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: Short-Cut For Ohm's Law

  1. Jul 21, 2003 #1
    What is the shortcut on solving problems that deal with ohms law?
     
  2. jcsd
  3. Jul 21, 2003 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You'll have to be more specific.
     
  4. Jul 21, 2003 #3

    Dx

    User Avatar

    E
    ------
    I | R



    Here is a pyramid so if you want to solve for eg; E = IR. Just cover up what you want to solve for and the remaining letters left are your formulas. Another eg; I = E/R. Is that what you mean, sir? Does that help?
    Dx :wink:
     
    Last edited by a moderator: Jul 21, 2003
  5. Jul 21, 2003 #4
    ok well i have problems here that im done with already. dealing with the calculating the current.

    used these kind of formulas:

    resistance = voltage/current

    and

    current = voltage/resistance

    now i was asking if there was easier shortcuts on solving these.

    now looking at the last problem i am stuck. instead of asking for ohms or amps it asked find the current of a watt.

    Code (Text):
    calculate the current in a 140-w electric blanket connected to a 120-v outlet
    now my question is how to solve that with a diffrent formula? what formula is this?
     
  6. Jul 21, 2003 #5

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's not short enough for you? [?]

    You need to learn basic units, and you need to start reading your book. I am not going to tell you the formula, because that gives away the whole thing. I am going to ask you a couple of questions that will lead you to the answer.

    1. What quantity is measured in Watts?
    2. Is there a formula in your book that relates the quantity in #1 to current (answer: YES, there is).
     
  7. Jul 21, 2003 #6
    Ohm's Law:

    The pyramid is "V" on top, with "I" and "R" on the bottom. It can be expressed as three complimentary equations:

    1) Voltage = Current * Resistance.

    2) Current = Voltage / Resistance.

    3) Resistance = Voltage / Current.

    Any time you have two of the factors from one side of the equation, you will know the third factor (the other side). In your question "calculate the current in a 140-w electric blanket connected to a 120-v outlet", you have Watts and voltage.

    Now you need to figure out what the "Watts" means. Watts is volts*amps. Again, you can draw a pyramid to help remember it all. Put Watts at the top, and across the bottom you have volts and amps (amps being current). Again, this allows three expressions:

    1) Watts = Voltage * Current.

    2) Current = Watts / Voltage.

    3) Voltage = Watts / Current.

    Now, Watts is voltage*amps. Since you know the Watts and voltage, you can divide Watts by voltage to get current. 140/120 = 1.15 Amps, roughly.

    Now you have two factors necessary for working with Ohm's Law. You have the voltage (120) and the current (1.15).

    The short version:

    Watts = voltage*amps.

    You've got voltage and watts: watts (140) = volts (120) * ?.

    So, by re-arranging the factors you can get: ? = watts (140) / voltage (120).

    Simply fill in the "?".
     
    Last edited: Jul 22, 2003
  8. Jul 22, 2003 #7

    Dx

    User Avatar

    remember P=IE, my friend.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook