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Electromagnets coursework

  1. Jun 16, 2006 #1
    Hey there,
    I am studying the effect of current on the strength of an electromagnet for my GCSE assessed practial.
    I set up the experiment with an iron c-core on a set of scales, and a electromagnet above, in theory, I should be able to see the weight of the c-core go down as it gets lifted off the scales. I varied the current using a variable resistor.
    My results are very slight at first, then they speed up (the graph is not very steep, then gets steeper).Could this be because at first the c-core is nearer the outside of the magnetic field induced by the electromagnet, so therefore the magnetic force pulling upon it is weaker, and as it gets closer the magnetic field gets stronger, and so pulls the c-core up more?
    Also, My teacher tells me that I am measuring the mass, and I need to link this to the weight. I have never really got my head round the whole mass/weight thing. How do I do this?
    I am very unclear about the two of the above,
    Any help very much appreciated.

    I wrote what I could in my 1st draft (I would put it on here if I knew how), and then gave it to my teacher, and these are the main points that he told me work on.
    Last edited: Jun 16, 2006
  2. jcsd
  3. Jun 16, 2006 #2


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    Staff: Mentor

    The magnetic force will definitely vary a lot with the separation, so you will need to control and maybe measure that. As for mass/weight, just remember that F=ma, where m is the mass. Since a scale measures force, IMO you are measuring the weight, not the mass. Gravity gives mass a weight (force). I wonder if your teacher misspoke?
  4. Jun 16, 2006 #3
    Its probably me that got it the wrong way round, thats a good help, so thank-you. How can I link weight to the mass, so that I can write about in my report? Would it be OK to state the conversion between grams and newtons, or should I explain some more, in which case, I will need help.
  5. Jun 16, 2006 #4


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    Staff: Mentor

    Yeah, the F=ma with units would be the simplest way to deal with it. Staying in mks units is generally the easiest for basic physics stuff. Advanced physics uses cgs units. If you want to get good and confused, you can try to explain it in the imperial (or whatever it's called) system of pounds and slugs and stuff, but that's probably not worth the reading time it would take to figure it out. mks is the way to go, IMO.
  6. Jun 16, 2006 #5


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    Staff: Mentor

    BTW, it just occurred to me that a good way to control the separation of the magnet and target is to put a rigid, non-ferrous material block between then and clamp the whole thing together. The non-ferrous block could be as simple as a piece of wood or plexiglass, for example.
  7. Jun 16, 2006 #6


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    Staff: Mentor

    Another BTW, I didn't notice your mixed units on my first read. Stick with consistent units in your work. For F=ma, it's Newtons [N], kilograms [kg], and meters per second squared [m/s^2].
  8. Jun 17, 2006 #7
    Thanks for all of the answers they are helping a lot.
    Also, could someone tell me if I have finally got my head round the mass and weight thing. Weight is the force exerted upon something by gravity. And mass is the amount of matter in an object. These are directly proportional. Is this right?
  9. Jun 18, 2006 #8


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    Gold Member

    Yes, mass and weight are related by Netwon's second law, F = ma, here the accleration is the 'accelertion due to gravity', which is usually taken to by 9.81 m.s-2 and is represented by the symbol 'g', thus; W = mg. This is of course at a constant distance from the earth, the force of gravity is inversly proportional to the square of the distance between to massive bodies. However, in your investigation you are keeping a sufficiently constant distance between the mass and the centre of the earth, so in this investigation it is sufficient to say that weight is directly proportional to mass.
  10. Jun 18, 2006 #9
    berkeman is right - you are measuring weight, but probably your top-pan balance READS in grams (i.e. mass).

    To "convert" grams to Newtons, first convert the grams to kg by dividing by 1000, then times by 'g' (if you are a GCSE student g can be taken as 10 rather than the more exact 9.81).

    For example:

    200g = 200/1000 = 0.2kg
    0.2 x 10 = 2N

    You might like to think about the forces on the C-core....what is pulling up, what is pulling down.

    What do you aim to plot on your graph?


  11. Jun 18, 2006 #10
    On my graph I am thinking about plotting the difference between the mass of the c-core a when there is no current flowing through the electromagnet, and the mass of the c-core when there is a certain current flowing through the electromagnet (for example, the mass of the c-core when there was no current flowing through the electromagnet is 213.9, but when there is 1A flowing through the electromagnet, it is 212.2, so the difference is 0.9), by doing this, I should hopefully get a positive correlation on the graph, which should go up as caurve, getting steeper, and this should prove that the strength of the electromagnet goes up as you increase the current, as it is picking up c-core by more each time I increase the current. Is this OK?
  12. Jun 18, 2006 #11

    The mass of the C-core does not change! The mass is the amount of "stuff" there. What changes is its apparent mass according to the weighing scale.

  13. Jun 18, 2006 #12
    Ok, so I measure the difference between the Apparent mass according to the weighing scales when there is no current through the electromagnet, to when there is a certain current. Is this ok?
  14. Jun 18, 2006 #13
    yes, I think so.

    I have PM ed you...
  15. Jun 19, 2006 #14
    Thanks everyone, you have all been a great help.
    Iceman123 :)
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