chapter 15 - electrostatics

Question 1

earthing

Answer 1

providing a conducting path for electric charges to flow

Question 2

electric field

Answer 2

a region in which an electric charge experiences an electric force

Question 2

electric line of force

Answer 2

path a positive charge would take if it was free to move

Question 3

charging by insulators

Answer 3

when rubbing a neutral polythene strip and a neutral piece of wool, electrons will be transferred from the wool to the polythene rod. the polythene rod becomes negatively charged while the piece of wool becomes positively charged

Question 4

obtaining oppositve charges on two conductors by induction

Answer 4

metal spheres A and B are in contact with each other. by bring a positively charged rod near A, electrons from B will be attracted to the left side of the sphere and be transferred from B to A. without removing the positively charged rod, separate the two spheres, then remove the rod.

Question 5

obtain negatively charged sphere

Answer 5

bring a positively charged rod close to the surface of the sphere, but not touching. electrons in the sphere will be induced to the left side of the sphere as unlike charges attract. connecting the sphere to an earth wire for earthing, electrons will flow up from the earth into the sphere, neutralizing the positive charges in the sphere. cut the earth-wire and remove the rod, leaving the sphere to be overally negatively charged

Question 6

obtain positively charged sphere

Answer 6

bring a negatively charged rod close to the surface of the sphere, but not touching. electrons in the sphere will be induced to the right side of the sphere as like charges repel. connecting an earth wire to the sphere for earthing, electrons will flow down from the sphere into the earth. cut the earth-wire and remove the rod, leaving the sphere to overally positively charged.

Question 7

neutralizing charges

Answer 7

place the charged insulators in a damp surrounding. electrons in the water molecules are transferred onto the charged insulator to neutralise the charges on the insulator.

heating the surrounding air allows the air molecules to lose electrons due to ionization, and the electrons will be attracted to the charged insulator, nuetralizing the charges.

Question 8

uses of electrostatics

Answer 8

electrostatic precipitator : wires are negatively charged to charge ash particles passing through. collector plates are positively charged to attract and collect the ash particles

spray painting : to achieve a even coating of paint, paint droplets from an aerosol become charged by rubbing against the nozzle of spray, and they obtain the same charge as the nozzle. the paint particles can evenly spread due to electrostatic repulsion.

Question 9

hazards of electrostatic

Answer 9

petrol tankers : friction between moving lorry and air can cause build up of electric charge. spark from electric charge can cause petrol vapour to explode. to prevent explosion, metal chain from lorry to ground is used for charge to flow into earth

operating theatres : gases used as anaesthesia are easily flammable. static charge due to rubbing of clothing or blankets. machines are linked to ground by metal chains. doctors wear anti-static conducting shoes.