Why are voltages usually stepped-up by transformers before power is transmitted over large distances?

Question

Ok thanks....

And it's a physics question:)

Answer 1

When the voltage is stepped up, the current is stepped down at the same time.  

The only way of losing power in the transmission line is by heat, so the longer the distance the higher the power loss. (Power loss = I^2R). This can be reduced by either transmitting at the lowest possible current, and reducing the resistance of the transmitting cables (by making them thicker, but that will increase the cost).

So the most economical way is to step-up the from a low primary voltage (thus high I) to a high secondary voltage (low I) to reduce the amount of power loss in the form of heat.

Answer 2

Not sure whether you want to know why step up or why transformers.

Current is what costs you.

If you send current down a transmission line, then the current generates losses.  I^2R loss, (& some effects to do with inductance & such as well)

Also the wire melts if you put too much current through it, so higher current lines, cost more: the wire have to be thicker.

Higher voltage lines also cost more than lower voltage as the wire have to be further apart, or better insulated, but overall high voltage, lower current wins.

To send a certain amount of power down a line, you can choose I & V such that I*V = P.

If you make I small & V large, you can send the required power, but pay the least for current.

For tee other part: transformers are convenient & do the job better than anything else. :)

Physics or engineering question?