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

Wattage and efficiency

  1. Sep 4, 2007 #1
    In an ordinary electrical appliance they usually tell you the wattage of it on a tag. Does the wattage include all power loss throughout the whole circuit or does it only tell you the amount of power dissipated at the main components. For example say I have a flashlight. Would the wattage on the tag tell me how much energy is dissipated throughout the whole circuit (wires) or will it only tell me how much energy is dissipated at the resistor (light bulb).

    I know they use the word "efficiency" to tell you how much energy is lost to resistance throughout the circuit.
     
  2. jcsd
  3. Sep 4, 2007 #2

    berkeman

    User Avatar

    Staff: Mentor

    The wattage rating on the tag refers to the power input jack. So it is Vrms * Irms for the whole product.
     
  4. Sep 4, 2007 #3
    Thanks alot.
     
  5. Sep 4, 2007 #4
    I have another quick question. Heres the explanation of power and watts in the book Teach Yourself Electricity and Electronics:

    I heard another explanation for power and watts which was confusing me. It explained that current flows in one direction from the negative terminal of the battery, does not dissipate at the resistor, and is measured in amperes. Power on the other hand travels at the speed of light from the power source to the resistor equally from both terminals and is dissipated at the resistor. It also explained that this power is not electricity but a form of magnetic energy measured in watts.

    Is the second explanation accurate?
     
  6. Sep 4, 2007 #5
    it is true. a battery has its curren flowing from both terminals, be t positive or negative. they can both be measured by the ammeter but the thing about dissipating the resistor im not sure. but power does travel fast and is is not a form of electricity but a term used for amount of ... in a circuit.

    ... i havent though of the word to be used yet.
     
  7. Sep 5, 2007 #6

    berkeman

    User Avatar

    Staff: Mentor

    What you quoted is reasonable. The other stuff is ridiculous garbage. Where did you see that?
     
  8. Sep 5, 2007 #7
    I can't remember exactly where I heard it. It might have been from someone here. I probably misinterpreted what I read.
     
  9. Sep 5, 2007 #8

    rbj

    User Avatar

    would that product Vrms x Irms be the VA which is bigger than the wattage if the power factor is less than one?

    it's probably not a big deal for consumer appliances.
     
  10. Sep 5, 2007 #9

    berkeman

    User Avatar

    Staff: Mentor

    I believe that is correct, but I don't work much with power factor. To the extent that the power factor is less than 1, you don't dissipate the total VA power. But the source power supply generally needs to be capable of supplying the VA rating, I believe. Probably wikipedia.org would have a clearer answer about power factor considerations.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Wattage and efficiency
  1. Resistor Wattage (Replies: 12)

  2. Resistor wattage (Replies: 5)

  3. Wattage of a blender (Replies: 5)

Loading...