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Homework Help: Practice exam question HELP PLEASE

  1. Mar 23, 2007 #1

    amy_a

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    practice exam question HELP PLEASE!!

    hi guys, ive been going through my past exam papers and i found this question. my lecturer says that a similar question will be on my exam but i havent got any idea how to do it :cry: can anybody help me please?

    i have to determine the current through RL, the voltage across RL and the power developed in RL for the following values of RL using thevenins theorem.

    (i) RL = 1 ohm
    (ii) RL = 2.83 ohm
    (iii) RL = 4 ohm


    Im so confused!!!!

    can anyone give me some pointers.... x
     

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    amy_a, Mar 23, 2007
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  3. Mar 23, 2007 #2

    mjsd

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    Homework Helper

    i think it is just an exercise of turning your circuit about terminals AB into a thevenin equivalent, and then do a simple analysis on the remaining circuit with different values of RL. Clearly, you must first learn how to convert everything left of AB into its thevenin equivalent. once that's done the rest would be simple.
     
    mjsd, Mar 23, 2007
  4. Mar 23, 2007 #3

    antoker

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    Yepp, mjsd is correct, leave the Rl alone and apply thevenin equivalent.
    Find thevenin voltage across the OPEN circuit.
    Replace batteries by short.
    Find thevenin resistance.... and so on.


    P.S I think this thread belongs in the homework-section.
     
    Last edited: Mar 23, 2007
    antoker, Mar 23, 2007
  5. Mar 23, 2007 #4

    berkeman

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    Staff: Mentor

    Yep, thread moved to Homework Help. Welcome to the PF, amy_a. It's a great place. Please keep in mind that homework and coursework questions must be posted in the appropriate Homework Help section, and not in the general forums. o:)
     
    berkeman, Mar 23, 2007
  6. Mar 23, 2007 #5

    amy_a

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    oops. im sorry bout that. i didnt realise there was a coursework section. thanks for moving the thread:approve: thank u for the replies as well guys. i managed to make two thevenin equivalent circuits from terminals a and b and c and d. is that correct? i found the voltage values from this but im still quite stuck on what to do next. how would i use the RL values and answer the question? i really am a dumb blonde i think!:rofl:
     
    amy_a, Mar 23, 2007
  7. Mar 23, 2007 #6

    antoker

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    Why are you getting two equivalents? ;) You're supposed to create a equivalent circuit between A-B terminals, so your circuit will be reduced to a single voltage source with a single series resistor, coupled toward Rl. After that finding voltage/current will be extremely easy. By simply using voltage divider rule ;) Or, you can do thevenin between C-D terminals and then apply loop equations to find the voltage across Rl.

    P.S please post your thevenin circuit.
     
    Last edited: Mar 23, 2007
    antoker, Mar 23, 2007
  8. Mar 24, 2007 #7

    amy_a

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    hi, i tried to make an equivalent circuit but im really quite useless at this. ive only just began studying this module! the problem is that my text books only have examples of thevenin circuits with one resistor parallel to the battery where as this question has 2. this is why im confused! im certain ive done this wrong so please dont laugh at my disastrous attempt at an equivalent circuit.:confused:
     

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    amy_a, Mar 24, 2007
  9. Mar 24, 2007 #8

    antoker

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    Ok, I'll take a look as soon as the image gets approved :cool:

    Update: ok, i can see it now, i'll take a look in an hour or so.
     
    Last edited: Mar 24, 2007
    antoker, Mar 24, 2007
  10. Mar 25, 2007 #9

    amy_a

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    thanks antoker, i really appreciate your help. im so useless at electrical engineering. its by far my weakest subject :uhh:
     
    amy_a, Mar 25, 2007
  11. Mar 25, 2007 #10

    antoker

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    amy_a: take a look at your first circuit. Remove the Rl, and compute the voltage across the 7Ohm resistance that is in parallel with Rl. That is your thevenin voltage(Hint: Use voltage divider equation, to obtain the voltage between C&D nodes). Then, replace your source battery by a short and compute total resistance, without the Rl present.
     
    antoker, Mar 25, 2007
  12. Mar 25, 2007 #11

    amy_a

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    hi antoker, thanks for all this help :smile: ive scribbled down what u told me and i found the thevenin voltage at terminals ab to be 9.94V. i then found the thevenin resistance to be 9.73 ohms. how would i go about adding RL back into the circuit so i can find the current across it etc?
     
    amy_a, Mar 25, 2007
  13. Mar 25, 2007 #12

    amy_a

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    hey, i think i figured it out ( i hope) i used the thevenin circuit and added in the RL resistor. i then found the total resistance by addin them both together and then found the current V/total resistance. with the current i found the voltage across RL and finally the power developed across RL. is that the right method? i hope so!
     
    amy_a, Mar 25, 2007
  14. Mar 25, 2007 #13

    antoker

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    applause :smile:


    P.S. I have not checked your calculations, but it seems to be correct.
     
    antoker, Mar 25, 2007
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