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: 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


    User Avatar

    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


    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


    User Avatar

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