the world of electricity

7/31/2019 THE WORLD OF ELECTRICITY

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WELCOME TO THE WORLD OFSCIENCE.


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SCIENCE PRESENTATION

WORK ON

ELECTRICITY


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CONTENTS

ELECTRICITY

ELECTRIC CURRENT

POTENTIAL DIFFERENCE

ELECTRIC CURRENT

SYMBOLS FOR ELECTRIC CIRCUIT OHMS LAW

VERIFICATION OF OHMS LAW

RESISTANCE

FACTORS FOR DEPENDENCE OF CURRENT RESISTORS IN SERIES

RESISTORS IN PARALLEL

ELECTRICAL ENERGY AND POWER

HEATING EFFECT OF ELECTRIC CURRENT

EXTRA QUESTIONS


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ELECTRICITY

Electricity is one of the basic forms of energy. Electricity is

associated with electric charge, a property of certain elementary

particles such as electrons and protons, two of the basic particles

that make up the atoms of all ordinary matter. Electric charges

can be stationary, as in static electricity, or moving, as in an

electric current.


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ELECTRIC CURRENT

IT IS THE AMOUNT OF CHARGE FLOWING THROUGH APERTICULAR AREA IN UNIT TIME. IT IS ALSO DEFINED AS

THE RATE OF FLOW OF ELECTRIC CHARGE.

CURRENT =CHARGE

TIMEOR I = Q

T


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LIGHTNING A CAUSE OF ELECTRIC CURRENT


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The SI unit of electric charge is coulomb (C). It is the charge

contained in 6x1018electrons .

The SI unit of current is calledampere (A).

One ampere is the current flowing through a conductor if I

coulomb of charge flows through it in 1 second.

1coulomb

1ampere =1 second

Electric Current is measured by an ammeter. It is always

connected in series in a circuit.


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ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE

Electric current will flow through a conductor only if there is a

difference in the electric potential between the two ends of the

conductor. This difference in electric potential between the two

ends of a conductor is called potential difference.

The potential difference in a circuit is provided by a cell or

battery. The chemical reaction in the cell produces a potential

difference between the two terminals and sets the electronsin motion and produces electric current .


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Work done W

Potential difference = or V =

Charge Q

The SI unit of potential difference is volt (V).

One volt is the potential difference when 1 joule of work is done tomove a charge of 1 coulomb from one point to the other.

1 joule 1J

1 volt = or 1 V =

1 coulomb 1C

Potential difference is measured by a voltmeter.It is alwaysconnected in parallel across the two point between which the

potential difference is to be measured.


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ELECTRIC CIRCUIT

It is a continuous and closed path of an electric current.

A

- +

+

-

bulb

ammeter

plug key

cell

A schematic diagram of an electric circuit comprising

of a cell, electric bulb, ammeter and plug key.


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SYMBOLS OF COMPONENTS USED IN ELECTRIC

CIRCUITS

1. An electric cell -+ -

2. A battery or combination of cells - + -

3. Plug key or switch (open) -

4. Plug key or switch (closed) -

5. Electric bulb -

6. A resistor of resistance R -


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7. Variable resistance or rheostat -

8. Ammeter - A

+ -

9. Voltmeter - v+ -

10. A wire joint -

11. A wire crossing over without joining -


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OHMS LAW

Ohms law is a relationship between the potential difference across

a conductor and the current flowing through it.

OHMS LAW STATES THATThe current flowing through a conductor is directly proportional tothe potential difference between its ends provided its temperature

remains constant.

I V

OR V I OR

V

I = constant

V

I= R

Where R is a constant called resistance for a given

metallic wire at a given temperature.


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VERIFICATION OF OHMS LAW

A

V

+

+ -

-

K

AR

B

+ -

Set up the circuit as shown in the circuit diagram. First use

one cell and note the current (I) in the ammeter and the

potential difference (V) in the voltmeter across the nichromewire AB. Repeat by using two cells, three cells and four cells

and note the readings in the ammeter and voltmeter. Then

plot a graph between the current (I) and potential difference

(V). The graph will be a straight line.


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This shows that the current flowing through a conductor

is directly proportional to the potential difference across

its ends.

IV OR ORV IV

I=

where R is a constant called resistance of the

conductor.

Potential difference ( V )

Current ( I )

R


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RESISTANCE

Resistance is the property of a conductor to resist the flow of current

through it.

According to Ohms law R =

The SI unit of resistance is ohm ().

