The study of charges at rest is called static electricity or electrostatics while the study of charges in motion is called current electricity. There are two types of electric charge: (i) Positive charge and

(ii) Negative charge

The magnitude of elementary positive or negative charge is same and is equal to [latex]1.6 × 10_{19} C[/latex] Charge is a scalar quantity and its SI unit is

(1) Similar charges repel and opposite charges attract.

(2) A charged body attracts light uncharged bodies.

(3) Accelerated charge radiates energy.

- The radio and TV require DC and this is obtained from AC mains by means of a rectifier which converts AC into DC.
- Current is a flow of charge, pressured into motion by voltage and hampered by resistance.

These materials are poor electrical conductors for the same reasons they are generally poor heat conductors such materials are good insulators.

The maximum permissible current in a circuit is determined by the size of the wires and the

- Metals are very good conductors of electric charge. Silver, copper and aluminium are some of the good conductors of electricity.
- The materials which do not allow electric charge to flow through them are called nonconductors or
**insulators**. - For example, most plastics, rubber, non-metals (except graphite), dry wood, wax, mica, porcelain, dry air etc., are
**insulators**.

[latex]F ∝ q_1 q_2[/latex]

[latex]F ∝[/latex][latex]\frac{1}{r^2}[/latex]

F = k[latex]\frac{q_1q_2}{r^2}[/latex]

K= [latex]\frac{1}{4πε_0}[/latex]

= [latex]9 × 10^9[/latex][latex]\frac{Nm^2}{Coul^2}[/latex]

[latex]⇒[/latex][latex]ε_0[/latex]= [latex]8.85 × 10^{-12}[/latex]

[latex]\frac{Coul^2}{Nm^2}[/latex]

- The S.I. unit of electric field intensity is N/coul or volt/metre.
- The S.I. unit of electric field intensity is N/coul or volt/metre.

or closed surface [latex]\phi_c[/latex] = [latex]\oint \vec E .d\vec s[/latex]

(Gaussian surface) [latex]\oint \vec E .d\vec s[/latex] = [latex]\frac{q_0}{ε_0}[/latex]

- It is a scalar quantity.
- Its dimensions:[latex][M L^2 T^{-3} A^{-1}][/latex]
- Its SI unit is volt or joule coulomb

[latex]Q∝V[/latex] or Q = CV or C = [latex]\frac{Q}{V}[/latex] The constant C is known as the capacitance of the conductor. Its SI unit is farad (F) or coulomb/volt Capacitance of the conductor depends upon: (i) Size of conductor

(ii) Surrounding medium

(iii) Presence of other conductors nearby

[latex]\frac{1}{C_eq}[/latex] = [latex]\frac{1}{C_1}[/latex] + [latex]\frac{1}{C_2}[/latex] +.....+[latex]\frac{1}{C_n}[/latex]

[latex]C_eq[/latex] = [latex]C_1 + C_2 + ....+ C_n[/latex]

(ii) If a hollow conductor is in contact with another conductor, then as charge is supplied to the conductor, the hollow conductor continues accepting the charge irrespective of the fact, howsoever large its potential may grow.

- It has constant amplitude and has alternate positive and negative halves.
- It is produced by
**ac dynamo**. - Its symbol is

[latex]R∝L[/latex] and [latex]R∝[/latex][latex]\frac{1}{A}[/latex]

so,[latex]R∝[/latex][latex]\frac{L}{A}[/latex]

or, [latex]R∝[/latex]=[latex] ρ[/latex][latex]\frac{L}{A}[/latex]

[latex] σ[/latex]= [latex]\frac{1}{ρ}[/latex]

[latex]V∝I[/latex] or V = IR where R= [latex]\frac{V}{I}[/latex] is the resistance of conductor.

