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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.

Extracts from this document...

Introduction

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:

image00.png

  R (Ohms) =image01.png

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:

image12.png

                 = a constant                 image24.pngimage19.pngimage18.png

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:

image21.pngimage20.png

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:

Voltage (volts)

Current (Amps)

Average reading

Resistance (ohms)

First

reading

Second reading

Third

reading

0.0

0.00

0.00

0.00

0.00

0.00

1.5

0.15

0.15

0.15

0.15

0.33

3.0

0.30

0.30

0.30

0.30

0.33

4.5

0.45

0.45

0.45

0.45

0.33

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:

  • 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.

Evaluation: ...read more.

Middle

Apparatus:

  • Micrometer – with this device, I shall measure the diameter of the wire. It is extremely accurate (0.01mm), and therefore I can take precise readings. The reason why I shall use this device is because the wire must have a constant cross-sectional area throughout, as a higher area will decrease the resistance, by providing electrons with more paths to take. Therefore to ascertain the experiment is fair, this will be controlled.
  • Mounted 1 meter ruler – accurate to 1mm therefore enabling precise measurements. It is the same instrument used in the preliminary work. I found it to be extremely useful, as I will not have to worry about holding the wire and the ruler. This therefore facilitates the process of obtaining results.
  • One voltmeter – this device is accurate to 0.2V, with a maximum reading of 5V. I will place it in parallel in the circuit to give me a reading of the potential difference between the two ends of the measured wire.
  • One ammeter – with this device I will be able to take a reading of the current (coulombs travelling per second) in the circuit. It is accurate to 0.05A and the maximum reading is 2A.
  • 1.03m of wire made from nichrome – The reason why I shall use constantan wire is because it is an ohmic conductor. I shall also use it, instead of for example copper, because its resistance is much larger. Therefore a small length of constantan will have the same resistance to a copper wire around 30 times longer. This option is therefore much more convenient to use as well as being enabling me to use a wide range of readings. The reason why I shall use 1.03m, although I only need 1m, is because the extra 30cm will be wrapped around the nails in the mounted ruler stick so as to be as tight and straight as possible. The range I will chose is 10 readings, which are 10, 20, 30, 40, 50, 60, 70, 80, 90, 100cm.
  • Circuit board – with this piece of apparatus I will be able to organise and hold the batteries in a fixed position throughout, therefore saving me the hassle of taping them together.
  • Six wires – I will use these wires to connect the equipment together. Some will be ‘plug-in’ wires, and others will be crocodile clips. I will use them according to the instrument I shall attach them to.
  • 2 x 1.5V batteries – this will be the voltage source. By having three (a maximum possible in the circuit board) I will be able to obtain a wide range of results, so therefore my final graph will be very accurate.

Safety:

In the experiment as part of my safety requirements I shall take the following precautions and procedures to enable the investigation to go through without harm to any of my classmates or I:

  • Before starting the experiment I shall clear my desk from any unnecessary materials
  • I will tuck my stool inside my table and remove bags away from the floor to avoid people or myself tripping when moving around the working area.
  • I will work away from a water supply to avoid contact with electricity and therefore ruling out electrocution.
  • I will check the wires for any tears in the insulation, because again I can be electrocuted
  • I will not touch the constantan wire with the current flowing through, nor any other metallic piece in the circuit.
  • I will take care with the crocodile clips to avoid them being clipped onto my fingers and so preventing myself from being cut.
...read more.

Conclusion

APPARATUS:

The apparatus I shall use for this experiment are:

  • 1.03m of Constantan wire 0.09mm thick
  • 1.03m of Constantan wire 0.18mm thick
  • 1.03m of Constantan wire 0.36mm thick
  • 1.03m of Constantan wire 0.54 mm thick
  • 1.03m of Constantan wire 0.72mm thick
  • 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
  • Variable resistor
  • 3x 1.5V batteries – for 5 readings (0.5, 1.0, 1.5, 2.0, and 2.5V)
  • Circuit board

METHOD:

I will first place the batteries in their compartment in the circuit board, and connect one wire from the positive terminal to the ammeter. I will then take measurements of the thickness of the 0.09mm thick constantan wire at six different points, to be sure of the average thickness. I will then wrap the constantan wire around the two nails as tightly as possible. Next, I will attach another wire from the positive terminal in the ammeter to the first nail on the mounted ruler, and a third wire from the same nail to the voltmeter. I will then connect the voltmeter to the other nail. After that I will connect another wire from the variable resistor to the constantan wire at the 50cm mark.

Lastly, to complete the circuit I will connect the last wire from the variable resistor to the negative terminal of the battery, ‘tune’ the variable resistor to 2V, and then jot down the reading on the ammeter in a table. I will then repeat this process for each wire thrice, for increased reliability.

...read more.

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