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Homework Help: Electromagnet, force! help!

  1. May 17, 2010 #1
    1. The problem statement, all variables and given/known data

    A coil with an iron coil is used as an electromagnet. With 500 loops and a current of 1.5A, it can exert a lifting force of 30N. What force would it be able to lift, if the following changes were made?

    a) The current is increased to 3.0A.
    b) The number of loops is increased to 750.
    c) Both the above changes are made together.

    2. Relevant equations

    F = IBL

    number of turns directly proportional to B
    current is directly proportional to B

    3. The attempt at a solution

    -ok well the qs does not give the length of the coil, without that infor how is it possible to find force...please help
  2. jcsd
  3. May 17, 2010 #2
    You can find the new force by making a ratio between the new force and the old force.
  4. May 17, 2010 #3
    i don't understand...so you use the info to find B... but what about F
  5. May 17, 2010 #4
    I am concerned with the relevant equation. You do have an equation that includes I, but that does not mean that you are using the equation correctly.

    Do you know which object the force that is calculated by the Lorentz Force (the equation that you used) acts on?

    I would look at the equations that you have for magnetism and see if a different equation would better describe the situation.
    Last edited: May 17, 2010
  6. May 17, 2010 #5

    -ok, the textbook I have does not give any equation relating the lifting force a coil can exert to the current and loops. My textbook only gives the proportionality of loops to magnetic field strength and current to magnetic field strength. However, the qs asks what force would the coil be able to lift it the current is increased to 3.0A and the number of loops is increased to 750. I have no idea how to do this. Please, please help. A step-by-step solution would be greatly appreciated. Thanks in advance.
  7. May 17, 2010 #6
    Alright, first, the equation that you listed as a relevant equation is used to calculate the force on a current carrying wire by a given magnetic field. I do not think that that is what the question is looking for.

    So, you and I will work through a step-by-step solution together.

    The first step is to understand what magnetic field is. So, when you have calculated a magnetic field, what is it?
  8. May 17, 2010 #7
    -magnetic field is magnetic force a magnetic can produce. the strength of the magnetic field (B) is directly proportional to current and directly proportional to number of loops if current is constant.
  9. May 17, 2010 #8
    can someone please help me indentify the original field strength, B, in the info given. Is it 30N? Isn't B supposed to be measured in telsa (T)....Please someone respond...before the morning...:eek:
  10. May 17, 2010 #9
    This is partially true. A magnetic field is a potential. When something is put into a magnetic field, some effect, such as attraction or repulsion, will be experienced by the object.

    Also, magnetic field is directly proportional to current and directly proportional to number of loops for a solenoid. This is a specific case. It is not bad to memorize results like this, but it is a problem to think that these specific results apply generally.

    *steps off soapbox*

    This is actually kind of an interesting problem. So, a magnetic field can create a pressure (thank you wikipedia). This pressure (force per unit area) is proportional to the magnetic field strength. The relation is [tex]P_B=B^2/(2\mu_0)[/tex]. From this, you might be able to determine the initial magnetic field. However, knowing that [tex]P_B[/tex] is proportional to a fixed quantity in the problem, you should then be able to solve the initial problem.
  11. May 17, 2010 #10
    this is what i did:

    a) 3.0A/1.5A
    = 2

    B, magnetic field strength, will be doubled.

    b) 750/500

    B, magnetic field strength, will be increased by a factor of 1.5.

    c) 2*1.5
    = 3

    B, magnetic field strength, will be tripled.

    -is that correct...
  12. May 17, 2010 #11
    ...the course i am in, is just university gr. 11 physics...am i supposed to use pressure in my calculations...what is the pressure...no value is given in the question
  13. May 18, 2010 #12
    Pressure, force per unit area, is most likely necessary to solve the problem. You know that you can assume that the area of the object does not change. Thus, the force that you are given is directly proportional to the magnetic field, which is directly proportional to the current and number of turns of a solenoid.

    Now, you know the magnetic field from a solenoid is proportional to the current and number of turns. Thus, you should be able to figure out the new force based on the old force.

    If you are having trouble seeing how to solve this problem, write down the equations you know, in this case the equation above for pressure from a magnetic field, pressure in terms of force, and magnetic field in terms of the current and number of turns for the solenoid. Then, plug one known equation into another equation. You should be able to reduce the equations so that you are equating one unknown to a set of knowns or constants.

    If you really get stuck, post what you have figured out so far. Please include equations.
  14. May 18, 2010 #13
    That is all correct, now you just need to relate that to force.
  15. May 18, 2010 #14
    ok so, force is directly proportional to magnetic field strength...so the answers for f are the same

    a) 2F

    b) 1.5F

    c) 3F

    so...but what is F
  16. May 18, 2010 #15
    Pressure is force per unit area. However, I incorrectly stated above that pressure is directly proportional to the magnetic field. It is related, but not by direct proportionality. You should be able to see the relation in the equation that I included.

    Now that you know the relation between magnetic field and pressure, as well as between pressure and force, you should be able to determine the force.
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