Electricity

Lesson

Part 1 — what flows in a wire, what pushes it, how we draw and measure circuits, and the simple rule that links them: Ohm’s law.

In this lesson you will

Define electric current and potential difference and state their SI units
Calculate charge, number of electrons and work done using Q = It and V = W/Q
Draw circuit diagrams with standard symbols and connect an ammeter and a voltmeter correctly
State Ohm’s law and find resistance from a V–I graph

Concept

Electric current: the rate of flow of charge

If a net charge $Q$ flows through any cross-section of a conductor in time $t$ , the electric current is $I = \dfrac{Q}{t}$ . Its SI unit is the ampere (A): 1 A = 1 C/s.

I = \frac{Q}{t}

Table: charge Q is measured in coulombs; current I in amperes, with milliampere equal to one thousandth and microampere one millionth of an ampere
Quantity	Symbol	SI unit	Smaller units
Charge	$Q$	coulomb (C)	—
Current	$I$	ampere (A)	1 mA $= 10^{-3}$ A, 1 μA $= 10^{-6}$ A

Diagram

Electrons move one way, current is shown the other

In metal wires the moving charges are electrons. By convention, electric current flows from + to −, opposite to the flow of electrons.

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