# Homewrk: electrical

1. Nov 6, 2006

### nekteo

a cable with total resistance of 2 ohms used to transfer 15000W of electrical power with electrical potential of 10000 V.
How much electrical energy is lose in the cable after transferring 2 hours of electrical power?

------------this is how my teacher taught me, but still confused----------
P = VI
15000 = 10000 I
I = 1.5 A

V = IR
V = 1.5 * 2
V = 3

E = VIt
E = 3 * 1.5 * (2 * 60 * 60)
E = 32400 J
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What i don't understand is, since the question has provided me with 10000V, why should i find another voltage (1.5V)? It's impossible for a cable to have 2 completely different voltage (1.5V and 10000V) is it?
My question is, what is 1.5V and 10000V?:yuck:
Why cant i just use,
E = Pt
E = 15000 * (2 * 60 * 60)?

HELP!!!!!!!!!!!!!!!!!! I know my English is not good enough... sorry for that.

2. Nov 6, 2006

### chroot

Staff Emeritus
The "1.5" is not a voltage, it is a current. 1.5 amperes (1.5 A) means that 1.5 coulombs of charge pass by a given point in the wire every second.

You cannot use power alone to find transmission loss, because transmission loss depends only on current, not on power. If you use a higher voltage to transmit the same power, you will have smaller transmission losses.

- Warren

3. Nov 6, 2006

### nekteo

i'm sorry, erm... not 1.5, it's the 3V actually...(i'm confused!!)
what is the voltage of 3 and 10000

The question told me the voltage is 10000, y still need to use formula, V = IR to find the V which is 3V?

What is the differences between the value of voltage provided by the question and the voltage get from the V = IR?

My physic is terrible huh??...

Last edited: Nov 6, 2006
4. Nov 6, 2006

### chroot

Staff Emeritus
The cable loses 3V over it's length. At one end, the voltage is 10,000V; at the other it's 9,997V.

- Warren

5. Nov 8, 2006

### nekteo

o... now I get it! thanks for your help...