The aim of this experiment is to investigate how a change in the length of wire made from nichrome will affect the potential difference (voltage) and the current across that length, hence affecting the resistance.

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PHYSICS INVESTIGATION

INVESTIGATING THE RESISTANCE OF A WIRE

AIM:

The aim of this experiment is to investigate how a change in the length of wire made from nichrome will affect the potential difference (voltage) and the current across that length, hence affecting the resistance.

Introduction:

Resistance is the opposing force to the current in a circuit. The unit of  resistance is the ohm (Ω). The electrical resistance of a conductor is defined by:

  R (Ohms) =

Where I is the current flowing through the conductor when the Potential Difference across it is V.

The ohm is defined as being:

“ The resitance of a conductor through which a current of one ampere is flowing when the PD across it is one volt, i.e. 1Ω = 1VA-1

Some conductors have resistances which depend on the current flowing through them, but the majority of conductors, notably metals, depends entirely on their physical condition. There are four factors which affect resistance in a metal wire:

  • Type of Material - The resistivity of various types of materials are different (see resistivity table further on). For instance, gold is a better conductor of electricity than copper, and therefore has less resistance.
  • Length – The longer a wire the more resitance it has as there is more matter for the electrons to collide with, so as to be able to pass.
  • Cross Sectional Area - The resistance of a material is inversely proportional to the cross sectional area. This means that the thicker the diameter of the wire, the lower the resistance. This is because the larger the cross sectional area is, the less friction there is over a given length.
  • Temperature - In various types of materials, resistance can vary inversely or directly with the temperature. This is because of the chemical properties of the material. In Carbon, for instance, the resistance decreases as the temperature rises. So we say it varies inversely. In copper, however, the opposite is true, with the rise in temperature, we have a rise in the resistance.

Such conductors are called ohmic conductors and are said to obey Ohm’s law. By rearranging the resistance equation and when R is constant we obtain the following expression:

                 = a constant                 

Therefore Ohm’s law may be stated as:

“The current through an ohmic conductor is directly proportional to the potential difference across it, provided there is no change in the physical conditions (e.g. temperature) of the conductor”

The current-voltage reationship of various non-ohmic conductors, together with that of an ohmic conductor is shown in the resistance equation.

The experimental determination of the resistivity of a material involves measuring the resistance of a specimen of the material. The specimen must be regularly shaped in order that its dimensions L and A can be measured and used in the resistivity equation. If the speciemn is in the form of a wire, its diameter should be measured at about six different points.

Preliminary Work:

Aim:

To investigate the ideal input voltage for the experiment, and to learn more about the apparatus provided.

        

Hypothesis:

I believe that no matter the input voltage the resistance will remain constant. I also expect an input voltage of 1.5V to be an ideal voltage.

        

Prediction:

A higher input voltage will mean there is a greater potential difference between the ends of the wire. Therefore that will mean there will be a greater current, as their relationship is directly proportional. As both of these variables are increased in fixed amounts the resistance will remain constant.

Apparatus:

The apparatus I shall use in this experiment are:

  • 1.03m of Nichrome wire – I shall use nichrome wire instead of the other option, which is copper, because its resistance is much larger. Therefore a small length of constantan will have the same resistance compared to a wire made of copper which is much longer. Therefore this option is much more convenient to use as well as enabling me to collect a wide range of readings.
  • A 1m ruler mounted onto a wooden board with 2 nails on either side.
  • A voltmeter – accurate to 0.2V
  • Micrometer – accurate to 0.01mm
  • An ammeter – accurate to 0.05A
  • 6 wires
  • 3x 1.5V batteries – for 3readings (1.5, 3.0 and 4.5V)
  • Circuit board

Method:        

I first placed the batteries in their slots in the circuit board, and connected one wire from the positive terminal to the ammeter. I then wrapped the nichrome wire around the two nails in the mounted ruler as tightly as possible. Next, I attached another wire from the positive terminal in the ammeter to the first nail on the mounted ruler. I then attached another wire from the same nail to the voltmeter, and connected the voltmeter to the second nail. After that I connected a fifth wire from the variable resistor to the nichrome wire at the 100cm mark:

 

Lastly I connected the sixth wire from the variable resistor to the negative terminal of the battery, ‘tuned’ the variable resistor to the required voltage, and jotted down the reading on the ammeter in a table.

Results:          

The following table lists the results obtained:

Conclusion:

Graph 1 shows that the material constantan, is in fact an ohmic conductor and so is suitable for the investigation. Graph 2 on the other hand proves that the resistance is constant, no matter the current, and that supports my earlier prediction. However from these results I cannot deduce which is the best input voltage as all have extremely accurate results. I will however determine the ideal voltage through the practicality of the values. In the experiment, I believe that an input voltage of 2V is the best, due to a number of reasons:

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  • Firstly, it wasn’t too high so as to produce a lot of heat and so increase the resistance, as well as needing a long length of wire, which is inappropriate to use.
  • With 2V, the resistance of a small length of constantan could be measured, as it would remain in the scales in the ammeter, and so would enable me to obtain a wide range of results.
  • Lastly, as I had to balance all the factors affecting resistance, i.e. the cross-sectional area, the shortest length possible to be measurable, as well as the scales of the apparatus available. ...

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