Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Series Circuit

  1. Jun 15, 2005 #1
    I have a series Circuit with a battery of 25 Volts and 2 Resistors, 1 is 250 Ohms and the other is 500 Ohms. What is wanted is

    a. I = _______mA
    b. V1 = _______
    c. V2 = _______

    To get I I took I = V 25 25
    ______ ______ _____ = .033mA
    R1 + R2 250 + 500 750

    The Current stays constant in a series circuit, so for V1 and V2 I got 16.5 and 8.25, and that only totals 24.75V, and the battery Voltage is 25, so what am I doing wrong?
  2. jcsd
  3. Jun 16, 2005 #2
    In series, the net resistance is additive, then use Ohm's Law to find the current. The sum of the voltage drops throughout the circuit must equal the voltage in the battery.

    The current is constant in any circuit via conservation of charge arguments.

    The discrepency is due to your rounding of the current.
  4. Jun 16, 2005 #3
    I dont understand because I took 25 Volts and divided it by 750 which is the sum of R1 and R2 and the answer is .033333333333333 ect. the answer was asked for in mA so I put .033mA. and like I said it comes to 24.75 Volts, so how can I fix this to keep current constant and my Voltage total to = 25?
  5. Jun 16, 2005 #4


    User Avatar
    Homework Helper

    Your unit is wrong. Volt/ohm = amp (A). So the answer is 25/750=0.033333... A=33.3 mA
    The other thing is rounding error. If you calculate with two significant digits the result will not be more accurate than two digits. That 24.75 rounded to two digits is just 25. The resistance data were given with 3 significant digits, you should give the result also with 3 digits. 750 (ohm) * 0.0333 (A) = 24.975 V. rounded to 3 digits it is 25.0 V.

  6. Jun 16, 2005 #5
    Thank you, I realized my first error on the mA's and then I just left the 3s go on the calculator and I was able to get it to equal 25 Volts!
  7. Jun 16, 2005 #6
    Like I said, the discrepency is due to the rounding in your current.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook