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November 01, 2025, 09:50:19 am
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Author Topic: Simple Transformers Q  (Read 1132 times)  Share 

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Andiio

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Simple Transformers Q
« on: August 02, 2011, 11:45:26 pm »
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Why is it that only P = I^2R can be used to calculate the power loss?

I recall knowing the reason a while ago.. but somehow it isn't coming back to me now :(

Thanks!
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Lasercookie

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Re: Simple Transformers Q
« Reply #1 on: August 03, 2011, 04:05:52 pm »
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It's to do with the definition of V in P=V^2/R.

V is defined to be the potential difference, not the voltage transmitted.

edit: got mixed up a bit with the definitions of V. Confused myself, I was right the first time.

« Last Edit: August 03, 2011, 04:19:55 pm by laseredd »
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kenhung123

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Re: Simple Transformers Q
« Reply #2 on: August 03, 2011, 04:09:20 pm »
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Yep, we want to use something that is constant throughout the transmission.
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Vincezor

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Re: Simple Transformers Q
« Reply #3 on: August 03, 2011, 06:39:34 pm »
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I remember someone asking this question in class the other day. I'm told that it is because current is the factor that causes power loss (not potential difference?), and hence using P=I^2*R would be necessary. 
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kenhung123

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Re: Simple Transformers Q
« Reply #4 on: August 03, 2011, 06:41:03 pm »
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Nah its because V varies throughout transmission and I and R are constant
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Vincezor

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Re: Simple Transformers Q
« Reply #5 on: August 03, 2011, 08:43:58 pm »
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Quote from: kenhung123 on August 03, 2011, 06:41:03 pm
Nah its because V varies throughout transmission and I and R are constant

Alrighty, thanks for the info! :)
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jgbs

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Re: Simple Transformers Q
« Reply #6 on: August 04, 2011, 04:53:40 pm »
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I thought it was because we are rarely given the voltage drop across the transmission cable.
If you're given that then wouldn't P= (V^2)/R work fine?
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AskQuestions

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Re: Simple Transformers Q
« Reply #7 on: August 04, 2011, 06:40:25 pm »
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Quote from: kenhung123 on August 03, 2011, 06:41:03 pm
Nah its because V varies throughout transmission and I and R are constant

How come I is constant?
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Graphite

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Re: Simple Transformers Q
« Reply #8 on: August 04, 2011, 06:42:15 pm »
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Coz you can't lose current, what goes in must come back out of the wire. Think about the definition of current.
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Lasercookie

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Re: Simple Transformers Q
« Reply #9 on: August 04, 2011, 06:55:23 pm »
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Quote from: jgbs on August 04, 2011, 04:53:40 pm
I thought it was because we are rarely given the voltage drop across the transmission cable.
If you're given that then wouldn't P= (V^2)/R work fine?
If you know the voltage drop, then you can use P=V^2/R.

Using an example from last years exam, question 16.

The information you know:
I = 2 A
R = 4 Ohms
V (Voltage Drop) = 8V

Using P=I^2R:
P = I^2R = 2^2 * 4 = 16W

If you used P = V^2/R:
P = 8^2/4 = 64/4 = 16W

However, if the voltage given was the voltage TRANSMITTED, then obviously the answer would be different if you applied P=V^2/R
In this case, the voltage transmitted was 10V (the answer to question 15)
P= 10^2/4 = 100/4 = 25W

Edit: To directly address the question, yes, P=V^2/R should work fine if you know the voltage drop.
« Last Edit: August 04, 2011, 07:16:59 pm by laseredd »
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kenhung123

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Re: Simple Transformers Q
« Reply #10 on: August 06, 2011, 09:26:11 am »
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I guess you can but if you get to choose it seems more convenient to use the latter equation and also seems less prone to error
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