Concept First Physics and Math Learning Centre In Singapore

1.    How does an ebonite rod become negatively charged when it is rubbed with a duster? What is the charge of the duster and how does it acquire that charge?

• The ebonite rod gains electron from the duster and becomes negatively charged as it now has an excess of electrons.

• The duster lose electrons to the ebonite and becomes positively charged as it now has more protons than electrons.

2.    Two plastic rods repel each other after they have been rubbed with a cloth. Suggest why the plastic rods repel each other.

• Rubbing the rods with a cloth cause them to acquire the same charges.

• Since like charges repel each other, the two rods will repel each other.

3.    A positively charged rod is brought near a piece of paper. Explain why the paper is attracted to the rod and sticks to it.

• When the positively charged rod is brought near the neutral paper, it induces a negative charge on the side of the paper nearer to the rod.

• Since unlike charges attract each other, the paper will be attracted to the rod.

• When the paper touches the rod, it does not get discharge as it is an insulator.

• The paper remains attracted to the rod.

4.    A negatively charge metal ball suspended on an insulating thread is brought near a neutral metal sphere held on an insulated stand. State and explain what will happen to the metal ball.

• The negatively charged metal ball will repel the electrons in the sphere to the far right side, hence inducing a positive charge in the near left side.

• Since unlike charges attract, the negatively charge ball will now be attracted to the positively charged side of the sphere.

State and explain what will happen to the ball if the metal ball is allowed to touch the sphere.

• Since both are conductors, the excess negative charges (electrons ) in the ball will flow to the sphere, and be redistributed among the ball and sphere.

• Both the ball and sphere will now be negatively charge.

• Since unlike charges repel, the ball will repel from the sphere.

5.    A positively charged rod is brought near a metal sphere held on an insulated stand. State and explain the movement of electrons in the sphere that occurs as the rod is brought near.

• The electrons, being negatively charged, are attracted to the positively charged rod.

• This leaves the left side of the sphere negatively charged and right side positively charged.

The metal sphere is earthed and the metal ball removed thereafter. State and explain what happens.

• Electrons will flow from the ground, through the earth wire to the right side of the sphere, discharging the right side of the sphere.

• When the positively charged rod is removed, the negative charges will be redistributed. A negatively charged sphere is obtained.

*Name this process.

• Charging by induction.

6.    Describe how the spray painting of objects makes use of electrostatic principle. State the advantages of using this method of spray painting.

• As the paint leaves the nozzle, the particles of paint become charged by friction.

OR

The nozzle is connected to a positive (negative) terminal so that paint that passes through it will become positively (negatively) charged.

• The paint particles being likely charged, will repelled one another, spreading out when being sprayed on an object.

• The charged paint particles will be attracted to the earthed object by induction.

• This method of spray painting ensures uniform coating of paint and reduces wastage of paint.