Charge, current, voltage and resistance — GCSE Physics practice questions

Question 1

A student tests strips of different materials by connecting each one in turn into a circuit with a cell and a lamp.
The lamp lights with a strip of aluminium foil but not with a strip of plastic film.

Which statement explains this?

Accepted Answer

A: Aluminium contains electrons that are free to move through it; plastic does not.. Metals such as aluminium contain free (delocalised) electrons that drift through the metal when a p.d. is applied, so there is a current and the lamp lights. In plastic the electrons are all bound to their atoms or molecules, so no charge can flow and the lamp stays off.

Question 2

A small polystyrene ball coated with metal paint hangs from a nylon thread.
A negatively charged rod is brought close to the ball, as shown, and the ball swings towards the rod.

What can be concluded about the charge on the ball before the rod was brought near?

Accepted Answer

A: The ball was either positively charged or uncharged.. Unlike charges attract, so a positively charged ball would swing towards the rod. An uncharged ball would too: the negative rod repels free electrons to the far side of the ball, leaving the near side positive, and the attraction on the near side is larger than the repulsion on the far side. So the ball was either positive or uncharged.

Question 3

The heating element of a heated cycling jacket is a wire 1.2 m long with a diameter of 0.40 mm.
A designer replaces it with a wire of the same metal with a diameter of 0.60 mm.
The new wire must have the same resistance as the old one.

What length of the new wire is needed?

Accepted Answer

D: 2.7 m. The new wire's area is (0.60 / 0.40)² = 2.25 times larger, so each metre has 2.25 times less resistance. To keep the same resistance the wire must be 2.25 times longer: 1.2 × 2.25 = 2.7 m.