# Short-Cut For Ohm's Law

yakabod
What is the shortcut on solving problems that deal with ohms law?

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Tom Mattson
Staff Emeritus
Gold Member
You'll have to be more specific.

Dx
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

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yakabod
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:
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?

Tom Mattson
Staff Emeritus
Gold Member
Originally posted by yakabod
now i was asking if there was easier shortcuts on solving these.
That's not short enough for you? [?]

calculate the current in a 140-w electric blanket connected to a 120-v outlet
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).

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 "?".

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Dx
remember P=IE, my friend.