x Turn on thread page Beta
 You are Here: Home >< Physics

# Power lost as heat in a cable. watch

1. Input voltage: 400kV.
Current carried: 440A.
Resistance per Km: 0.058 Ohms.

The input power to one cable = VI.

Yet for some reason, the power wasted as heat = I^2 x R.

According to the formula booklet, P = VI and P = I^2 x R.

So shouldn't VI = I^2 x R?

Please could someone help explain this, and explain how I would normally find the power lost as heat in a cable.
2. Imagine the cable had no resistance. Then it could carry all the input power to its destination without loss. The power loss occurs because the resistance of the cable imposes a voltage drop proportional to the current carried. So the 'V' you need in P=VI is the voltage drop in the cable. Which is why it's easier to use I^2R. You'll get an answer in Wm^-1

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 9, 2011
Today on TSR

### Unconditional offer...

But going to get a U!

### Can I date a girl with no boobs?

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE