CBSE PHYSICS: ELECTRIC CURRENT AND CIRCUIT+ Revision Notes

CBSE PHYSICS Notes
ELECTRIC CURRENT AND CIRCUIT

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. 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

SI unit of electric charge = coulomb (C)= 6 X 10¹⁸ electrons

Electric Current is also showed by Ampere (A)

1 A = 1 C/1s or flow of one coulomb charge per second.

Instrument Measuring Electric Current is called Ammeter. It is always connected in series in a circuit

ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE

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

The SI unit of electric potential difference is volt (V),

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.

OHM’S LAW

Georg Simon Ohm (1787–1854) The electric current flowing through a metallic wire is directly proportional to the potential difference V, across its ends provided its temperature remains the same. This is called Ohm’s law. In other words –

v µ I

or V/I = constant = R

or V = IR

R is a constant for the given metallic wire at a given temperature and is called its resistance. It is the property of a conductor to resist the flow of charges.

As per Ohm’s law the resistance of the conductor depends

(i) on its length,

(ii) on its area of cross-section, and

(iii) on the nature of its material.

The SI unit of resistivity is Ω m. It is a characteristic property of the material. The metals and alloys have very low resistivity in the range of 10^–8 Ω m to 10^–6 Ω m. They are good conductors of electricity. Insulators like rubber and glass have resistivity of the order of 10¹² to 10¹⁷ Ω m.

Both the resistance and resistivity of a material vary with temperature. Resistivity of an alloy is generally higher than that of its constituent metals.

Alloys do not oxidise (burn) readily at high temperatures. For this reason, they are commonly used in electrical heating devices, like electric iron, toasters etc. Tungsten is used almost exclusively for filaments of electric bulbs, whereas copper and aluminium are generally used for electrical transmission lines.

HEATING EFFECT OF ELECTRIC CURRENT

If the electric circuit is purely resistive, that is, a configuration of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of electric current. This effect is utilised in devices such as electric heater, electric iron etc.

Applying Ohm’s law, we get

H = I² x Rt

This is known as Joule’s law of heating. The law implies that heat produced in a resistor is

(i) directly proportional to the square of current for a given resistance,

(ii) directly proportional to resistance for a given current, and

(iii) directly proportional to the time for which the current flows through the resistor.

Practical Applications of Heating Effect of Electric Current

1. Electric Heaters

2. Bulb's Filaments

3. Electric Fuse

4. Electric Kettle

5. Sandwich Maker