Exercises: Charging and Conservation of Charge

A student in Galway rubs a polythene rod with a woollen cloth. The rod becomes negatively charged.

1. a) Explain, in terms of electrons, how the rod became negatively charged.
2. b) What charge, positive or negative, does the woollen cloth acquire? Justify your answer using the principle of conservation of charge.

State whether each of the following materials is normally classified as a conductor or an insulator: copper, polythene, dry wood, aluminium, glass, graphite. Explain in one sentence why conductors allow charge to move through them.

A negatively charged rod is brought near, without touching, the cap of an uncharged gold-leaf electroscope.

1. a) Describe what happens to the gold leaf.
2. b) Explain the observation in terms of the movement of electrons in the electroscope.

Two identical small metal spheres on insulating stands have charges of $+6.0\ \mu\text{C}$ and $-2.0\ \mu\text{C}$. They are brought into contact and then separated.

1. a) Calculate the charge on each sphere after separation.
2. b) State the principle you used.
3. c) How many electrons were transferred, and in which direction? Use $e = 1.6\times10^{-19}\ \text{C}$.

A student at a school in Cork wants to charge a metal sphere on an insulating stand positively by induction, using a negatively charged rod.

1. a) Describe, step by step, the procedure, including the role of the earth connection.
2. b) Explain why the rod loses none of its charge in this method.
3. c) Explain why the sphere ends up with the opposite charge to the rod.

A hollow metal sphere carries a net charge of $-8.0\ \text{nC}$. A small uncharged metal ball is touched to the outside of the sphere and then removed.

1. a) Explain why the ball picks up charge from the outside surface.
2. b) Where does the excess charge on a conducting sphere reside?
3. c) The charge is on the surface and is more concentrated at a sharp point on a charged conductor. Explain how this is used in a lightning conductor.

Three identical metal spheres A, B and C are on insulating stands. A carries $+9.0\ \mu\text{C}$, B carries $-3.0\ \mu\text{C}$ and C is uncharged. A is touched to B and separated. Then A is touched to C and separated.

1. a) Calculate the final charge on each sphere.
2. b) Verify that the total charge is conserved.
3. c) Calculate the number of electrons by which sphere C's charge differs from neutral, and state whether C has gained or lost electrons. Use $e = 1.6\times10^{-19}\ \text{C}$.

A petrol tanker in Dublin Port is earthed before filling. A technician explains that friction between the fuel and the pipe can build up charge.

1. a) Explain how charge builds up by friction.
2. b) Explain the danger of charge accumulating on the tanker.
3. c) Explain how the earth connection removes the danger, stating the direction of electron flow if the tanker is positively charged.
4. d) Why is the pipe nozzle often made of a conducting material rather than an insulator?

A positively charged rod is brought near an uncharged gold-leaf electroscope, and the leaf rises. While the rod is still near, the cap is touched briefly with a finger, then the finger is removed, and then the rod is removed.

1. a) Explain the behaviour of the leaf at each of the four stages: rod approaching, finger touching, finger removed, rod removed.
2. b) State the sign of the final charge on the electroscope and justify it.
3. c) Explain why bringing a negatively charged rod near the electroscope afterwards would cause the leaf to diverge further, and why this is a more reliable test for the sign of the charge than bringing up a positive rod (which would make the leaf fall).