Prediction
If I double the current the strength of the electromagnet will double, therefore it will exert double the force. If I halve the current the strength of the electromagnet will halve so it will exert half the force.
The iron core has a maximum strength of the electromagnetic field it can exert and so the force exerted will stop increasing after a certain current is reached and will remain at this maximum value even if the current is increased further. This is because the magnetic domains of the metal atoms can exert magnetic fields only up to a maximum strength when all of them are aligned with the field, irrespective of whether the current is increased further.
Preliminary
I did a number of preliminary experiments, to give me a rough idea of what the actual experiment would be like, and to find a suitable range of readings. The first experiment I did, I had 56 turns of coil on my rod, and the power pack setting was on 2V. I tried to do the experiment but I found that the current going through the solenoid was too low, because when I altered the variable resistor to actually get a reading on the Newton meter I had almost pushed the sliding contact to the fullest extent. I tried it again, this time with 121 coils on the rod. This time I got a reading but the current was still too low, and the meter was not picking up anything. I then altered the power pack setting to 4V, and used the same amount of turns. I got a reading on the meter very early. So, I decided to use these settings in the experiment. Also I found that when I clamped the electromagnet. I had to use a wooden clamp, because if I used a metal clamp then the clamp would get magnetised, thus affecting the results. I used sellotape to keep the wire in place, so that it will not fall off.
In the main experiment I will start from 0A to 5.5A with intervals of 0.5A in between, so in all I will have 12 readings, and then I will repeat the experiment two more times, to ensure accuracy.
The preliminary experiments helped me a lot because I now know what settings I need to use and how many readings I can use.
Plan
Materials
- Constantan wire (insulated) for solenoid
- Iron Rod
Apparatus
- Power pack
- Connecting leads
- Variable resistor
- Ammeter
- Wooden clamp & stand
- 2 Newton Meters (1 going up to 1N, and the other to 10N)
- Crocodile clips
Circuit diagram / set-up of experiment:
Method
- Set up circuit as in diagram.
- Check that the power setting on the power pack says 4V and on DC (Direct current).
- To make the electromagnet, coil the wire around the magnet as tightly as you can so that all the wire touches the core. Leave space at each end of the wire.
- With sellotape secure the wire in place at each end of the core, but at the end tip of the magnet where you will be placing the Newton meter, leave that free of sellotape.
- Clamp the electromagnet using the wooden clamp stand.
- Using the variable resistor, measure 0.50A.
- Using the 1N Newton meter, place on top of electromagnet, which is sellotape free, and pull gently.
- Record result.
- Repeat steps 6→8, using the different currents.
- Once done, repeat whole experiment twice so that you have done it three times.
How to keep the experiment a fair test
- While I alter my independent variable I will have to keep all the other variables constant, e.g. the number of coils: - if you change the number of coils while doing the experiment it can affect your results by either increasing or decreasing the force exerted.
- I will have to use the same equipment all the way through, including the electromagnet because any piece of equipment may have a flaw in it, and if changed, can affect the results.
Obtaining Evidence
I have done my results to 3 S.F. (Significant figures) to keep my table consistent.
Analysing Evidence
My results agree quite well with my quantitative prediction. As the current was increased the electromagnet exerted a larger force and the force nearly doubled when the current was doubled. As seen from the graph, the force approximately doubled, i.e. increased from 0.6N – 1.3N when the current was doubled from 1.0A to 2.0A and was increased to 1.3A – 2.7A when the current was doubled from 2.0A to 4.0A.
The conclusion is that the strength of an electromagnet will rise in a directly proportional manner with rise in current.
The explanation of my conclusion is that as you increase the current, more electrons flow around the electromagnet in a given time. The iron atoms in the coil aligned in a more potent manner – they are induced to align themselves by the current – the electromagnet becomes stronger and the electromagnetic field becomes stronger and may encompass a greater area of its surroundings. As explained in the introduction, the reasons for the electromagnet producing the results I obtained, on which my conclusion is based, are the following:
When the power was on, the current flowed through the circuit and through the coil that was wound around the core. This magnetised the core by aligning the iron atoms into magnetic domains and hence a magnetic field was created – exerted by the magnetised iron core. When the current was increased, the strength of the magnet increased – the strength of the electromagnet increased, and the strength of the magnetic field increased. Therefore the Newton meter experienced greater force acting on it. So, I plotted a graph of force exerted by the electromagnet against Current.
The possible reasons for the results not showing a maximum value of the force, i.e. a levelling off of the force, are:
The size of the electromagnet used in the experiment may have been greater than the size of the electromagnet that I assumed in my prediction and there might have been more turns on the coil of the electromagnet in the experiment, than on the coil of the electromagnet that I based my prediction on. The highest value of the current used in my experiment may not have been sufficient to reach a point at which the force would level off.
The theory behind my results however is in good agreement with the theory of my planning, as was quantitatively anticipated. The most important factors of the experiment were identified in my planning. My results reflect these factors: the strength of the electromagnet, the force exerted, the maintenance of a fair test and accuracy, e.g. the maintenance of steady, well chosen currents which enabled a clear graph of results to be made.
Evaluating Evidence
The plan worked very well – a clear pattern can be seen from the graph and the line of best fit is well justified by the results obtained. However, the theory of the experiment should have made for even more uniform results – the plot of points on the graph should have been more of a straight line. The reason they were not because of the finer points on how the experiment should have been carried out. I found out, even though the sellotape was securing the wire to the core, the coil was gradually slipping of the core. This could have affected my results in a way.
Also, I did not know whether the Newton meters that I had used were calibrated. Calibration would have involved comparing the piece of equipment, which I had used, to a piece of equipment, which was known to be accurate. Then do the experiment twice, first time with the piece of equipment, which I used, draw a graph of those results, then, with the calibrated piece of equipment, do the experiment with that, and draw a graph. Then, compare both graphs, and now you can find the difference between the equipment. A better design for the experiment would have been to connect a circuit where I would have been safe to pass higher currents through the electromagnet. This could have probably been done best by using higher current carrying capacity connecting wires.
The experiment could be extended by doing it with a larger spread of currents and up to a higher level of currents. This would have tested the prediction even further and may have proved or disproved it conclusively.
The experiment was a fair test because all the variables were kept constant except for the current (the test variable). The experiment was accurate in my opinion.
There were no anomalous results. The evidence is definitely good enough to support my conclusion as they are clear and simple, but I have also explored the detail of my conclusion, and any contrast between my prediction and results.
The prediction stated that the iron core held a certain limit for the amount of magnetic field it could exert, irrespective of whether I increased the current further or not. This would explain the predicted levelling off, which was expected by me to appear on the graph.
