In an experiment to investigate electrostatic charge, a student rubs a polythene rod with a dry woollen cloth. The charged rod is then brought near the cap ...

(a) The gold leaf rises (deflects away from the central rod). [1]

The charged rod induces a redistribution of charge in the electroscope: electrons in the cap are repelled down the rod and onto the leaf, giving both the rod and the leaf the same sign of charge, so they repel.

(b)

[1] for polythene correct. [1] for both glass and copper correct.

Polythene gains a negative charge and glass gains a positive charge when rubbed, both of which cause the leaf to rise. Copper is a conductor, so any charge produced immediately flows away through the student's hand, and the leaf shows no response.

(c) When polythene is rubbed with wool, friction transfers electrons from the wool onto the polythene rod. [1] The polythene gains extra electrons and therefore acquires an overall negative charge. [1]

(d) Any one valid improvement: repeat each test three times and check for consistency / ensure the rod is fully discharged (e.g. by touching it to a metal surface) before each test / carry out the experiment on a dry day or in a dry environment to reduce charge leakage through moisture. [1]

(e) A glass rod rubbed with silk acquires a positive charge. [1]

Electrons are transferred from the glass to the silk during rubbing, leaving the glass with a deficit of electrons and hence a net positive charge.