resistance of a wire

Authors Avatar

Physics – “Resistance in a 18 SWG Constantan Wire”

Aim

The aim of this coursework is to investigate how the length of a constantan (18SWG) wire affects the resistance in a circuit.  

Preliminary Testing

Before I can conduct the experiment I need to investigate further into what I am going to do for the investigation.  By doing the preliminary testing I can find the easiest and the best method of doing the experiment.  I can also make any alterations to the investigation.  

The first problem I had was the material that I was going to use as a conductor.  I decided to choose constantan wire because it is cheap and it is easily found.  Another reason for choosing this was because the resistivity was not that small or large to work with.  The table below shows the possible materials we could have used.

The next decision I had to make was the type of constantan wire we could have used.  I chose to use 18 SWG because its diameter was not to small and not to large.  I had other alternatives as well the table below shows this.

 

Now that I have decided on my apparatus I now need to work on the factors.  There are many factors that could be taken in to account.  I need to pick the right one because it is vital that I pick the correct one.

The spider gram on the next page shows all the possible factors that I could use in the investigation.

       

For the power supply in the circuit I chose to use a 24 Volt Power Pack set to A.C.  24 volts is a large voltage and I can see the changes in the readings because they are not too small.  The voltage wasn’t high enough to burn the circuit and so it was safe.  I decided to keep the temperature the same so we kept it at room temperature (around 27 degrees).   I chose to investigate using the length of the wire because it was easy and the results are effective.  I can also change the length of the wire.  The other alternative that I had was the thickness of the wire.  This option was too hard as it was difficult to measure the thickness of the wire.

Plan

In this investigation I plan to investigate how the length of an 18SWG constantan wire affects the resistivity of the circuit.  The constantan wire is the conductor in the circuit.  The constantan wire will be placed in parallel to the voltmeter.  To make a complete circuit I am using crocodile clips and wires.  

The electronic diagram for the circuit is shown below:

Join now!

                       A 24-Volt Power Pack

        An Ammeter

          18 SWG Constantan Wire

        A Voltmeter

By changing the length of the 18 SWG constantan wire we can see the differences that happen.  The ammeter is used in the circuit to see how much current there is in the circuit.  The voltmeter in the circuit will be used to see how much voltage there is in the circuit.  The voltmeter is placed parallel in the circuit because otherwise the readings will be wrong.  After the experiment has ...

This is a preview of the whole essay

Here's what a star student thought of this essay

The author mentions that they did the experiment in a group of four boys, so a point worth noting is that generally it is better to write your experiment up as though you have completed it individually, even if you completed it in groups. Additionally, grammar errors such as using “to” instead of “too” make the work appear lower quality and give a bad impression. I assume that graphs were included in the final piece handed in, make sure these are accurate and include error bars if you can. The points you mark should be small (less than a little box on the graph paper). At my school we were given to understand that hand drawn graphs got better marks than those drawn by computer. One final point: if you don’t KNOW it, don’t put it in, or better still keep looking until you are SURE any science you put in is correct. If it’s beyond the syllabus you’ll gain marks for putting it in, but wrong science loses you marks.

The author’s explanation of the science behind resistance was good, but could have been more detailed. For example, the author does not explain that metals are made up of positive metal ions (which are large) and free electrons (which are small). When explaining resistance, they talk about electrons hitting other electrons, a useful point. However, they also talk about electrons hitting “the wire itself” which is rather vague, so the examiner cannot give you marks for this. A better explanation would have been “the electrons hit the large positive ions” and explaining how this deflects them, so they don’t move as much, so the current is smaller. A really positive aspect of their work is how they have checked their experimental results with those that the equation for resistivity predicts. However, the formula they provide for this appears to be wrong (although this may be the formatting when it was uploaded. The formula they provide is R = ρ^(L/A), while the actual equation is R= (ρL)/A. The author should also explain the meaning of ρ; it is a value of resistivity that is specific to that metal. (While resistance is a property of the object and changes with the length and cross-sectional area, resistivity is a property of the material and accounts for length and area.) Talking about resistivity shows the author has read beyond the syllabus.

The author does answer the question, and includes many valid points in their answer, but needs to ensure they write down equations accurately (see R= ρL/A) and explain scientific concepts clearly (see explanation of resistance). The writer should explain why the resistivity of constantan is neither too small nor too large; is it easily measurable with the equipment available? They should also explain why they chose the diameter they did. This does not need to be a complicated explanation; that it made the wire easy to see, or stopped it from snapping etc would all be suitable explanations. The author might have found that one of their problems, the wire being bent, would have been easier to solve had they used a thinner wire. When describing possible causes of errors such as “the wire being bent” the author should explain why this might cause an error, as this shows you understand the science. In this case, a suitable explanation would be “Being unable to straighten the wire may be a cause of error in my results as the length of wire may really be longer than that I measured, so the resistance will be higher than that predicted.” A possible improvement would be to cut the wire longer than required and marking the lengths on with permanent marker. This would ensure an accurate measurement of length. When cutting the wire, keeping it taut between two clamps would ensure the author cut the correct length. When talking about improvement always mention “doing more repeats”, as this is an easy way to tick boxes on the mark scheme, especially when you explain that this improves the reliability of the experiment by reducing the effect of outliers and experiment errors and showing repeatability.