Question

how is electric field is measured(which formula) near power lines where we have moving charges? is it depends on length of a power cable

Answer 1

Using Faraday's Law if we wrap a coil around the power line, we can measure the electric field near the power line using the following equation:
$$
\Large{
\oint_{Closed~Loop}\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\overline{E}\cdot\overline{dl}=-\cfrac{d}{dt}\oint_{Closed~Surface}\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\overline{B}\cdot\overline{dA}
}
$$
Since charges induce currents and if currents are also moving (which it will be for AC) we will have a changing magnetic field, which will, according to Faraday's Law, induce an electric field in the coil we've placed in the vicinity.

What we need to know is the perimeter of the loop of wire and the rate of change of the magnetic field enclosed within the open surface of this loop.