Homework Help: Increase in voltage = decrease in current?

1. May 24, 2012

chwl3

Increase in voltage = decrease in current??

1. The problem statement, all variables and given/known data
Electric power is transferred over large distances at very high voltages. Explain how the high voltage reduces power losses in the transmission lines.

2. Relevant equations
V = IR
P = VI

3. The attempt at a solution
From searching on the web, many people say increase in voltage will naturally lead to decrease in current, thus reducing the power losses in the lines. With those equations, I cannot understand why a rise in voltage would lead to a drop in current, and not rise? Also, how does drop in current reduce the power loss?

2. May 25, 2012

azizlwl

Re: Increase in voltage = decrease in current??

p=I2R

The line with resistance will consume power too. If less current means less power loses due to line resistance.

P=IV
With same power usage, either we increase current and decrease voltage(means more power consume by the lines) or more voltage less current.

3. May 25, 2012

truesearch

Re: Increase in voltage = decrease in current??

With these equations you can show the ida with a numerical example:
Suppose you need to supple 2kW of electrical power along cables having a total resistance of 1 ohm.
If the supply voltage is 1000V then the current needs to be 2A and the power wasted in the cables is I^2.r = 4 x 1 = 4Watts....negligible!!
If the supply voltage is 100V then the current needs to be 20A and the power loss = 400W...a 10 times lower voltage means 100 times greater power loss.
The principle is to use the greatest possible voltage for transmission but for safety and convenience reasons the voltage at the generator and the user should be low.
This means some way of 'stepping up' voltage for transmission and then 'stepping down' voltage for use is needed. This stepping up and stepping down is easilydone with transformers and this is why electricity is generated as AC....so that transformers can be used.

4. Sep 16, 2012

asitiaf

Re: Increase in voltage = decrease in current??

more voltage means more power dissipation in transmission

5. Sep 17, 2012

Jobrag

Re: Increase in voltage = decrease in current??

No because you don't drop your entire voltage over the transmission line.

6. Sep 17, 2012

CWatters

Re: Increase in voltage = decrease in current??

chwl3 - transmission lines are used to transmit power from one area to another.

Power transmitted = Voltage * Current...............(1)

So you can choose any combination of voltage and current that meets the above equation.

The losses in the cable are

Power Loss = current2 * resistance .............(2)

So there is an incentive to use a low current and high voltage in (1).

7. Sep 17, 2012

asitiaf

I think that
P=VI; so, for constant power voltage and current are inversely proportional.
P=I^2R; it is the power dissipated (loss)
during transmission. So, lesser the current, lesser the dissipation and more power delivered.
P=V^2/R; it is the power delivered (carried) during transmission. So, more the voltage, more the power delivered.
Please confirm, whether I am right.