Electric Current - Definition and SI unit
ELECTRIC CURRENT
The motion of electric charges (electrons) through a conductor (e.g., copper wire) will constitute an electric current. This is similar to the flow of water through a channel or flow of air from a region of high pressure to a region of low pressure.
In a similar manner, the electric current passes from the positive terminal (higher electric potential) of a battery to the negative terminal (lower electric potential) through a wire as shown in the Figure 4.1.
1. Definition of electric current
Electric current is often termed as ‘current’ and it is represented by the symbol ‘I’. It is defined as the rate of flow of charges in a conductor. This means that the electric current represents the amount of charges flowing in any cross section of a conductor (say a metal wire) in unit time. If a net charge ‘Q’ passes through any cross section of a conductor in time ‘t’, then the current flowing through the conductor is
I = Q/t
2. SI unit of electric current
The SI unit of electric current is ampere (A). The current flowing through a conductor is said to be one ampere, when a charge of one coulomb flows across any cross-section of a conductor, in one second. Hence,
Solved Problem-1
A charge of 12 coulomb flows through a bulb in 5 second. What is the current through the bulb?
Charge Q = 12 C, Time t = 5 s. Therefore, current I = Q/t = 12/5 = 2.4 A
ELECTRIC CIRCUIT
An electric circuit is a closed conducting loop (or) path, which has a network of electrical components through which electrons are able to flow. This path is made using electrical wires so as to connect an electric appliance to a source of electric charges (battery). A schematic diagram of an electric circuit comprising of a battery, an electric bulb, and a switch is given in Figure 4.2.
In this circuit, if the switch is ‘on’, the bulb glows. If it is switched off, the bulb does not glow. Therefore, the circuit must be closed in order that the current passes through it. The potential difference required for the flow of charges is provided by the battery. The electrons flow from the negative terminal to the positive terminal of the battery.
By convention, the direction of current is taken as the direction of flow of positive charge (or) opposite to the direction of flow of electrons.
Thus, electric current passes in the circuit from the positive terminal to the negative terminal.
Electrical components
The electric circuit given in Figure 4.2 consists of different components, such as a battery, a switch and a bulb. All these components can be represented by using certain symbols. It is easier to represent the components of a circuit using their respective symbols.
The symbols that are used to represent some commonly used components are given in Table 4.1. The uses of these components are also summarized in the table.
