I expect the results from the test on diameter to show an increase in diameter and so cross sectional area to bring a decrease in resistance. This is because with a wider “passage” for the electrons to pass through they can move more freely and there is les chance of there being a bottleneck. A larger scale version of this would be lots of people trying to all rush through some narrow doors. I think the resistance will be proportional to π r2 and this is what I will plot the results against
Results:
Conclusion:
The results for the test of length matched my prediction. The graph shows a strong positive correlation with the resistance roughly doubling when the length of wire is halved
The test on diameter gave some patchy results. Although the overall trend matched my prediction, that the resistance decreases with increased cross sectional area, or to be more precise the resistance halves when the area doubles, as the graph shows.
With these two sets of results we can find the resistivity of the material in ohm mm. If you times the resistance of the wire by the cross sectional area you get the resistance of the same wire if the area = 1. Then divide this by the length of the wire to get the ohm/mm resistivity.
P=RA/l
Where
ρ= resistance of the material
R= resistance of wire
A= cross section area of wire
l=length
I have then used this formula to test the accuracy of my test results as shown in the table. If the results are accurate then all the calculations done on the copper wires should be the same because the material is the same. As is shown, the majority of the copper wires have a material resistivity of around 2x10-5.
There are a few outliers to this, the main one being the result for 16 swg copper. I think that because the wire is so thick it has less resistance than the leads that make up the rest of the circuit, and the current flowing through it doesn’t take up its “capacity”. This would make its resistance seem higher than it actually is, because R= V/A. So when this higher resistance is put into the formula for material resistance it makes it look less efficient than the other copper wires.
I have also applied the formula to the last test, and as the graph and table show the two other wires have much higher resistance due to having a higher material resistance.
Copper is a better conductor than these other metals because of its chemical makeup. It has more free electrons that can flow as a current in relation to ions which hinder the flow of electrons. The other alloy wires are designed to have a low temperature coefficient and high resistivity. This makes them ideal for use in resistors, for which they are most widely used
Evaluation:
For this first set of experiments I managed to obtain some reasonable results however I found they were not as accurate as I had hoped. part of this is due to using an ac to dc power supply, it was not of a very high quality and the rails weren’t very stable, making it hard to get the right voltage.
If I had more time I got take measures to further reduce any anomalies in the circuit. One of these is the contact point between the clips and the test wire. Even though I cleaned the surface of the wire it would maybe be better to solder the wire to the circuit instead
Due to my outlier result for the 16 swg wire I would also consider using a higher voltage for the thicker wires. Either this or try to use thicker leads in the rest of the circuit if possible, so that the point with most resistance is always the test wire.
My results for the test of diameter gave fitting results however more points would be useful on the graph to plot a better fit line, so I would defiantly consider using a wider variety of wire thicknesses in the future.
As part of a new experiment to move on to I could look at more factors that effect resistance, such as temperature or maybe magnetic fields, however with the possibility of there being very small discrepancies in the data, I think that it would not work well given the accuracy and quality of the current given out by the power supply.
Instead I have decided to apply the wires to perform a task similar to one that could be found in a real world situation, and to test variables involved in it.
Part 2 (extended investigation)
Aim:
Wires are a vital variable component of electromagnets. The relationship between magnetism and electricity is very important, it is how out electricity is produced and is used to help distribute it all over the world.
Electro magnets are the force behind all electrically driven machines, but they also have many other varied uses. One of the simplest of these is an electro magnet crane, as used on scrap yards for lifting tonnes of metal.
In this experiment I aim to look at the variables of an electromagnetic lifting device. I will focus on the number of wire coils, the type of core and items being lifted, and the current going through the wires.
Plan:
During preliminary tests I found that hand made electro magnets were only capable of lifting a small amount of weight on their own. However the attraction between the power iron “horse shoe” core and another un powered identical core (when places so the poles are together) is so much stronger it can lift many times more weight.
I will use the same basic setup for three experiments. This setup is shown in figure 2.
Equipment list:
DC power supply.
Leads
Crocodile clips
Ammeter
26swg insulated copper wire
2 horse shoe iron cores
Many 100g weights and hangers
Steel bar
Firstly I will make up the electro magnet by tightly and neatly wrapping the wire around its centre. Starting with 5 coils the magnet will be held in the clamp stand and the power supply switched on with the current set to 0.5 amps. I will then start buy placing the steel bar under the poles of the horseshoe core and proceed to hang the weights from underneath it. The total weight that can be hung will be recorded, then this is repeated but with the other iron core instead of the steel bar.
With the two ends of the core attached to the two ends of the magnet I will repeat hanging weights on it to find the maximum it can hold and record this. This process will be repeated at .5, 1, 1.5, 2, 2.5 and 3 amps.
Next the magnet will have the wire coiled round it 5 more times and I will repeat all the previous tests but whilst keeping the current at 2 amps. I will continue adding coils until I reach 25.
I am using the steel bar and iron core and hanging weights off them instead of letting the weights be attracted to the magnet directly because it doesn’t let other factors alter the main experiment, such as the material of the weights, their shape and surface smoothness etc.
Safety and fair testing:
Many of the safety measures from the first experiment apply to this one also however there are a few additions.
Firstly electro magnets can damage sensitive electronics, so I will make sure no watches, phones, laptops or other devices go near my experiment. Secondly because my experiment may potentially involve lifting not only a heavy iron core but many weights, I will put padding under the magnet to prevent weights falling around and causing damage.
Too keep this test as fair as possible I will be keeping all the experiment other than the variables the same. I will also use the same bit of coil wire constantly, just changing the number of times it is coiled for each test. I will weigh the total amount of weight including the iron core/steel bar that is lifted, not just the weights.
