PPU 960 Physics Note [Sem 2 Chapter 14 - Electric Current]

7/29/2019 PPU 960 Physics Note [Sem 2 Chapter 14 - Electric Current]

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2013

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Chapter 14 Electric Current y : Josh, LRT

2013 LRT Documents Copyrighted. All rights reserved. Page 1 of 7

Chapter 14Electric Current

14.1 Conduction of electricityConsider a simple closed circuit consists of wires, a battery and a light bulb as shown inFigure A.

Figure A

From the Figure A,

Direction of electric field, or current, :Positive tonegativeterminal Direction of electron, flows:Negative topositiveterminal The electron acceleratesbecause of the electric forceacted on it. It is defined as the total nett charge, flowing through the flowing through the area per

unit time, .

SI unit for 1 ampere of current is defined as one coulomb of charge passing through the surface area in

one second.

14.2 Current densityIt is defined as the current flowing through a conductor per unit cross-sectional area.

It is a vector quantity

Unit = The direction of is the same with the direction of the .


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From, ( )

Electrical conductivity, is the measure of a materials ability to conduct electric current.

SI unit for is

14.3 Drift velocityThe drift velocity is the average velocitythat a particle, such as an electron, attains due to anelectric field.

When the electric field is applied to the metal , the freely moving electron experience an

electric force and tend to drift with constant average velocity(called drift velocity) towards

a direction opposite to the direction of the field as shown inFigure B.

Figure B

**1 The magnitude of the drift velocityis smaller than the random velocitiesof the free

electrons.

**2 The electric current is flowing in the opposite directionof the electron flows.

Drift velocity of charges, L = length of metal rodA = cross-sectional area

J = current density

e = electron

E = Electric filed

I = current


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Set, number of free electrons per volume in metal rod. the number of free electron, is given by:

The total charge of the free electrons that pass through the area along the rod is

Time, required for electron move along the rod is

Then,

: number of free electron per unit volume [DENSITY of free electron]: charge of the electron

14.4 Resistivity and Ohms law Resistance, R

Ratio of the potential difference across an electrical component to the current passing

through it.

# It is a measure of the components opposition to the flow of the electric charge.

# Scalar quantity with unit or

Resistivity, The resistance of a unit cross-sectional area per unit length of the material .

# It is a measure ofmaterials ability to oppose the flow of an electric current.

# Scalar quantity with unit # Known as specific resistance

# # Good conductors[low resistivity] ; Good insulator[high resistivity]

Q

t


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Conductivity, The reciprocal of the resistivity of a material.

# Scalar quantity with unit

Ohms LawStates that the potential difference across a metallic conductoris proportional to the

current flowing through it if its temperature is constant.

It can be stated in other form,

is maintainedacross the conductor sets up by an electric field, and this fieldproduce a current, that is proportional to the potential difference.

If the field is assumed to be uniform,

From ohms law, where and ( )

or and


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Graphs ofpotential difference, against current, for various materials.

METAL SEMICONDUCTOR

CARBON ELECTROLYTE

14.5 Dependence of resistivity on temperatureMetal

o Temperature (T) = , Number of electrons per unit volume (n)= UNCHANGED.o Metal atomsin the crystal lattice vibrate with greater amplitudeand cause the number of

collisions between the free electrons and metal atoms increases. Hence the resistance in themetal increases.

Superconductor

o Temperature (T) = The resistancedecreases to 0.

Temperature coefficient of resistivity, Fractional increase in resistivity of a conductor per unit rise in temperature


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Since,

The resistivity of a conductor varies approximately linearly with temperature

# Unit for or Thus,

Graphs ofresistance, against temperature, for various materials.

METAL SEMICONDUCTOR

SUPERCONDUCTOR CARBON


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Problems

1. Resistivity of a wire depends on

2. A silver wire carries a current of .Determine

a) The numbers of electrons per second pass through the wire.b) The amount of charge flows through a cross-sectional area of the wire in 55 s.

(Given charge of electron,

9

)

3. A high voltage transmission line with a diameter of and a length of carries asteady current of . If the conductor is copper wire with a free charge density of8 electrons , calculate the time taken by one electron to travel the full length of the line.( 9 )

4. When a potential difference of 240 V is applied across a wire that is long and has a radius, the current density is 9 .Calculate

a) The resistivity of the wire.b) The conductivity of the wire.

5. A copper wire carries a current of . The cross sectionof the wire is a square of side and its length is .The density ofthe free electron in the wire is 8. Determinea) The current density

b) The drift velocity of the electronsc) The electric field intensity between both end of the wired) The potential difference across the wiree) The resistance of the wire(Given the resistivity of copper is

8

and charge of electron,

9 )

A. Length

B.Material

C. Cross-section area

D. Electrons

Answers:

1. B

2. a) 9b) 165 C

3. 8s4. a)

8

b) 75. a) 6A

b) 4c) d) e)