ElectricityElectric Current and Circuit
You already know two kinds of current: an air current, which we feel as wind, and a water current, the flow of water in a river. In the same way, when electric charge flows through a conductor such as a metal wire, we say there is an electric current in the conductor.
A torch is the simplest example. Its cells, placed the right way round, make charge flow through the bulb, and the bulb glows — but only while the switch is on. A switch simply makes or breaks a conducting link between the cell and the bulb. A continuous and closed path along which an electric current flows is called an electric circuit. Break the path anywhere, or turn the switch off, and the current stops at once.
Build one yourself: a circuit with a cell, a bulb, an ammeter and a plug key. Tap a part, then the gap it belongs in.
Notice two things. First, the order of the parts did not matter: in a single loop, every part carries the same current wherever it sits. Second, the bulb lit up the moment you closed the key, and went dark the moment the loop was broken.
How Much Charge Flows?
How do we measure a current? We count how much charge crosses a cross-section of the wire every second. Electric current is the rate of flow of electric charge. If a net charge flows across any cross-section of a conductor in time , the current through it is
The SI unit of charge is the coulomb (C). The unit of current is the ampere (A), named after the French scientist André-Marie Ampère (1775–1836). One ampere is a flow of one coulomb of charge per second, that is, . Small currents are measured in milliamperes and microamperes:
In a metal wire the charge is carried by electrons. Each electron carries a negative charge of C. So the number of electrons in one coulomb is
Look back at your glowing circuit. The blue arrows point out of the + terminal of the cell, through the bulb and the ammeter, and back into the − terminal: that is the direction of the electric current. But the grey electrons drift the
Why the mismatch? When electricity was first studied, nobody knew about electrons. Current was thought of as a flow of positive charge, and the direction in which positive charge would move was taken as the direction of the current. That choice stuck. So, by convention, the direction of electric current is opposite to the direction of flow of electrons, which carry negative charge.
Note: measuring current
An ammeter measures the current in a circuit. It is always connected in series, in the loop itself, so that the whole current passes through it — exactly as you placed it in the circuit above.
Example 11.1 : The filament of an electric bulb draws a current of 0.5 A for 10 minutes. Find the amount of electric charge that flows through the circuit.
Solution : We are given A and min
From , the charge is