This topic is the starting point for all of electricity. You learn how objects become charged and how the flow of charge is measured as a current. You also learn why current direction is a convention, and how to draw a circuit that anyone can read.
The exact content and depth depend on your syllabus year, so check the current Cambridge IGCSE Physics 0625 page for your exam year. The Physics learning guide shows where this module sits in the whole subject.
What do I need to know first?
You need the idea that everything is made of atoms with a positive nucleus and negative electrons around it. You also need comfortable unit conversion between seconds, minutes and hours, and between milliamperes and amperes. The measurement habits in measurement and quantities help here.
The next module, potential difference, resistance and circuits, assumes everything on this page.
One orienting worked example
A steady current of 0.30 A flows through a lamp for 20 s. (Invented example data.) How much charge passes through the lamp?
Step 1, choose the relationship: current is charge divided by time, I = Q ÷ t, so Q = I × t.
Step 2, check the units: current is in amperes (A), time is in seconds (s) and charge comes out in coulombs (C).
Step 3, calculate: Q = 0.30 × 20 = 6.0 C.
Step 4, check: 6.0 C ÷ 20 s = 0.30 A, which matches the current we started with.
Every lesson here is a variation on that pattern: say what is moving, pick the relationship, use compatible units, then check.
In what order should I study the lessons?
- Explain charging through electron transfer: where charge comes from, and why only electrons move.
- Calculate current from charge and time: the core equation and the unit habits it needs.
- Compare conventional current and electron motion: why the arrows on a diagram point the “wrong” way for electrons.
- Use a circuit symbol consistently: how to draw and read a circuit without ambiguity.
- Distinguish current from charge stored: separates a rate of flow from an amount that sits on an object.
Then test yourself with the charge and current practice set.
Which traps catch most students?
- Using minutes in I = Q ÷ t when time must be in seconds.
- Saying that protons move in a wire, or that a rod gains positive charge by gaining protons.
- Saying that current is “used up” by a lamp, so a later ammeter reads less.
- Drawing conventional current arrows from the negative terminal.
- Treating charge (in coulombs) and current (in amperes) as the same thing.
How should I use the practice set?
Write each equation and unit on the answer line, then open the worked answer. Use the routing section at the end to return to the right lesson. Afterwards, add any repeated slip to the mistake log and retest queue so it returns later as a fresh question.
The circuit reasoning simulator, described here, shows ideal low-voltage circuits on screen and is a safe place to test your reading of a diagram. If you understand each idea but still lose marks on units or current direction, a teacher can watch your working and fix it. That is the work of an assigned teacher in online one-to-one Physics tuition.