Emf of a battery

  1. 1. The problem statement, all variables and given/known data
    determine the terminal voltage in each battery.
    Emf1=18V
    r1=1 ohm
    EMF2=12V
    r2=2 ohm
    R=6.6 ohms
    also the batterys currents are opposing each other.
    2. Relevant equations Vab=emf-Ir


    3. The attempt at a solution i used kirchnoffs rule to find the current I , 18-I6.6-12-I2-I=0 and got I= .625 then i thought i would just use Vab=emf-Ir to find the terminal voltage of each battery however im not getting the correct answer... could someone please explain this to me i have a test tomorrow morning and i really need to figure this out! the answers i got were 17.375 for the 18V batt and 10.75 for the 12V batt
     

    Attached Files:

    • tn.jpg
      tn.jpg
      File size:
      1.2 KB
      Views:
      12
    Last edited: Mar 31, 2011
  2. jcsd
  3. gneill

    Staff: Mentor

    Watch out for the current direction and the thus the polarity of the voltage drop it creates on the internal resistors. If current is being forced into a battery's positive terminal it's possible for it to have a higher terminal voltage than its internal EMF.
     
  4. so on the larger voltage batt would i use equation Vab=Emf-Ir = 18-(.625*1) and on the batt with voltage being forced across it use Vab=Emf+... idk
     
  5. what are the equations you would use in this situation?
     
  6. gneill

    Staff: Mentor

    Yes. It can be helpful to sketch in your current and the polarities of the voltage drops across the resistors. You did fine writing the KVL loop equation.
     

    Attached Files:

Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?