no guarantee that the temperature stayed constant. The water bath reduced the difference in temperature, but there could have been a difference in the temperature, which may have affected the results. I doubt it made a large impact, as we got the results we expected that follow scientific logic.

        The power packs used are not the most accurate of power packs, meaning that the e.m.f produced differs from the amount it is set to. They fluctuate in their production of e.m.f, meaning that the results may have been inaccurate. Again, this will have made a negligible impact.

        Improvements to the method could be made. Instead of using the power packs we were using, we could use more up-to-date, sophisticated power packs, to avoid fluctuations in the current.

We could refridgerator the water beforehand, resulting in a cooler wire, and a smaller variation in temperature.

        The results we got were accurate. We know this because they follow a clear trend, highlighted by the line of best fit. We do not have any radical anomalous results, as they are all close to the line of best fit. The greatest point of inaccuracy is at 25cm, where the value is 0.2Ω away from the line of best fit, which is within our tolerance margin.

...read more.

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