O lvl physics transformers

donkhead333 SG

20kW of power is transmitted by a cable of resistance 0.2ohms from the power station to a house at 2 possible voltages, 100V and 200 000V.

Calculate the power loss in the cable at these 2 voltages.

Question..why must use P=I^2R? (Or rather, why is finding the current neccessary at all when you can use P=V^2/R directly?)

when I use P=V^2/R I get a wrong answer..

Answer for both parts, 8000W and 0.002W respectively

donkhead333 SG

-_-"

Nvm..solved

Voltage of 100/200000 is of the power station..current constant throughout, rest are variables

eagle SG

Good!!! You solved it partly yourself :)

I have challenged many students (as well as some tutors in physics) on this question before... Almost all could not come out with the reason why...

To add on, 100/200000 is not just about the voltage of the power station... The power station can be treated like a battery. What happens is that the potential difference across the wires is not 100/200000 V, but rather, the potential difference across the wires and all the electrical applicances of all the households connected to the power station itself :)

donkhead333 SG

but rather, the potential difference across the wires and all the electrical applicances of all the households connected to the power station itself :)

Meaning that if I use P=VI, it would take into consideration the power loss within the household itself?

eagle SG

Meaning that if I use P=VI, it would take into consideration the power loss within the household itself?

yes

not power loss... u call it power dissipated ;)

yiha093 SG

OR .

canot use v2/r because it is not V that causes resistance, or power dissipated.

BUT

ITS THE CURRENT AND RESISTANCE ! =)

eagle is this the reason u looking for ?

yiha093 SG

hi .

Question..why must use P=I^2R? (Or rather, why is finding the current neccessary at all when you can use P=V^2/R directly?)

lets break ur qn down , '' why must use P = i^2 R ?

read my logic

act of current passing through wire and because of RESISTance which gen heat as current faces resistance in moving thru wire

do u get the 'flow'

. yes u can use v^2/ R * eagle say 1*

but u must use the P. D accross Wire only. I assume u use P. D of 2kw , hence ur wrong answer.

WHY?

this is because not all power that is sent through wire is dissipated in the wire. ( obviously)

HENCE. i suggest u use

(P. D ACROSS WIRE only)^2 / R

because u r trying to find how much heat produced by the (P. D across wire) right?

hence saying that ur p. d of 2kw is wrong.