Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Still no confident with these answers.

  1. Mar 2, 2004 #1
    I appreciate all of the help I have recieved on these review problems. I feel pretty confident going into the test. Here are a few problems, I am still having trouble with. (some have been asked before, but I still dont understand)

    11. The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank theses wires according to the magnitude of the magnetic forces, from least to greatest.

    . x .
    1 2 3

    a) 1,2,3
    b) 2, 1 & 3 tie
    c) 2 and 3 tie, then 1
    d) 1 and 3 tie, then 2 (I believe it is d, but am not sure)
    e) 3,2,1

    18) The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to:
    a) R
    b) 1/R
    c) 1/R^2
    d) 1/R^3
    e) 1/R^3/2

    I choose 1/R b/c of this formula B=uI/2piR

    20) A uniform magnetic feild is directed into the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius. A reasonable explanation is:

    a) the charge is positive and slowing down
    b) the charge is negative and slowing down
    c) the charge is positive and speeding up (I choose this one)
    d) the charge is negative and speeding up
    e) none of these

    21. Electrons are going around in a circle counterclockwise direction. At the center of the circle they produce a magnetic field that is:

    a) into the page
    b) out of the page (I believe it is this one)
    c) to the left
    d) to the right
    e) zero

    24) Two parralle wires, 4 cm apart, carry currents of 2 A and 4 A resp;ectively, in opposite directions. The force per unit Lenght in N/m of one wire on the other is:

    a) 1 x 10^-3 repulsive
    b) 1 x 10^-3 attractive
    c) 4 x 10^-5 repulsive
    d) 4 x 10^-5 attractive
    e) none

    I choose c used this forumula F = uI1I2/2pid

    25) An electron (q = -1.6 x 10^-19)nis moving at 3 x 10^5 m/s in the positive x direction. A magnetic field of 0.8 Tesla is in the positive z direction. The magnetic force on the electron is:

    A) zero
    b) 4 x 10^-14 in the positive z direction
    c) 4 x 10^-14 in the negative z direction
    d) 4 x 10^-14 in the positive y direction
    e) 4 x 10^-14 in the negative y direction

    I dont even know which formula to use on this one.

  2. jcsd
  3. Mar 2, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    No. 1 and 3 are tie, but what is the force on 2? Figure it out this way: What is the B field at 2 caused by 1? caused by 3?
    No. There are two parts to consider. First, what is the direction of the force: that is what determines whether it will spiral clockwise or counterclockwise. That depends on the charge. (F = q V X B) A positive charge will spiral counterclockwise. To find the size of the spiral, consider centripetal acceleration.
    No. Once again, you must know how a moving charge creates a magnetic field: magnitude and direction. Learn the right hand rule.
    The force on a moving charge in a magnetic field is:
    F = q V X B
  4. Mar 2, 2004 #3
    Re: Re: Still no confident with these answers.

    I am coming up with the right number using this formula, but the direction is confusing.
  5. Mar 2, 2004 #4
    25) I am coming up with the correct #, but I am confused about the direction of the force.
  6. Mar 2, 2004 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Re: Re: Re: Still no confident with these answers.

    Answer my question. What it the net magnetic field at 2?
    F=ma = qVxB=mV2/r
    so... V/r is constant. If the radius is getting smaller, the speed must be getting smaller.
    Here's how I use the right hand rule: Hand spread out, fingers point in direction of V, palm in direction of B, thumb points towards force on a positive charge. Reverse the direction of force for an electron.
    Keep practicing the right hand rule until you get it straight.
  7. Mar 2, 2004 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Right hand rules

    This site may help: http://www.physics.brocku.ca/faculty/sternin/120/slides/rh-rule.html [Broken]

    There are several versions; pick one and stick with it. (I suspect you are just mixing up signs: positive and negative charges feel opposite forces.)
    Last edited by a moderator: May 1, 2017
  8. Mar 2, 2004 #7
    I think I got it. should be in the negative y.
    Last edited by a moderator: Mar 2, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook