Electricity has an important place in modern society. It is a controllable and convenient form of energy for a variety of uses in homes, schools, hospitals, industries and so on.

What constitutes electricity? How does it flow in an electric circuit? What are the factors that control or regulate the current through an electric circuit? In this Chapter, we shall attempt to answer such questions.

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.

What makes the electric charge to flow? Let us consider the analogy of flow of water.

Charges do not flow in a copper wire by themselves, just as water in a perfectly horizontal tube does not flow. If one end of the tube is connected to a tank of water kept at a higher level, such that there is a pressure difference between the two ends of the tube, water flows out of the other end of the tube.

For flow of charges in a conducting metallic wire, the gravity, of course, has no role to play; the electrons move only if there is a difference of electric pressure called the potential difference along the conductor.

This difference of potential may be produced by a battery, consisting of one or more electric cells. The chemical action within a cell generates the potential difference across the terminals of the cell, even when no current is drawn from it.

When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current.

In order to maintain the current in a given electric circuit, the cell has to expend its chemical energy stored in it.

We define the electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other :-

Potential difference (V) between two points = Work done (W)/Charge (Q)

` V = W//Q` .......(12.2)

The SI unit of electric potential difference is volt (V), named after Alessandro Volta, an Italian physicist. One volt is the

potential difference between two points in a current carrying conductor when `1` joule of work is done to move a charge of `1` coulomb from one point to the other.

Therefore, `1` volt `= text( 1 joule)/text ( 1 coulomb)` (12.3)

`1 V = 1 J \ \ C^(–1)`

The potential difference is measured by means of an instrument called the voltmeter. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.

We know that an electric circuit, as shown in Fig. 12.1, comprises a cell (or a battery), a plug key, electrical component(s), and connecting wires.

It is often convenient to draw a schematic diagram, in which different components of the circuit are represented by the symbols conveniently used.

Conventional symbols used to represent some of the most commonly used electrical components are given in Table 12.1.