V

I


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If the potential difference across the two ends of a wire is 1 V

and the current flowing through it is 1 A then the resistance R of

the conductor is 1 ohm (1 ).

V

Since I =

R

The current flowing through a resistor is inversely

proportional to the resistance.

So if the resistance is doubled, then the current gets halved.


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FACTORS ON WHICH THE RESISTANCE OF A

CONDUCTOR DEPENDS

The resistance of a conductor depends upon its:-

i) Length

ii) Area of cross section

iii) Material of the conductor.Resistance is directly proportional to the length of the conductor

and inversely proportional to the area of cross section of the

conductor.

R l

R I /A

or R l

A

or R = l

A


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Where (rho) is a constant of

proportionality called Resistivity of thematerial of the conductor.

The SI unit of resistivity is ohm meter ( m).

Conductors like metals and alloys have low

resistivity 10-8 m to 10-6 m.

Insulators like rubber, glass etc. have high

resistivity 1012 m to 1017 m.


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RESISTORS IN SERIES

-( ) A

V

A BR1 R2 R3

V1 V2 V3

+ +

+ -

-

When three resistors R1, R2 and R3 are connected in series across ABi) The current in all the resistors is the same.

ii) The total voltage (PD) across the resistors is equal to the sum of the

voltage across each resistor.

V = V1 + V2 + V3

iii) The equivalent resistance is the sum of the resistances of eachresistor.

RS = R1 + R2 + R3


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RESISTORS IN PARALLEL

V

A( )+ - - +

+ -

R3

R2

R1

BA

I1

I2

I3

When three resistors R1, R2 and R3 are connected in parallel across AB,

i) The voltage (PD) in all the resistors is the same.

ii) The total current in all the resistors is the sum of the current in each

resistor. I = I1 + I2 + I3iii) The reciprocal of the equivalent resistance is the sum of the

reciprocals of each resistance.

1 1 1 1

= + +

Rp R1 R2 R3


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ELECTRICAL ENERGY AND ELECTRIC POWER

ELECTRICAL ENERGY

It is the work done to maintain the flow of current in

a conductor.

W = Q X V I = Q / t Q = I X t

W = I X t x V V = IR

W = I2Rt

The unit of electrical energy is joule (J).

ELECTRIC POWER


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ELECTRIC POWER

It is the rate at which electric current is used.

Power = Work done P = W W = I2Rt = I2Rt

Time t t

Power = I2R R = V = I2 X V = I X V

I Ior Power = I X V

The SI unit of power is watt (W).

One watt is the power when 1A of current flows across a

potential difference of 1V.1000 W = 1kW 1kWh = 1000 watt x 3600 seconds

= 3.6 x 106 joules

The commercial unit of power is watt hour (Wh) or kilo

watt hour (kWh). One kWh is the power consumed

when 1W of power is used for 1 hour.


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HEATING EFFECT OF ELECTRIC CURRENT

+ -( ) A- +

V+ -

RA B

I I

If a current I flows through a resistor of resistance R and t be the

time for which a charge Q flows through it, then the work done to

move the charge through potential difference V

W = Q X V

P = W = Q X V Q = I or P = V X I

t t t

or Heat energy supplied = P X t = V X I X t

According to Ohms law V = IR

Heat produced H = I2Rt


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PRACTICAL APPLICATIONS OF HEATING EFFECT

OF ELECTRIC CURRENT

The heating effect of electric current has many useful applications.

i) It is used in electric iron, electric toaster, electric oven, electric kettle,

electric heater etc.

ii) It is also used to produce light in electric bulbs. The filament used

in electric bulbs should retain heat and should not melt. So a metal

having high melting point like tungsten is used as the filament formaking electric bulbs.


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iii) It is used as fuse in electric circuits. The fuse protectscircuits and appliances from damage due to high current.

It is a wire having a low melting point. If high current

flows through the circuit the fuse wire melts and breaks

the circuit and protects the circuit and appliances from

damage. The fuse wire is made of a metal or an alloy of

metals like aluminium, copper, iron or lead.


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TIME FOR QUESTIONS

TEST YOUR KNOWLEDGE


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1. What does an electric circuit mean?

It is a continuous and closed path through which current

flows.


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2. Define the unit of current.

The unit of electric current is ampere (A). 1 A is defined as

the flow of 1 C of charge through a wire in 1 s.


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3. Calculate the number of electrons constitutingone coulomb of charge.

1 C of charge is contained in 1 = 6.25 x 1018

1.6 x 10-19

= 6 x 1018electrons.

