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Motor Principle and Magnet on Balance

  1. Dec 29, 2014 #1
    Hi everybody. This is my first thread ever on this forum :S

    I'm in a first year physics class, and the other day we did a lab involving a c.c.c (current carrying conductor) and a magnet on a balance. The c.c.c was a wire which completed a circuit. One had a magnet assembly which was placed on an electric balance, with a gap between the North and the South poles. Before beginning, one measured the weight of the magnet. After this, the wire was positioned between the two poles, and the power source was turned on. As the amount of current was increased, the weight of the magnet on the balance decreased.

    This experiment combined the motor principle with Newton's Laws. Because the c.c.c. was within the magnetic field of the magnet, it experienced a force. The equation for this is F = IL*B. Using Newton's Third Law, one could then determine that the magnetic field exerted a force on the magnet, and vice versa, which caused the magnet to be lifted a bit, causing this loss of weight.

    We were given the equation of W = mg +/- ILBsin(90) to represent this.

    I understand all of this, however, I don't understand how the movement of the c.c.c. causes a force on the magnet. Obviously, the magnet is being pulled upwards in order to have less weight, but I don't understand where this pull comes from?

    Thank you for any help!
     
  2. jcsd
  3. Dec 29, 2014 #2

    BvU

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    Hello Bag, and welcome to PF :)

    The ccc experienced a force: it was attracted towards the magnet. Could it be that the ccc couldn't move towards the magnet, so that instead, the magnet wanted to move towards the ccc ?

    If you would have used the template (instead of -- accidentally?-- erasing it), you would have told us what L stands for. W is probably a force ?
     
  4. Dec 29, 2014 #3
    Thank you! That answer makes complete sense to the situation: the wire was attached to a fixed supporting arm.

    I'm sorry about omitting the template. I've copied and pasted it here, so hopefully it will help. Although I think the answer you already gave helped me a lot:


    1. The problem statement, all variables and given/known data

    The purpose of the lab is to discover the value of the magnetic field of the wire and the value of the force acting on the magnet. (However my question focuses more on how the interactions of the two magnetic fields affects the magnet)

    W = the equilibrium weight (The weight that the scale reads: So the resultant force on the scale)
    F= the force that the wire experiences
    L = the length of the wire
    I = the current
    B = the value of the magnetic field of the wire
    m = the mass of the magnet
    g = the Earth's gravitational field strength

    - As the current increases in strength, the equilibrium weight of the magnet decreases.

    Several values were:

    Wire Length: 0.042 m
    Current: 0.500 A
    Weight: 1.617 N

    Length: "
    Current: 1.009 A
    Weight: 1.616 N

    Length: "
    Current: 1.500 A
    Weight: 1.614 N

    2. Relevant equations

    F= IL×B (both F, L and B have vectors)

    The magnitudes only can be represented by:

    F= ILBsin(90°)

    -This describes the force resulting from the two magnetic fields interacting (motor principle)

    W = mg ± ILBsin(90°)

    -This describes the resultant force (equilibrium weight, or weight shown by the scale) which is created by adding the force on the wire to the weight of the magnet (if the wire pushes down) or by subtracting this force (if the wire pulls up)
    -The angle is 90° because the wire is perpendicular to the magnet

    3. The attempt at a solution

    I haven't begun trying to calculate the value of the force and the magnetic field yet because I'm just starting my report. I'm asking this question about why the magnet moves up, and therefore loses weight, so that I have a full understanding of what happened before I try to explain it on my pre-lab. I understand that the c.c.c. was supposed to experience a force due to the motor principle, and this force, using Newton's Third Law, was supposed to result in a force that pulls on the magnet and ∴ decreases the weight of the magnet. But where I was stuck was in why and how the movement of the c.c.c caused the magnet to be pulled upwards.

    I hope this helped!
     
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