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Homework Help: Electricity DC assignment

  1. Apr 23, 2009 #1
    Hi out there! I've been handled 3 questions which for me is rather hard and therefore I'm asking the experts.
    Any help would be great:)
    Donkey Kong

    Question 1)
    A circuit consists of a 15Ω and a 30Ω resistor connected in parallel across a battery of internal resistance 2 Ω. If 60W is dissipated by the 15 Ω resistor, calculate
    (a) the current in the 30 Ω resistor
    (b) the terminal potential difference and emf of the battery
    (c) the total energy dissipated in the external circuit in one minute
    (d) the quantity of electricity (charge) through the battery in one minute.

    Question 2)
    A coil of 20 Ω resistances is joined in parallel with a coil of R Ω resistance. This combination is then joined in series with a household apparatus A. This whole circuit is connected to 100V mains. What must be the value of R so that A shall dissipate 600W with 10A passing through it?

    Question 3)
    A person wants to move a 20W car stereo system from an old car with a 6V battery to a new car with a 12V battery. What is the resistance of the resistor that should be connected in series with the radio to limit the current, and what is its minimum power rating?
  2. jcsd
  3. Apr 23, 2009 #2


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    Welcome to Physics Forums.

    What have you attempted thus far? What are your thoughts on the problems?
  4. Apr 23, 2009 #3
    Thank you:)
    I'm not sure how to do these, in Question 1 I thought about the thing that the resistors were parallel connected. Which should give a resistance on 10 ohm. BUt then i'm not sure what to do.
    In question 2 i've tried to use the law of Ohm, but without any succes

    In question 3 I have no idea!

    I'm sorry my english isn't very good, i'm from Denmark were i'm studying as a Global Business Engineer.. Usually I don't like people doing my assignments, but I have to admit that i' lost at this area
  5. Apr 23, 2009 #4
    I've been trying articles all over the website but nothing will help me
  6. Apr 23, 2009 #5


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    Okay, let's start with question one.

    Can you use Kirchoff's laws to write expressions for the potential differences around the two loops of the circuit?
  7. Apr 23, 2009 #6
    The amount of I running into the circuit is the same running out..But how am I going to find the find the current running into the circuit before the loops?
  8. Apr 23, 2009 #7


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    I asked for the potential differences, but we can do the currents first if you like.

    Have you drawn a circuit diagram? If not, it would be a good idea to draw one at this point and post it so that we can both reference it in our subsequent posts.
  9. Apr 23, 2009 #8
    I'm not quite sure how it would be.. But if 60 W is dissipated from r1 then the current running through r2 should be 60w/r2= 60w/30ohm = I=2 ?
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