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: Current and resistors

  1. Feb 6, 2006 #1
    The little speaker in a portable radio is labeled 16 Ohm's , 0.1 W. To what current does that correspond?

    I'm kind of confused where to go with this one. what i know is this, 1 watt=1volt*1amp, but where do i go from there?

    Do I have to use Ohm's law somehow? V=IR.

    So does VI=.1 and R=16 ohms? am i supposed to be able to solve for I from that or something?

    im just confused as to where to go next with this problem. This is the first problem i've had with Watts.

    Thanks for any tips.
  2. jcsd
  3. Feb 6, 2006 #2
    if you know V=IR and that P=IV

    then you know that P=I^2 R
  4. Feb 7, 2006 #3


    User Avatar

    Staff: Mentor

    Actually, it's a little more complicated than Andrew's hints. You are asking about an audio speaker, and the DC resistance is not the 16 Ohm number. Its DC resistance is likely very low, since it's just a coil of wire around a magnet.

    The 16 Ohm number is the impedance at some audio frequency. I didn't know what frequency is used for the standard, so I googled the following:

    16 ohm speaker impedance resistance

    and got a lot of useful hits. Here is a very good tutorial on speaker impedance:


    From that article it looks like the DC resistance isn't super-low as I'd assumed, but it is still less than the quoted AC impedance, like the 16 Ohms of your problem. So I guess that Andrew's hints are still valid, but just keep in mind that the I and V that you are using in your calculations are RMS values, not DC or peak or peak-to-peak.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook