The brilliance of lamps in a room noticeably drops as soon as a high power electric iron is switched on and after a short interval , the bulbs regain their original brilliance . Why ?

Ans. The cold resistance of the coil of the iron is much smaller than when it is hot . So when the iron is switched on , there is a large drop of voltage and consequently the bulb do not receive the proper voltage . As the coil gets heated , its resistance increases and so the voltage drop is made up when it is fully heated . This is why after some time the bulbs regain their brilliance .

When moulded plastic parts are removed from metal dies , they develop a high voltage . Why ?

Ans : When the plastic parts is removed , the capacitance of the metal dies decreases but the charge (developed by friction ) remains unchanged and so the voltage increases in compliance with the formula Q = CV

A metal box is placed in a space which has an electric field .What is the field inside ? Explain

Ans : The field inside is zero (there is electrostatic screening )

Q . An electron moves along a metal tube with variable cross – section . How will its velocity change when it approaches the neck of the of the tube ?

Ans : Positive induced charges will appear on the neck of the tube and accelerate the electron

An inflated balloon is charged by rubbing it with fur . Will it stick readily to a conducting wall or to an insulating wall ? Explain

Ans : It will stick readily to the conducting wall because the same amount of charge is induced on the wall . But on the insulating wall , less amount of charge is induced . Due to greater force of attraction , it will stick more readily to the conducting wall.