Current Electricity

The idea

Static electricity is charge sitting still. Current electricity is charge flowing steadily — and that flow can do useful work.

Quantity	Symbol	Unit	Measures
Current	$I$	ampere (A)	Rate of charge flow
Potential difference	$V$	volt (V)	Energy given per unit charge
Resistance	$R$	ohm ($\Omega$)	Opposition to flow
Charge	$Q$	coulomb (C)	Amount of charge

$$I=\frac{Q}{t}$$

If $12\mathrm{C}$ of charge pass a point in $4.0\mathrm{s}$:

$$I=\frac{12\mathrm{C}}{4.0\mathrm{s}}=3.0\mathrm{A}$$

The water analogy, and where it breaks

Voltage is like pressure, current like flow rate, resistance like a narrow pipe. It is a good analogy — but water can spill out of an open pipe, while charge cannot flow at all unless the circuit is complete. Every analogy has an edge; know where yours is.

Conventional current

By convention, current is drawn flowing from + to −, even though electrons actually drift the other way. Franklin guessed the sign before anyone knew electrons existed, and the convention stuck.

Curriculum connection

D2.3

identify the components of a direct current (DC) circuit and explain their functions, and identify electrical quantities, their symbols, and their corresponding International System of Units (SI) units

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D2.7

explain the difference between electricity and electrical energy

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