A transformer is an electrical gear box. Changing transformer taps is like shifting gears in your car. You have a given amount of power and that power can be expressed as a lot of force and a little speed, or, a lot of speed and a little force, a lot of voltage and a little current, or, a lot of current and a little voltage. Voltage is electrical pressure and current is electrical flow rate. The product of the two remains the same, save losses to inefficiency.

If the voltage goes up, then (for a constant resistance) the current goes up too- Ohm's law.

Power in = power out for an ideal transformer. If the secondary *voltage* is *higher* than the primary voltage, then the secondary *current* must be *lower* than the primary current for the power (i*V) to be the same. But one current is in the primary, and one is in the secondary. In either side individually, a higher voltage leads to a higher current for a given resistance.

It's simple...power does not appear out of thin air, so if you use an ideal transformer to (say) double the voltage, no extra power is created, so the current must halve. If you input 100 volts at 15 amps, that's 1500 watts (V*I...100*15 = 1500). If the output is 200 volts, the current must be 7.5 amps (V*I...200*7.5 = 1500).

There is no such thing as a free lunch!

i know p=v*i*cos(t) ... but how is it relevant to ohm's law ?

p=v*i*cos(phi) is a formula for calculating power in an AC circuit. where theta is the

For a resistive load you can use ohms law to work out the current.

Heres an example

A 100 ohm resistor is connected across a 240V AC supply. What current will be drawn?

I = V/R = 240/100 = 2.4A

How much power will this use?

p=v*i*cos(phi)= 240 *2.4 * 1 = 576 Watts

phi - the phase angle - is 1 for a resistive load.

i know that p=v*i*cos(t) ... but how is it relevant to ohm's law .. I mean if you increase potential drop across a conductor , then how come the current flowing through it gets lower down .This doubt comes to my mind every time i read power transmission or transformers.. but i m unable to relate the two laws ..