We are familiar with air current and water current. We know that flowing water constitute water current in rivers.
Similarly, if the electric charge flows through a conductor (for example, through a metallic wire), we say that there is an electric current in the conductor. In a torch, we know that the cells (or a battery, when placed in proper order) provide flow of charges or an electric current through the torch bulb to glow.
We have also seen that the torch gives light only when its switch is on. What does a switch do? A switch makes a conducting link between the cell and the bulb.
`"A continuous and closed path of an electric current is called an electric circuit."`
Now, if the circuit is broken anywhere (or the switch of the torch is turned off ), the current stops flowing and the bulb does not glow. How do we express electric current?
`"Electric current is expressed by the amount of charge flowing through a particular area in unit time."`
In other words, it is the rate of flow of electric charges. In circuits using metallic wires, electrons constitute the flow of charges.
However, electrons were not known at the time when the phenomenon of electricity was first observed.
So, electric current was considered to be the flow of positive charges and the direction of flow of positive charges was taken to be the direction of electric current.
Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.
If a net charge Q, flows across any cross-section of a conductor in time t, then the current I, through the cross-section is
`I = Q/t` ........(12.1)
The `SI` unit of electric charge is coulomb (C), which is equivalent to the charge contained in nearly `6 xx 10^(18)` electrons. (We know that an electron possesses a negative charge of `1.6 xx 10^(–19) C`.)
The electric current is expressed by a unit called ampere (A), named after the French scientist, Andre-Marie Ampere . One ampere is constituted by the flow of one coulomb of charge per second, that is, `1 A = 1 C// 1 s`.
Small quantities of current are expressed in milliampere `(1 mu A = 10^(–3) A)` or in microampere `(1 μA = 10^(–6) A)`. An instrument called ammeter measures electric current in a circuit.
It is always connected in series in a circuit through which the current is to be measured. Figure 12.1 shows the schematic diagram of a typical electric circuit comprising a cell, an electric bulb, an ammeter and a plug key.
`"Note"` that the electric current flows in the circuit from the positive terminal of the cell to the negative terminal of the cell through Figure 12.1 the bulb and ammeter.