Lesson 1 of 12 · 7 min

Electric current

NCERT § "Electric Current"

Riya switches the heater on and the meter says 5 A. Five of what, passing where, every second?

The story this chapter follows: Riya's room on a winter night

Riya studies late in a room on the 220 V supply. A 1100 W heater draws 5 A through 10 m of 1.5 mm² copper wire from the switchboard, a 60 W and a 100 W bulb light the desk, and an emergency torch with 1.5 V cells waits for the power cut. Every idea in this chapter, from drifting electrons to a balanced bridge, happens somewhere in that room.

The lesson in notes

In short

Electric current through a cross-section is the net charge crossing it per unit time: I = q/t for a steady current, or I = dq/dt in general.

The SI unit is the ampere (A), a base unit; 1 A corresponds to 1 C of charge passing per second.

By convention the direction of current is the direction in which positive charge would flow, opposite to the motion of electrons in a metal.

Current has a direction along the wire but is a scalar, because currents at a junction add algebraically, not by vector addition.

Examples span a huge range: lightning carries very large currents for a short time, while currents in electronic devices can be of the order of microamperes or less.