finding current

Discussion in 'Homework Help' started by phantomvs, Jan 24, 2016.

  1. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    01.05.053.jpg

    I am stuck on this question, that asks me the following :

    If R2 is short circuited , the amps in R3 is : ?

    Here is what i did :
    I redrew circuit without R2. So it becomes a series circuit R1in series with R3
    added the resistance. r1 + r3 so 40 +10 = 50
    Found the amps so 30v/50 ohm = 0.6 amps
    so 0.6 amps times R1 =6 volts on R1 and 30-6 is 24 volts on R3
    Since this becomes a series circuit it should be the same current.

    What am i doing wrong ?
     
    Last edited: Jan 24, 2016
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    609
    120
    Draw R2 as a straight line (short circuit) and look again.
     
    phantomvs likes this.
  3. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Review what it means for something to be short-circuited. Does removing it from the circuit satisfy that meaning?
     
    phantomvs likes this.
  4. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    from what i am reading ,short circuit on R2 automatically makes R3 0 ohms as well.
    So pratically removing R3 from circuit.

    So 30v/0 ohms is 0 amps

    did i get that right ?
     
  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,790
    827
    Automatically makes R3 = 0 Ω?

    I don't understand that, do you?

    If R2 is shorted, what is the voltage across R2? R3?
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Yes, since shorting R2 also shorts R3. But you need to be careful about saying that it "removes" them from the circuit, since that is basically the mistake you made originally. They are replaced by a wire (the short circuit).

    How do you get that 30 V / 0 is 0 amps? What would 30 V / 0.1 Ω be? How about 30 V / 0.000001 Ω? Does that jive with 30 V / 0 Ω being 0 A?
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    HINT: The key is to ask what the voltage is across R3.
     
  8. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    The voltage across R3 is 30 volts.
    As if there is a short on R2 , then there is a short on R3

    So to find amps in R3 would be 30 v / 0 ohms as no resistance just a straight wire .
    Voltage at R3 is 30 volts ?
     
    • cur.jpg
      cur.jpg
      File size:
      18.5 KB
      Views:
      7
  9. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    No. What is the voltage across a short (or across an ideal piece of wire)?
     
  10. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    I get it, 0 volts ?
     
  11. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Correct. So how much current is flowing in R3?

    For extra credit, how much current is flowing in the short?
     
  12. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    None as well ?
     
  13. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    No current ? Are you sure ? Maybe instead of a short circuit replace R2 with a 0.01Ω resistor.
     
    phantomvs likes this.
  14. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    As Jony130 already mentioned, consider the case when instead of a dead short (0 Ω) it is just a very low valued resistance such as 1 Ω, or 0.1 Ω, or 0.001 Ω.

    Another way to come at it is to ask how much voltage is across R1 and, as a result, how much current is flowing in R1. Where does that current go? How does it get over to the other side (which, presumably, is at 0 V)?
     
    phantomvs likes this.
  15. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    So since it would be only 1 resistor left as the other 2 become short circuited would it be 3 amps .
    Voltage on R1 30 volts

    If not i am really lost now :)
     
  16. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    So, after this 3 A of current leaves R1, where does it go?
     
  17. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    Well wouldn't it go through R2 /R3
     
  18. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    What is the voltage across R2 and R3?
     
  19. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    0 volts
     
  20. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    This is like pulling teeth!

    So what is the current through R2 and R3?
     
Loading...