Homework Help: Which is power loss = I^2R

1. Oct 25, 2016

victorhugo

[Moderator note: Thread moved from technical section hence no template. Question text from later in the thread added to this post to make things clear]

"A power station generates electric power at 120kW. It sends this power to a town 10 km away through transmission lines that have a total resistance of 0.40 ohms. If the power is transmitted at 240 V, calculate:

a. the current in the transmission lines
b. the voltage drop across the transmission lines
c. the power loss in the transmission lines
d. the power loss in the transmission lines
"

[Since question d looks an awful lot like question c, I suspect a miscopy from the original source]

Since power is in joules, i'd imagine we could get the energy in - energy out (using p=vi=i^2r).
But it doesn't work like that. My textbook states that power loss = I^2R, but why?
Could someone explain please? thank you

Last edited by a moderator: Oct 26, 2016
2. Oct 25, 2016

Staff: Mentor

The unit of power is Watts...

3. Oct 25, 2016

victorhugo

Oh... Oops.

4. Oct 25, 2016

victorhugo

so e=pt?

5. Oct 25, 2016

victorhugo

also i still don't see the process where i can get power loss by I^2R...

6. Oct 25, 2016

Staff: Mentor

Yes, Energy = Power * Time
Can you say more about your specific question? The energy loss in wires leading to a load is I^2 * R.

7. Oct 25, 2016

victorhugo

"A power station generates electric power at 120kW. It sends this power to a town 10 km away through transmission lines that have a total resistance of 0.40 ohms. If the power is transmitted at 240 V, calculate:

a. the current in the transmission lines
b. the voltage drop across the transmission lines
c. the power loss in the transmission lines
d. the power loss in the transmission lines
"

I haven't gone over this stuff in a veryyy long time but it helps to understand it for my last course. I honestly don't get the way they do *any* of those questions, so a logical explanation to any assumed knowledge would definitely help :)

8. Oct 25, 2016

victorhugo

*btw this is not homework but me revising some basic stuff i need to know for my upcoming course*

9. Oct 25, 2016

Staff: Mentor

You have been here long enough to know that all schoolwork-type questions go in the Homework Help forums, and use the HH Template. Sigh.

10. Oct 25, 2016

victorhugo

Well it's been a while since i had to trouble doing homework questions so it didnt even pop into mind.
"A power station generates electric power at 120kW. It sends this power to a town 10 km away through transmission lines that have a total resistance of 0.40 ohms. If the power is transmitted at 240 V, calculate:

a. the current in the transmission lines
b. the voltage drop across the transmission lines
c. the power loss in the transmission lines
d. the power loss in the transmission lines
"

a. I don't like to pick out the formulas i have and plug in the numbers I have. I want to see how the formula works. So, current being C/s, I don't see what i can do here.
same for b, c and d.
please help

11. Oct 25, 2016

Staff: Mentor

You are given the power input and the resistance of the transmission line. What is the current to deliver that power at that voltage? What would be the power dissipated in the wires to deliver that current?

12. Oct 26, 2016

Chan Pok Fung

We actually get power from
$$P = εI$$
where ε is the emf. ε is the energy per unit charge. Current is the charge passing through per unit time. Therefore, the product of them means how much energy is passing through per unit time. You also have the relation between I, ε, R. You can then find the answer you want

13. Oct 26, 2016

Staff: Mentor

That's not helpful in this question.

Quiz Question -- Why not?

14. Oct 26, 2016

Staff: Mentor

Actually it can help if @victorhugo considers this fundamental enough to start with. The current delivered by the power source can be extracted from the source EMF and the power it generates, both given values:
So the power station is putting out 120 kW with an EMF of 240 V.

15. Oct 26, 2016

Windadct

In short the issue is P(w) = V(v) * I(a) Since V= I * R(ohms) we have
=( I * R ) * I = I^2R.

In this problem - for clarity it should state this is a single phase system, and V and I units in RMS values.

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