CHSE CURRENT ELECTRICITY

 Chse current electricity notes |chse current electricity long questions|+2 2nd year  current electricity |chse physics notes|+2 2nd year physics 

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Electric Current:The rate of flow of charge through any cross-section of a wire is called electric current flowing through it. 

Electric current(I) = q / t. Its SI unit is ampere (A).

Types of Electric Current

(i) Direct Current (DC) Its magnitude and direction do not change with time. A ceil, battery or DC dynamo are the sources of direct current.

(ii) Alternating Current (AC) An electric current whose magnitude changes continuously and changes its direction periodically is called alternating current. AC dynamo is source of alternating current. 

Current Density the electric current flowing per unit area of cross-section of conductor is called current density.

Current density (J) = I / A

Its SI unit is ampere metre-2 and dimensional formula is [AT-2].

It is a vector quantity and its direction is in the direction of motion positive charge or in the direction of flow of current.

Drift Velocity When a potential difference is applied across the ends of a conductor, the free electrons in it move with an average velocity opposite to direction of electric field. which is called drift velocity of free electrons

MobilityThe drift velocity of electron per unit electric field applied is mobility of electron.

Mobility of electron (μ) = vd / E

The SI unit of velocity is m/s, and the SI unit of electric field is V/m. Therefore the SI unit of mobility is (m/s)/(V/m) = m²/(V⋅s).

Its SI unit is m²s-¹V-¹and its dimensional formula is [M-¹T²A].

Current Density: It is defined as the amount of current flowing pér unit area of cross-section of the conductor, provided the area is held in a direction normal to the current.

Ohm’s Law:It states that the current flowing through a conductor is directly proportional to the potential difference across its ends at constant temperature

If physical conditions of a conductor such as temperature remains unchanged, then the electric current (I) flowing through the conductor is directly proportional to the potential difference (V) applied across its ends.

I ∝ V

or V = IR

where R is the electrical resistance of the conductor and R = Ane2 τ / ml.

 Resistance

The obstruction offered by any conductor in the path of flow of current is called its electrical resistance.

Electrical resistance, R = V / I

Its SI unit is ohm (Ω) and its dimensional formula is [ML2T-3A-2].

Electrical resistance of a conductor R = ρl / A

where, l = length of the conductor, A = cross-section area and

ρ = resistivity of the material of the conductor.

(i) Increases with increase in concentration of the electrolyte.

(ii) Increases with increase in distance between the electrodes.

(iii) Decreases with increase in area of electrodes dipped in electrolyte.

Relation between E. V and r

E = V + Ir

r = (E / V – I) R

If cell is in charging state, then

Resistivity Resistivity of a material of a conductor is given by

ρ = m / n2 τ

where, n = number of free electrons per unit volume.

Resistivity of a material depend on temperature and nature of the material.

It is independent of dimensions of the conductor, i.e., length, area of cross-section etc.

Resistivity of metals increases with increase in temperature as

For metals α is positive, for some alloys like nichrome, manganin and constantan, α is positive but very low.

For semiconductors and insulators. α is negative.

Resistivity is low for metals, more for semiconductors and very high alloys like nichrome, constantan etc.

In magnetic field the resistivity of metals increases. But resistivity of ferromagnetic materials such as iron, nickel, cobalt etc decreases in magnetic field.)

Ohmic Conductors

Those conductors which obey Ohm’s law, are called ohmic conductors e.g., all metallic conductors are ohmic conductor.

For ohmic conductors V – I graph is a straight line.

Non-ohmic Conductors

Those conductors which do not obey Ohm’s law, are called non-ohmic conductors. e.g., diode valve, triode valve, transistor , vacuum tubes etc.

For non-ohmic conductors V – I graph is not a straight line.

Superconductors

When few metals are cooled, then below a certain critical temperature their electrical resistance suddenly becomes zero. In this state, these substances are called superconductors and this phenomena is called superconductivity.ex Mercury

Combination of Resistors

1.In Series

(i) Equivalent resistance, R = R1 + R2 + R3

(ii) Current through each resistor is same.

(iii) Sum of potential differences across individual resistors is equal to the potential difference, applied by the source.

2. In Parallel

Equivalent resistance

1 / R = 1 /R1 + 1 / R2 + 1 / R3

Potential difference across each resistor is same.

Sum of electric currents flowing through individual resistors is equal to the be electric current drawn from the source.

Electric Cell

An electric cell is a device which converts chemical energy into electrical energy.

Electric cells are of two types

(i) Primary Cells Primary ceUs cannot be charged again. Voltic, Daniel and Leclanche cells are primary cells.

(ii) Secondary Cells Secondary cells can be charged again and again. Acid and alkali accumulators are secondary cells.

Electro motive Force (emf) 

The energy given by a cell in flowing unit positive charge throughout the circuit completely one time, is equal to the emf of a cell.

Emf of a cell (E) = W / q.

Its SI unit is volt. 

Potential Difference of a Cell

The energy given by a cell in flowing unit positive charge through till outer circuit one time from one terminal of the cell to the other terminal of the cell.

Terminal potential difference (V) = W / q.

Its SI unit is volt.

Kirchhoff’s Laws

(i) Junction Rule The algebraic sum of all currents meeting at a junction in a closed circuit is zero, i.e., Σ I = O.This law follows law of conservation of charge.

(ii) Loop Rule The algebraic sum of all the potential differences in any closed circuit is zero, i.e.,ΣV = 0 ⇒ ΣE = ΣIR

Potentiometer is an ideal device to measure the potential difference between two points. It consists of a long resistance wire AB of uniform cross section in which a steady direct current is set up by means of a battery.

Wheatstone bridge a simple circuit for measuring an unknown resistance by connecting it so as to form a quadrilateral with three known resistances and applying a voltage between a pair of opposite corners.

Meter Bridge: a meter bridge also called a slide wire bridge is an instrument that works on the principle of a Wheatstone bridge. A meter bridge is used in finding the unknown resistance of a conductor as that of in a Wheatstone bridge.