1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Electric circuit

  1. Aug 18, 2010 #1
    1. The problem statement, all variables and given/known data
    Two light bulbs are rated as 60 W and 75 W at 220 V. If these are connected in series to a source of 220 V, what will the power in each be? Assume a constant resistance for the light bulbs.

    2. Relevant equations
    W=VQ P=IV V=IR

    3. The attempt at a solution
    I was solving a similar question, which was dealing with bulbs connected in parallel. By dividing the work (W) by the Volt (V) I got the current (A), which seems a little strange to me since I thought I'd get the charge. Could anyone explain this please? (I then used V=IR and then P=IV to figure out the power dissipated) When it comes to light bulbs connected in series I don't know how to approach the problem or what differs between the bulbs that are connected in series and those that are connected in parallel.
  2. jcsd
  3. Aug 18, 2010 #2


    User Avatar
    Homework Helper
    Gold Member

    Hello poskhare,

    Welcome to physics Forums!
    But there isn't any 'work' value given in the problem. There are power values however: the 60 [W] and 75 [W].

    Power is the time derivative of work. In other words, power is the change in energy per unit time.

    So when you divide the power by the emf (a.k.a. voltage), you get the change in charge per unit time. Charge per unit time is current.

    In summary, the important variables to this problem involve some or all of the following:

    o Power, P (and two different values were given, one for each bulb). Power has units of Watts [W].
    o EMF (sometimes called 'voltage'), V. Voltage has units of Volts [V].
    o Current, I. Current has units of Amperes (sometimes abbreviated as 'Amps') [A].
    o Resistance R. Resistance has units of Ohms [Ω].
    My first recommendation is to find the resistance of each bulb, based on each bulbs rated power and rated voltage.

    Then study the difference between putting resistors in parallel and putting them in series. It makes a difference. :wink:
  4. Aug 19, 2010 #3
    Okay, now I get it. Thanks a lot!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook