Answer:

Explanation:
Given data
Ex = Ey = 0
Ez=(4.3V/m)cos(π×10¹⁵s⁻¹)(t-x/c)
To find
The amplitude of the magnetic field component
Solution
The electric field is given by:
Ez=(4.3V/m)cos(π×10¹⁵s⁻¹)(t-x/c)
Where Em=4.3 V/m
The magnitude of magnetic field is given by:

Where Em is amplitude of electric field and c is speed of light.
Substitute the given values
So
The heat energy released from a piece of wire or any other section of a circuit is:
Energy = (voltage between its ends) x (current through it) x (time it's been going)
light speed in vacuum = 3.8 * 10^8
Distance (Given) = 3.5 ft
now, time = distance/speed = 3.50 / 3.8 * 10^8 = 9.21 * 10^-9 s = 9.21 nanoseconds
So, your answer is 9.21 nano-seconds...
The return stroke (the current that cause the visible flash) moves upward at a speed of about 320,000,000 ft per second or about 220,000,000 miles per hour (about 1/3 the speed of light).