ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: Andiio on August 02, 2011, 11:45:26 pm

Title: Simple Transformers Q
Post by: Andiio on August 02, 2011, 11:45:26 pm: 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!
Title: Re: Simple Transformers Q
Post by: Lasercookie on August 03, 2011, 04:05:52 pm: 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.
Title: Re: Simple Transformers Q
Post by: kenhung123 on August 03, 2011, 04:09:20 pm: Yep, we want to use something that is constant throughout the transmission.
Title: Re: Simple Transformers Q
Post by: Vincezor on August 03, 2011, 06:39:34 pm: 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.
Title: Re: Simple Transformers Q
Post by: kenhung123 on August 03, 2011, 06:41:03 pm: Nah its because V varies throughout transmission and I and R are constant
Title: Re: Simple Transformers Q
Post by: Vincezor on August 03, 2011, 08:43:58 pm: 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! :)
Title: Re: Simple Transformers Q
Post by: 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?
Title: Re: Simple Transformers Q
Post by: AskQuestions on August 04, 2011, 06:40:25 pm: 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?
Title: Re: Simple Transformers Q
Post by: Graphite on August 04, 2011, 06:42:15 pm: Coz you can't lose current, what goes in must come back out of the wire. Think about the definition of current.
Title: Re: Simple Transformers Q
Post by: Lasercookie on August 04, 2011, 06:55:23 pm: 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.
Title: Re: Simple Transformers Q
Post by: kenhung123 on August 06, 2011, 09:26:11 am: 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