Some Questions from Electrostatics posted by visitors

1. An infinite no. of charges each equal to 4 micro coulomb are place along the x-axis at x=1m,2m,4m and so on.find the electric field at the origin due to given sets of charges.


2. What is meant by a cavity and what is the electric field on the circumference of a cavity whose centre is at a distance 'a' from the centre of a positively charged sphere of radius 'r' and 'rho' volume charge density?


3. when a steady current flows through a conductor, the electron in it move with a certain drift speed which can be calculated for a typical copper wire carrying a small current ( ~ 1 mm/s) which is very small. How is it then that an electric bulb lights up as soon as we turn the switch on?  Please explain

http://plustwophysics.com/differences-between-mass-and-charge/