Circuit diagrams use standard symbols so anyone can read them. Learn these fourteen:

• A battery is two or more cells joined together. The long plate is the positive side.
• Arrows pointing out mean light leaves (LED). Arrows pointing in mean light arrives (LDR).
• An ammeter measures current. It goes in series, in the same loop as the component.
• A voltmeter measures potential difference. It goes in parallel, across the component's two ends.

Draw wires as straight lines with a ruler, and leave no gaps: a gap means the circuit is broken.

A current is a flow of electric charge. In a metal wire the charge is carried by electrons. For charge to flow you need a closed circuit and a source of potential difference, such as a cell.

Current is the rate of flow of charge: how many coulombs pass a point each second.

charge flow = current × time
Q = I t

Charge in coulombs (C), current in amperes (A), time in seconds (s). You must recall this one for AQA, Edexcel and OCR: it is not on the equation sheet.

1. A lamp has a current of 0.4 A for 30 s. Charge?
2. Q = I × t
3. Q = 0.4 × 30 = 12 C

In a single closed loop the current is the same at every point. Charge is not used up by a lamp: it carries energy to the lamp and keeps going.

Questions hide the work in the units. Change everything to A and s before you multiply.

1. An LED takes 250 mA for 2 minutes. Charge?
2. 250 mA = 250 ÷ 1000 = 0.25 A
3. 2 minutes = 2 × 60 = 120 s
4. Q = 0.25 × 120 = 30 C

To find a current, rearrange: I = Q ÷ t. To find a time: t = Q ÷ I.

The classic slip is leaving the time in minutes. If a question says minutes or hours, convert to seconds first, then convert back at the end if it asks for minutes.