Therefore, 6 x 1018electrons constitute one coulomb of charge.

One electron possesses a charge of 1.6 1019 C, i.e., 1.6 1019 C

of charge is contained in 1 electron.


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4. Name a device that helps to maintain a potentialdifference across a conductor.

A source of electricity such as cell, battery, power supply, etc.

helps to maintain a potential difference across a conductor.


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5. What is meant by saying that the potential differencebetween two points is 1 V?

If 1 J of work is required to move a charge of amount 1 C

from one point to another, then it is said that the potentialdifference between the two points is 1 V.


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6. How much energy is given to each coulomb of charge

passing through a 6 V battery?

The energy given to each coulomb of charge is equal to the amount

of work required to move it. The amount of work is given by the

expression,

Potential difference = Work done

Charge

Work done = Potential difference x Charge

Where,

Charge = 1 C

Potential difference = 6 V

Work Done = 6 x 1 = 6JTherefore, 6 J of energy is given to each coulomb of charge passing

through a battery of 6 V.


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7. On what factors does the resistance of a conductordepend?

The resistance of a conductor depends upon the following factors:

(a) Length of the conductor

(b) Cross-sectional area of the conductor

(c) Material of the conductor

(d) Temperature of the conductor


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Resistance of a wire,

Where,

= Resistivity of the material of the wire

l = Length of the wire

A = Area of cross-section of the wire

Resistance is inversely proportional to the area of cross-section of the wire.

Thicker the wire, lower is the resistance of the wire and vice-versa.

Therefore, current can flow more easily through a thick wire

than a thin wire.

8. Will current flow more easily through a thick wire or a thin

wire of the same material, when connected to the samesource? Why?


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9. Let the resistance of an electrical component remains

constant while the potential difference across the two ends ofthe component decreases to half of its former value. What

change will occur in the current through it?

The change in the current flowing through the component is given by Ohms

law as,

V = I x RWhere,

Resistance of the electrical component = R

Potential difference = V

Current = I

The potential difference is reduced to half, keeping resistance constant.Let the new resistance be R' and the new amount of current be I '.


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Therefore, from Ohms law, we obtain the amount of new current.

Therefore, the amount of current flowing through the electrical component

is reduced by half.


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10. Why are coils of electric toasters and electric irons made

of an alloy rather than a pure metal?

The resistivity of an alloy is higher than the pure metal. Moreover, at

high temperatures, the alloys do not melt readily. Hence, the coils of

heating appliances such as electric toasters and electric irons aremade of an alloy rather than a pure metal.


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11. Use the data in Table to answer the following

Electrical resistivity of some substances at 20C

Material Resistivity ( m)

Conductors Silver 1.60

108

Copper 1.62 108

Aluminium 2.63 108

Tungsten 5.20 108

Nickel 6.84

10

8

Iron 10.0 108

Chromium 12.9 108


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Mercury 94.0 108

Manganese 1.84

106

Constantan

(alloy of Cu and Ni)

49 106

Alloys Manganin

(alloy of Cu, Mn and Ni)

44 106

Nichrome

(alloy of Ni, Cr, Mn and Fe)

100 106

Glass 1010 1014

Insulators Hard rubber 1013 1016

Ebonite 1015 1017Diamond 1012 1013

Paper (dry) 1012


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(a) Which among iron and mercury is a better conductor?

(b) Which material is the best conductor?

(a) Resistivity of iron = 10.0 x 10-8 m

Resistivity of mercury = 94.0 x 10-8

mResistivity of mercury is more than that of iron. This implies that iron is a

better conductor than mercury.

(b) It can be observed from Table that the resistivity of silver is the lowest among

the listed materials. Hence, it is the best conductor.


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12. Draw a schematic diagram of a circuit consisting of a battery

of three cells of 2 V each, a 5 resistor, an 8 resistor, and

a 12 resistor, and a plug key, all connected in series.

Three cells of potential 2 V, each connected in series, is equivalent to a

battery of potential 2 V + 2 V + 2 V = 6V. The following circuit diagram

shows three resistors of resistances 5 , 8 and 12 respectively

connected in series and a battery of potential 6 V.


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13. What determines the rate at which energy is delivered

by a current?

The rate of consumption of electric energy in an electric appliance is

called electric power. Hence, the rate at which energy is delivered by

a current is the power of the appliance.


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PRESENTED BY

MAHESH PRASAD PATI

the world of electricity

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