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
-_-"
Nvm..solved
Voltage of 100/200000 is of the power station..current constant throughout, rest are variables
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 :)
Originally posted by eagle:..
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?
Originally posted by donkhead333: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 ;)
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 ?
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.