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Homework Help: Some physics work, show me what i am doing wrong.

  1. Sep 15, 2005 #1
    Hey guys, here is the pic for one problem. http://www.webassign.net/pse/p30-17.gif
    the question reads:In Figure P30.17, the current in the long, straight wire is I1 = 9.00 A and the wire lies in the plane of the rectangular loop, which carries the current I2 = 10.0 A. The dimensions are c = 0.100 m, a = 0.150 m, and l = 0.550 m. Find the magnitude and direction of the net force exerted on the loop by the magnetic field created by the wire.
    I know that the direction is to the left. As for the magnitude i tried to use (munot(I1)(I2)/(2pi)) * ln(1+a/c). when i plug in my numbers i get 1.0363e-4 which is not correct. What am i doing wrong.

    next, i have this problem:
    Figure P30.23 is a cross-sectional view of a coaxial cable. The center conductor is surrounded by a rubber layer, which is surrounded by an outer conductor, which is surrounded by another rubber layer. In a particular application, the current in the inner conductor is I1 = 1.06 A out of the monitor, and the current in the outer conductor is I2 = 3.14 A into the monitor. Determine the magnitude and direction of the magnetic field at point a.
    Determine the magnitude and direction of the magnetic field at point b.

    I tried to use the equation (munot*I)/(2pi*r) but i get 1.06 which is not correct.
    For magnetic field at b, i tried to use((munot*I)/(2piR^2))*r which gets me to 6.98 but is not correct.

    Then for this 3rd problem it reads:
    A 1.00 A current is charging a capacitor that has square plates, 2.00 cm on a side.
    (a) If the plate separation is 6.00 mm, find the time rate of change of electric flux between the plates.

    I don't know what to do for this one, so any equation or hint someone can give me would be great, thanks.

    For this 4th problem:The magnetic field 41.0 cm away from a long, straight wire carrying current 9.00 A is 4.39 µT.
    (a) At what distance is it 0.800 µT?
    (b) At one instant, the two conductors in a long household extension cord carry equal 9.00 A currents in opposite directions. The two wires are 3.00 mm apart. Find the magnetic field 41.0 cm away from the middle of the straight cord, in the plane of the two wires.

    Your answer differs from the correct answer by orders of magnitude. nT
    (c) At what distance is it one tenth as large?
    (d) The center wire in a coaxial cable carries current 9.00 A in one direction and the sheath around it carries current 9.00 A in the opposite direction. What magnetic field does the cable create at points outside?

    I know A is 225 cm by usingb(2pi*r)= munot Iinc, in which i found Iinc then used that and the new value of b into the same equation in which i got 225 which according to webassigned is correct.
    I don't know what to do for b,c,and d. I was told to use b= (munot8I/2pi)(1/r1-1/r2) but i can not get the correct answer.
  2. jcsd
  3. Sep 16, 2005 #2


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    Homework Helper

    I think you have to use formulae for magnetic field

    [tex] B = \frac{\mu _0 I_1}{2 \pi d}[/tex]

    for the magnetic field and then the magnitude of force d = c and c + a

    [tex] F = BI_2 l [/tex]

    for the two wires separately

    the current in both wires is in opposite direction and therefore the resultant force is the difference of the two.
  4. Sep 16, 2005 #3


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    Homework Helper

    For part (a) it appears that you have used correct formula. r = 1mm and I = I1.

    For part (b) again the same only I = I1 -I2 and r = 3 mm

    Go through Ampere's circuital law again.
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