O Level / IGCSE Physics · Electricity and magnetism. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
There are two kinds of charge, positive and negative, measured in coulombs. Like charges repel and unlike charges attract. Rubbing two insulators together moves electrons from one to the other: the one that gains electrons becomes negative and the one that loses them becomes positive.
Current is the charge passing a point each second. In a metal it is a flow of free electrons from negative to positive, but conventional current is drawn from positive to negative.
The e.m.f. of a source is the work it does per coulomb in driving charge round the whole circuit. The potential difference across a component is the work done per coulomb passing through it. Resistance tells you how much p.d. is needed for each ampere of current.
Saying that an object becomes positive because it gains positive charge. Only electrons move when solids are rubbed, so a positive object is one that has lost electrons.
Answer: 24 Ω
Try each one, then open the answer.
Answer: B. The filament heats up; the metal ions vibrate more and impede the electrons, so resistance rises. This is why the I–V graph of a lamp curves.
Answer: B. I = Q ÷ t = 60 ÷ 20 = 3.0 A.
Answer: B. 250 mA = 0.25 A and 2.0 min = 120 s. Charge = current × time = 0.25 × 120 = 30 C. Leaving the current in mA gives 30 000 C; leaving the time in minutes gives 0.50 C.
← Simple magnetism and magnetic fieldsElectric circuits →All rules on one pageStuck? Ask a question