[latex]R_s[/latex] = [latex]R_1 + R_2 + R_3 + ....+ R_n[/latex]

[latex]\frac{1}{R_P}[/latex] = [latex]\frac{1}{R_1}[/latex] + [latex]\frac{1}{R_2}[/latex] + [latex]\frac{1}{R_3}[/latex] +.....+[latex]\frac{1}{R_n}[/latex]

- 1 unit of electrical energy = 1 Kilowatt hour (1 KWh) = [latex]3.6 × 10^6 joule[/latex] This is known as Board of trade (B.O.T) unit of electrical energy.
- Energy liberated per second is called its power.
- The electrical power P delivered or consumed by an electrical device is given by P = VI, where V = Potential difference across the device and I = current.

- An ammeter is a low resistance galvanometer used to measure strength of current in an electrical circuit.
- An ammeter is always connected in series in a circuit because, when an ammeter is connected in series it does not appreciably change the resistance of circuit and hence the main current flowing through the circuit.

- A voltmeter is a high resistance galvanometer used to measure potential difference.
- A voltmeter is connected in parallel to a circuit element because, when connected in parallel it draws least current from the main current. So it measures nearly accurate potential difference.

(ii) The generation of A.C. is found to be economical than that of D.C.

(iii) Alternating current can be controlled by using a choke coil without any significant wastage of electrical energy.

(iv) Alternating current may be transmitted at a high voltage from the power house to any place where it can again be brought down to low voltage. The cost in such a transmission is low and energy losses are minimized. Transformers cannot be used for D.C. Hence the cost of D.C. transmission from one place to other is quite high.

(v) A.C. equipments such as electric motors etc are more durable and convenient as compared to D.C. equipment.

K = [latex]\frac{Number of turns in secondary N_s }{Number of turns in primary N_p }[/latex] K > 1, for step-up transformer. K < 1, for step-down transformer input power = output power i.e,[latex]E_p × I_p = E_s × I_s [/latex] or [latex]\frac{I^p}{I^s}[/latex]= [latex]\frac{E^s}{E^p}[/latex]= [latex]\frac{N^S}{N^p}[/latex]

(ii) In the induction furnaces.

(iii) Step down transformer is used for welding purposes.

(iv) In the transmission of ac over long distance.

(v) Step down and step up transformers are used in electrical power distribution.

(vi) Audio frequency transformers are used in radiography, television, radio, telephone etc.

(vii) Radio frequency transformers are used in radio communication.

e[latex]∝[/latex][latex]\frac{Δ\phi}{Δt}[/latex]

or e = k[latex]\frac{Δ\phi}{Δt}[/latex]

In the S.I. system, emf ‘e’ is measured in volt and [latex]\frac{d\phi}{dt}[/latex] in Wb/sec.

On combining Lenz’s law with Faraday’s laws e = -[latex]\frac{d\phi}{dt}[/latex]

The Lenz’s law is consistent with the law of conservation of energy.

(1) Dead beat galvanometer.

(2) Energy meter.

(3) Speedometer.

(4) Electric brakes.

(5) Single phase AC motor.

(6) Induction furnace.

[latex]\phi∝I[/latex]

[latex]\phi[/latex]= LI The constant L is called coefficient of self induction or self inductance of the coil.

The S.I. unit of self inductance or inductance is

Then, [latex]\phi_{s}∝I_p[/latex] or [latex]\phi_{s}[/latex] = [latex]M I_p[/latex] M = constant of proportionality called mutual inductance or coefficient of mutual induction.

[latex]η[/latex] = [latex]\frac{EI}{VI}[/latex]= [latex]\frac{E}{V}[/latex]= [latex]\frac{Back e.m.f.}{Applied e.m.f.}[/latex]

2. They are used for pumping water.

3. Big D.C. motors are used for running tram-cars and even trains.

- A. Gravitation field
B. Gravitation force
C. Potential energy
D. Electric potential

- A. rate of flow of an electrical charge
B. rate of flow of an electric potential
C. rate of flow of an alternating current
D. rate of flow of an Direct current

- A. Low boiling point
B. High melting point
C. Low melting point
D. High boiling point

- A. Electric current
B. Magnetic effect
C. Magnetic energy
D. Special current

- A. Aluminium
B. Platinum
C. Potassium
D. Sodium

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