electricity.holiday hw.june 2012

7/30/2019 Electricity.holiday Hw.june 2012

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Q1.Which material is best conductor ?

Ans-Silver is generally regarded as the

best conductor. Its resistivity is very much

less than those of the other metal and

alloys .

Q2.What does an electric circuit mean ?

Ans-An electric circuit as a continuous and

closed path through which an electric

current can follow.

Q3.How do we define electric current ?

Ans-Electric current is define as the rate of

flow of electric charge. It equals the

amount of charge flowing through a

particular area in a unit time .

Q4.What type of charges flow through a

metallic wire ?

Ans-It is negative charge electrons that

flow though a metallic wire.

Q5.What is the SI units of electric charge

and electric current ?

Ans-The unit of electric charge is

coulomb(C) and electric current is

coulomb per second or ampere.

Q6.Name a device that help to maintain a

potential difference across a conductor.

Ans- We can use a batterymade from

one or more electric cells- for producing a

potential difference .

Q7.How much energy is given to each

coulomb of charge passing through a 6 V

battery ?

Ans- Energy given = Working done

=p.d. x charge

= 6 V x 1 C = 6 J

Q8.How is a voltmeter connected in the

circuit to measure the potential difference

between two points?

Ans-The voltmeter is always connected in

parallel, across the two points between

which the potential difference is to be

measured .

Q9.Why is much less heat generated in

long electric cables than in filaments of

electric bulbs ?

Ans-Filaments of electric bulbs flow of

electric current whereas cable are made ofcopper which has a very low resistance.

Q10.Why is tungsten metal selected for

making filaments of incandescent lamp

bulb ?

Ans-Tungsten is selected for making

filaments of incandescent light bulbs

because of its high melting point and

(relatively) higher resistivity.

Q11.Should the heating element of an

electric iron be made of iron, silver or

nichrome wire ?

Ans- The heating element of an electric

iron should be made of nichrome wire.

Q12.Will current flow more easily through

a thick wire or a thin wire of the same

material and length when connected to the

same source ?

Ans- Current will flow more easily

through the thick wire. A thick wire has a

higher value of area of cross-section than a

thin wire. For the same length and

material, the resistance of the thicker wire

is therefore, less than that of a thinker

wire. Current, therefore , flow more easilythrough the thicker wire.


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Q13.What are the advantage of connecting

electrical devices in parallel with the

battery instead of connecting them in

series ?

Ans-There are three advantages of

connecting electric devices in parallel with

the battery:

(i)All the devices have the full voltage of

the battery acting on them.

(ii)Each device can be switched on or

off independent of the other.

(iii)If one of the devices gets damages or

brunt, the other devices will still keep on

working.

Q14.Why does the cord of an electric

heater not glow while the heating element

does ?

Ans-The cord has a very low (almost zero)

resistance compared to the heatingelement. Hence when the same current

flow through both of them, the heat

produced (=I2R)in the cord is very small

compared to that in the heating element.

Hence only the latter glows.

Q15.Two wires of equal length, one of

copper and the other of manganin (an

alloy) have the same thickness. Which one

can be used for (i)electric transmissionlines (ii) electrical heating devices ? why?

Ans-(i)copper wire can be used for

electrical transmission lines because

copper has very low resistivity and hence

it is very good conductor of electricity.

(ii) Manganin can be used for electrical

heating devices because the resistivity of

manganin is about 25 times more than that

of copper and hence it produces a lot of

heat on passage of current through it.

Q16.In a factory, an electric bulb of 500 W

is used for 2 hours and electric motor of

0.5 horse power is used for 5 hours every

day. Calculate the cost of using the bulb

and motor for 30 days if cost of electrical

energy is there rupees per unit.

Ans- Energy consumed by bulb in a day =

500 W x 2 h = 1000 kWh

Energy consumed y the motor in a

day = 0.5 x 746W x 5h = 1865 Wh =1.865kWh (1 hp=746)

Total energy consume in a day =1 +

1.865 = 2.865 kWh

Energy consumed in 30 days = 30 x

2.865 kWh = 85.95 kWh

Cost of electrical energy =Rs. 3 x

85.95 = Rs.257.85

Q17.Compute the heat generated while

transferring 96000 coulomb of charge in

one hour through a potential difference of

50 V.

Ans- Total heat generated = Work done

= Charge

transferred x p.d. = 96000 x 50 J

=

4800000 J = 4.8 x 106

J

Basic Concepts:1.Charge: It is an inherent property of the

body due to which the body feels attractive

and repulsive. There are two types of

electric charges : (i) Positive (ii) Negative.


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2.Coulombs Law:The Force between

two electric charges at rest is directly

proportional to the product of the

magnitudes of the charge and inversely

proportional to the square of distance

between them. If we have two

electric charge q1 and q2 placed at a

distance r from one another, then

according to coulombs law, the force F

between them is given by

F = k .q1q2 /r2

, k is a

constant.

3. 1 Volt : One volt is defined as the

potential difference between two points in

a current carrying conductor when 1 joule

of work is done to move a change of 1

coulomb from one point to another.

There, 1 volt = 1 joule / 1 coulomb

4.Voltmeter: The potential difference is

measure by mean of an instrument called

voltmeter. A voltmeter has high resistance.

5.Electric Current : The electric current

is the rate of flow of electric charge (called

electrons) in a conductor. Current , I =

Q/t. SI unit of electric current is Ampere

.electric current is a scalar quantity.

6.Ohms law:At constant temperature,

the current flowing through a conductor is

directly proportional to the potentialdifference across its end. IfI is the current

flowing through a conductor and V is the

potential difference across its end, then

according to Ohms law :

. I V

(At constant temperature)

This can also be written as, V I or

V= RI , Where R is a constant called

resistance of the conductor.

Some Important Formulae :1.Coulombs law- F =k. q1q2 /r

2

2.Electric current - I = Q /t

3.Potential difference- V = W/ Q

4.Ohms law V = IR

5.Resistivity - = RA/l

6.Resistors in series, R = R1 + R2 + R3

electricity.holiday hw.june 2012

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