IGCSE Physics / Current and voltage in circuits
Current is the rate of flow of charge revision notes
Edexcel International GCSE Physics (4PH1), chapter 7: Current and voltage in circuits, specification point 2.14, 2.15, 2.16. Free lesson.
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Current is the rate at which electric charge flows past a point. You can calculate charge, current or time using Q = I × t, and charge is not used up as it passes through a lamp.
What current measures
A current tells you how much charge passes a point each second. In a metal wire, the moving charges are negative electrons. One amp means one coulomb of charge passes each second. Charge is measured in coulombs (C). Current is measured in amps (A). 1 A means 1 C of charge flowing past each second.
Which way current flows
In circuit diagrams, conventional current is shown flowing from the cell’s positive terminal to its negative terminal around the circuit. The cell’s long line is positive and its short line is negative. Electrons in the wire move the opposite way. Conventional current: from + to −. Electron flow: from − to +. Both directions can be described, as long as you say which one you mean.
Why current is not used up
Charge is not destroyed when it passes through a lamp. The same number of electrons leave the lamp each second as arrive, so the current is the same on both sides. The lamp transfers energy, not current. Current is not used up anywhere in the circuit. The lamp lights because it transfers energy.
Using charge, current and time
The equation Q = I × t links charge, current and time. Time must be in seconds before you use the equation. The equation is on the equation list, but you need to know what each symbol means. Q is charge in coulombs (C). I is current in amps (A). It uses the letter I, not C. t is time in seconds (s). Convert minutes to seconds by multiplying by 60. Convert hours to seconds by multiplying by 3600.