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Resistance and current problems

  1. Jan 13, 2004 #1
    Problem 5.
    A total charge of 12mC passes through a cross-sectional area of a nichrome wrie is 3.1s.
    If the number of charges that pass through the cross-sectional area furing the given time interval doubles, what is the resulting current? In units of A.
    Note: What formula(s) should I use?

    Problem 7.
    How long does it take for 7.5 C of charge to pass through a cross-sectional area of a copper wire if I=19A? In units of s.
    Note: I don't know where to start?

    Problem 11.
    A typical color television draws 2.8 A of current when connected across a potential difference of 121V.
    What is the effective resistance of the television set? Answer in omega.
    Note: What formula(s) should I use?
  2. jcsd
  3. Jan 13, 2004 #2
    Current can be defined as the charge which passes a given point in one second. Specifically, 1 ampere of current is 1 coulomb of charge passing a given point in one second.

    If you have 12 milli-coulombs pass in 3.1 seconds then it's a simple matter to calculate how many coulombs are passing in 1 second. That would be the current in amperes. Current (in amperes) equals Charge (in Coulombs) divided by time (in seconds). I=C/T

    If charge doubles, then it's easy to see what happens to current by looking at the formula. Current is directly proportional to charge.


    Here we are given 19 amperes of current and are asked to find the time it takes 7.5 coulombs of charge to pass a given point. First ask yourself how many coulombs are passing a given point in one second. If we have 19 amperes of current, we have 19 coulombs of charge passing any given point in one second. If it takes 1 second for 19 coulombs of charge to pass, how long will it take for 7.5C of charge to pass?


    Simple ohms law problem:

    Voltage(V)= Current(I) * Resistance(R)

    To find resistance, R=E/I.
    Last edited: Jan 13, 2004
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