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Help with magnetism/electromagnetic inducion/ac circuits

  1. Mar 31, 2006 #1


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    i need help understandings these concepts. i just dont get the relationships between things like force, magnetic field (B), velocity of a particle (eg a proton), EMF, induction of a field, etc...

    can someone please get me started on these problems and help me understand the concepts and equations.

    1) a proton falls through a potential difference of 10^5 volts - what speed will it have if it starts from rest?

    2) A wire 30 m long moves at 2 m/s perpendicularly through a field of magnetic induction 1 N/Am. a) what EMF is induced in the wire? b) the total resistance of the circuit of which the wire is part of is 15 ohms - what current flows?

    3) Double-ionized helium atoms (alpha particles) are traveling at right angles to a magnetic field at a speed of 4.0 x 10^4 m/s and the induction of the field is 5.0 x 10^-2 N/Am. what force acts on each particle?

    4) An oil drop weighs 9.0 x 10^-15 N. It is suspended in an electric field of 2 x 10^4 N/coul. a) what is the charge on the oil drop? b) how many excess electrons does it carry?

    5) what energy is given to an electron to transfer it across a difference in potential of 4.0 x 10^5 volts?

    now i dont expect the answers of course, but i really need some help getting started and just understanding all the different relationships between the terms and concepts.

    and did i do this one right:

    A wire weighing .5 N/m is exposed to a magnetic field of strength .25 N/Am for a distance of 1.3 m. What current must flow in the wire to suspend it in the field?

    I used the equation B = (u x I)/(2pi x d)

    sorry i dont know how to use the symbols but pi is obviously pi (3.14) and u is suppose to be that constant 4pi x 10^-7). i plugged all of those in and solved for I to get 1.625 x 10^6 A.

    although im not sure what to do with that .5 N/m (and how is that a weight anyways? isnt weight in N?)
  2. jcsd
  3. Mar 31, 2006 #2


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    Staff: Mentor

    On 1 and 4, when you have the electric potential, how do you get the electric field? When you have the electric field and the charge, how do you get the force?
  4. Apr 1, 2006 #3


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    i know that F=qE and E=V/d but we dont know the distance and how do you get velocity involved?
  5. Apr 2, 2006 #4


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    anyone? just a little help. not looking for answers but rather explanations so i can better understand the topic.

    even a few links would be much appreciated.
  6. Apr 2, 2006 #5
    If you can find F and you know m, what's a?

  7. Apr 2, 2006 #6
    That isn't a weight and the problem shouldn't have stated it that way. It is a "linear weight density." To get the weight of a length of wire you mulitply the length by .5 N/m.

  8. Apr 2, 2006 #7
    This is a charged particle moving in a constant magnetic field. How do you find the force on a moving charge in a magnetic field? (Big hint: Think Lorentz Force.)

  9. Apr 2, 2006 #8


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    F=ma...but the question doesnt ask for a.
  10. Apr 2, 2006 #9


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    is there a good website that gives all the related equations used in these topics?
  11. Apr 2, 2006 #10
    dnt, this is what is called a "synthesis" question. You have to put a few concepts together from different places. Let's go through the steps you need to run through to solve this.

    First, categorize what you've got. It's a proton, so you know the mass and charge. There's a potential difference, so you know there's an electric field present. The proton is starting from rest.

    Second, label what you are looking for. In this case the speed of the proton after it goes through the potential difference.

    Third, what kind of problem is it? Well, you are in an Electricity chapter, so odds are kind of high that you are going to be using Electro-static methods.

    Let's start there. You have a charged particle in an electric potential. What happens to such a particle? Well, it moves. How does it move? Well, it's in an electric field and we know that a charged particle in an electric field has a constant force applied to it.

    Reminder time: You are looking for a final speed for the proton. Force doesn't give speed. Synthesis #1: But we know that a constant force implies a constant acceleration. And we know, Synthesis #2, how to calculate the motion of a particle under a constant acceleration.

    This programme does get a bit involved. (You have to assume a distance d that the proton moves over, and then later use the acceleration equation to give a second equation for d and solve simultaneously.)

    The other, vastly simpler, method is to use energy techniques, but I don't know if you've hit those yet. You can find the energy the proton gains by moving across the potential difference (W = qV). Since the proton started from rest, that energy is all kinetic. Hence you can get the speed.

  12. Apr 3, 2006 #11


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    so could i say W=qV and plug in the q of a proton and the voltage given? then set that equal to 1/2mv^2 (KE) and solve for v that way?

    thanks for your help topsquark. really appreciate it.

    any more key equations to know? where does EMF fit in with the other concepts?
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