I will make sure the core/bar are always attached at the same position and that the weights are loaded onto them evenly so as not to put any excess strain on them.
To make sure the magnet has really reached its limit when the weights fall off I will attempt hanging them again twice to rule out the chance of a freak event causing the magnet to drop the load.
Prediction:
I predict to see the magnet lift more with the core rather than the bar because of its better permeability, and also because of its better contact with the magnet.
I also expect to see the weight the magnet can lift increase in proportion to current and number of coils going up.
Results:
Conclusion:
I was surprised by the results I saw, although they fitted the general trends I was expecting, they were far from uniformed and there was no close relationship between coils, current and weight.
Firstly looking at the iron core results, for 5 and 10 coils the graphs show a clear increase in weight held with an increase in current. And the graph curves upwards towards its end. I think this is evidence of the current being multiplied by the number of coils in any possible formula to describe the power of the magnet.
However on the graphs for 15 and 20 coils show the weight tailing off towards the maximum current as if heading to a level. This may be because the core is a ferromagnetic material. These and other metals or alloys like soft iron have the ability to change the magnetic alignment of their domains. However once all of its crystal structure has been aligned in the direction of the field the core becomes known as “saturated” and it can increase its field strength no more. The only way would be to use a bigger, most importantly, fatter core.
I believe I didn’t see this happening with the steel bar because the iron core acted as part of the electromagnet when it was attached, so saturation plays a part in both the magnet and the extra core, whereas the steel rod seemed to be governed more buy the power of the coil. Although it was hard to tell with the minimum weight increases I was able to use (100g) being much larger in proportion to the total weight the steel bar could hold compared to the iron core. This may have hidden and levelling out towards the end of the graphs.
The steel bar seems to show much more uniformed and steady weight increases through all the graphs compared to the iron core. This is also shown in the average weight lifted against number of coils graph. This graph also clearly shows the iron core weights tailing off towards a level at the end of the graph.
The average weight per coil against number of coils graph shows some interesting results, in fact contradicting what I though I saw in previous graphs. Basically showing the efficiency in the magnets ability to lift with added coils, it shows it steadily dropping for the steel bar, suggesting it will eventually tail off. The plot for the iron core in this graph is however inconclusive.
My prediction about the iron core lifting more than the steel bar was correct. One of the key components in the strength of a magnetic field is how “permeable” the material that the field lines travel through. My electro magnet is essentially a solenoid with an iron core through the middle of it. This means I can use the formula from gauss’ law:
Magnetic field = permeability x turn density x current
Turn density is basically the number of coils divided by the length of the solenoid (or core) in meters. The permeability of air is pretty much 0, and that of steel and soft iron being 50 and 200 respectively. When put into gauss formula it makes a big difference to the power of the field (measured in gauss)
For example for my 10 coil magnet at 2 amps the field for the steel core would be:
50x(10/0.11)x2=9090gauss
And the iron:
200x(10/0.11)x2=36363gauss
Magnetic field lines will always prefer to pass along a more permeable material and this is what causes the attraction between magnets and magnetic materials. It also means that even the smallest gap or air space in the path of the field will reduce its power. This is another reason for the iron core performing better, because the contact between it ant the electromagnet was far better, being a lot smoother and a perfect fit.
Evaluation:
It is clear from my results that although the basis of my predictions were proved correct, I would have a long way to go to find and prove a formula that gives a relationship between coils amps and weight.
One of my first problems was the range of results. In the end my testing range and scope proved too little to make any good interpretations from. Also the weights I used were too big to make more accurate results.
In the future I would use container holding sand suspended under the iron core or steel bar. This would act as my weight and because I can add just grains of sand at a time if really necessary, I know the total weight the magnet can hold will be accurate to the nearest gram when I weigh it.
I would improve the data range by changing the number of coils by one each time, this should allow me to maybe see more of a pattern evolving if I had enough time. The same could be said for the current, maybe increasing in smaller increments of 0.1 amps would aid in finding a pattern.
I think the decline in the efficiency of the magnet (at least before the core became saturated) could be due to the coil heating up, thus increasing resistance, and decreasing current. The could maybe be fixed by attaching heat sinks to the coil, or cooling it with water, although it is hard to control.
Another concern of mine is again to do with the power supply. Because it converts AC to DC the actually current isn’t true constant power like that which comes from a battery, but intact the power is going in the same direction all the time, but it has a wave pattern. This causes the magnet to vibrate very quickly which makes the whole setup very delicate, if the contact between the magnet and the bar or core is just a fraction out of place the vibrations will shake it off. It also accentuates imperfections in the contact surface, increasing the chances of the weights dropping off prematurely.
I would therefore consider using batteries to power the circuit, giving a try DC current, although I would then have to take into consideration the drop in power as the battery drains.
As is shown in my results, the one major failure in my test was with using the iron core with 25 coils. I found during testing that the amount of weight it could hold became unpractical to measure and also made it possibly dangerous should something break. I also found that with it being so awkward to attach that amount of weight to the core all I was getting was random anomalous results rather than something that had any logic two it. I think not only did I reach the maximum capabilities of my apparatus there, but I also may have been at the very limits of the magnets strength too.
As gauss’ formula shows, the power of the magnetic field is not so much governed by the number of coils as the density of them. This means there is more power available from many layers of coil rather than one big long one. This explains why electrical motors have lots of layers of copper wire. In future tests my magnet could be improved by using these methods.
Overall I feel I did the best I could in regard to the limitations of the equipment I had, although a true DC setup with flawless magnet cores and a larger number of tests would hopefully lead to a better conclusion should I repeat this experiment in the future.
