Watts from ohms one line

[ggunn]

I deal with a lot of electricians but few if any inspectors.

Lucky you.

 

[BillyMac59]

One final comment .... this type of problem was covered in my first year of trade school (1990) as DC circuit analysis. When trying to determine the total resistance in a circuit, remember these two basic tenets: 1 - The total resistance of a series circuit is the sum of their resistances. The total resistance of a parallel circuit is quite different and requires a little math. In a parallel circuit, the reciprocal of the total resistance equals the sum of the reciprocals of the individual branch resistors. When you apply this to a basic parallel circuit with only two branches, the math breaks down to the "product over the sums" that I noted in a previous post. In your example problem, with switches S1 and S3 closed, all resistors are shorted except the two parallel 10 ohms . Using the reciprocal method : the sum of the reciprocals = 1/10 + 1/10 = 2/10 = 1/5. This equals the reciprocal of the total resistance. Reciprocal of 1/5 is 5. This is your total circuit resistance when S1 and S3 are closed.

 

[ggunn]

The book says the answer is 500 watts, please explain how they got that.

P=(I^2)R
I = 10A, and R = 5 ohms.

 

[RCC1]

To determine the resistance of two resistors in parallel try using the "product" over the "sum" formula.

Product / Sum in this case is: 10 * 10 / 10 +10 = 100/20 = 5 ohms

Now try using power formula from wheel above:

Power = I² * R

Current² (squared) * Resistance = 10 Amps * 10 Amps * 5 ohms = 500 watts

 

[ggunn]

To determine the resistance of two resistors in parallel try using the "product" over the "sum" formula.

Product / Sum in this case is: 10 * 10 / 10 +10 = 100/20 = 5 ohms

Now try using power formula from wheel above:

Power = I² * R

Current² (squared) * Resistance = 10 Amps * 10 Amps * 5 ohms = 500 watts

In this case the question writer made it easy; for two identical resistors in parallel the combined resistance is simply half that of either one of them.

 

[retirede]

In this case the question writer made it easy; for two identical resistors in parallel the combined resistance is simply half that of either one of them.

Works for any number of identical resistors. Just divide by the quantity in parallel.

 

[Strathead]

Yes I know all about that , what I didn't know is how to find total resistance of parallel loads. The equation made no sense to me , now I understand it. Thanks for the help

Oddly no one seems to want to give you the actual Kirchoff's law on parallel resistance. Feel free to search Google for a more in depth study, but

when you only have two resistors in parallel you can use R total= R1xR2/R1+R2 Hope this helps

 

[Strathead]

I am always amazed at how poorly electrician apprentices are taught the really basic stuff about electricity. This should have been something the op figured out in his head in about ten seconds.

The other issue that just makes me crazy is how few electricians can actually read electrical drawings and understand them.

I teach my students as well as time allows and still only about 1 in 5 will retain it beyond the test.

 

[LarryFine]

I am always amazed at how poorly electrician apprentices are taught the really basic stuff about electricity. This should have been something the op figured out in his head in about ten seconds.

The other issue that just makes me crazy is how few electricians can actually read electrical drawings and understand them.

I agree, too. I have been interested in electronics since I was little. I built "foxhole" and crystal radios when I was six from books I found in the school library, and ran phone lines at home.

When I first became a helper, I was amazed about how little the "mechanics" I worked under knew about electrical theory. I probably did as much teaching as I did learning back then.