1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Power wasted/delivered to a resistor

  1. Nov 9, 2014 #1

    chris858

    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data
    two 8 ohm resistors and a battery of emf 12.0V and internal resistance of 8ohms are connected in series. Calculate the power delivered to each external resistor and then calculate the power wasted due to internal resistance.
    I keep coming up with the answer 18 but apparently that is not right, and I am strugglying, so help would be much appreciated.

    2. Relevant equations
    W=IV
    I=V/R

    3. The attempt at a solution
    VxV/R
    12 X 12/8 =18...
    current = 1.5A
     
  2. jcsd
  3. Nov 9, 2014 #2

    gneill

    User Avatar

    Staff: Mentor

    Sketch the circuit. How many resistors does the current flow through? What's the total resistance that the current flows through? So what then the current?
     
  4. Nov 9, 2014 #3

    chris858

    User Avatar
    Gold Member

    flows through two resistors, total resistance = 16 ohms. Current = 12/16 =0.75A
     
  5. Nov 9, 2014 #4

    gneill

    User Avatar

    Staff: Mentor

    What about the battery's internal resistance?
     
  6. Nov 9, 2014 #5

    chris858

    User Avatar
    Gold Member

    16+8=24 so 12/24=0.5A
     
  7. Nov 9, 2014 #6

    gneill

    User Avatar

    Staff: Mentor

    Right. So now what will you do now that you know the current through each resistor?
     
  8. Nov 9, 2014 #7

    chris858

    User Avatar
    Gold Member

    pwer= voltage times voltage over current. p=VxV/R so 12x 12/0.5=2
     
  9. Nov 9, 2014 #8

    gneill

    User Avatar

    Staff: Mentor

    R is not current in the formula P = V2/R, it represents resistance.

    There are other formulas for power. What are they?

    Oh, and be sure to include units with all results.
     
  10. Nov 9, 2014 #9

    chris858

    User Avatar
    Gold Member

    power = work/time
    P=IV
    P=V²/R
    P=RI²
     
  11. Nov 9, 2014 #10

    chris858

    User Avatar
    Gold Member

    power is measured in watts
    current in amps
    resistance in ohms
    voltage in volts
     
  12. Nov 9, 2014 #11

    gneill

    User Avatar

    Staff: Mentor

    Okay, so what is the power dissipated by one of the resistors?
     
  13. Nov 9, 2014 #12

    chris858

    User Avatar
    Gold Member

    I get it now, so basically the equation i was using gave the total power watts, which is actually 6 not 2, and so there are 3 resistors, including the internal resistance, so therefore you have to divide 6 by 3 which gives 2.0Watts per resistor.
    Thankyou very much!!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Power wasted/delivered to a resistor
Loading...