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Homework Help: Electromagnetism help.

  1. Mar 17, 2012 #1
    A 5.0nC point charge travels in a circle of radius 1.0cm. What is its speed so that the current it generates 1.0mA?.

    I tried to use I=nqvA, but for some reason it didnt work because i dont know how to calculate n. any hints? thanks!
  2. jcsd
  3. Mar 17, 2012 #2
    You need to know that a current of 1A means a flow of charge of 1 Coulomb per second.
  4. Mar 17, 2012 #3
    i know the definition of Ampere, but hows that relevant to the question?
  5. Mar 17, 2012 #4
    So you know the current is 1 x 10^-3 coulombs per second.
    The charge that is moving is 5 x 10^-9 couloms along a circumference.
    Can you calculate the speed along this circumference that relates to 1 x 10^-3 coulombs in 1 second?
  6. Mar 17, 2012 #5
    OK i think I am missing a really important thing here because I still not sure what formula to use... to derive velocity.....
  7. Mar 17, 2012 #6
    my professor said that I = frequency x charge.. and i dont know from where he got that...
  8. Mar 17, 2012 #7


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    Hello spock9000. Welcome to PF.

    What is the definition of electric current ?
  9. Mar 17, 2012 #8
    Current is just charge counting to determine the rate of charges passing a given point in a circuit. Imagine that you are watching a very strange horse race that has only one contestant in the match, and imagine that you are standing at the finish line. How many horses per second are passing you? Well, at the start of the race, none are. However, at the end of the race that single horse goes by you very suddenly and quickly. If you define "horse-current" to be the rate of change of horses, it's difficult to come up with one number of the current. First the number is zero, and then it is not zero, but it's hard to count when the horse goes by.

    Now, instead of this very strange horse race, imagine one of those horse carousels (or merry-go-rounds) that children ride on at an amusement park. Imagine one child is on a horse going round and round. If you define the "child-current" to be the rate of change of children going by, what is the current. Since, the event of the child going by is cyclic, we can say that the cyclic frequency times the number of children (in this case one) is the current. It might be 1 child per 20 seconds (or 0.05 children/s). Still, this concept might be a little confusing because the cyclic rate is so low. There are moments where it seems there are no children going by, and suddenly the child is there going by. This is the discrete nature of charge and current, and it does not destroy the nature of the definition if we understand that the value is an average value.

    In the case of real electrical charge current, the definition is the same, we imagine counting the charges per unit time period. If it helps, take a very long time period so that you can count many events, then do the calculation of number divided by total time, and you have your rate of change of charge, which is current (averaged current).

    Tying back to your starting question. The child on the carousel has a particular linear speed (radius times angular frequency). Obviously, the faster the frequency, the faster the speed. Greater speed implies higher current. More children on the carousel also implies higher current, and this relates to the number n in your original formula.
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