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CBSE

Subject

Physics

Class

CBSE Class 12

CBSE Physics 2012 Exam Questions

Short Answer Type

1.

An electric dipole is held in a uniform electric field.

(i) Show that the net force acting on it is zero.

(ii) The dipole is aligned parallel to the field. Find the work done in rotating it through the angle of 180°.


i)  Dipole moment of dipole is given by,  

i) 

open vertical bar straight p with rightwards harpoon with barb upwards on top close vertical bar space equals space q space x space left parenthesis 2 a right parenthesis

Force on –q at A = -qE

Force on +q at B = + qE

So,

Net force = qE – qE = 0

 

ii)

Work done on dipole is given by, 

straight W space equals space increment straight U space equals space PE space left parenthesis cos space straight theta subscript 1 space minus cos space straight theta subscript 2 right parenthesis

space space space equals space PE space left parenthesis cos space 0 to the power of straight o space minus space cos space 180 to the power of straight o right parenthesis

straight W space equals space 2 space PE

4459 Views

2.

A charge ‘q’ is placed at the centre of a cube of side l. What is the electric flux passing through each face of the cube? 


Using Gauss theorem,

straight ϕ space equals space integral straight E with rightwards harpoon with barb upwards on top. ds with rightwards arrow on top space equals space straight q over straight epsilon subscript straight o

Total space flux space through space all space faces comma space
straight ϕ space apostrophe space equals space 6 space straight ϕ space

So comma

6 space straight ϕ space space equals space straight q over straight epsilon subscript straight o space
rightwards double arrow straight ϕ space space equals space fraction numerator straight q over denominator 6 space straight epsilon subscript straight o end fraction

1034 Views

3.

A bar magnet is moved in the direction indicated by the arrow between two coils PQ and CD.

Predict the directions of induced current in each coil. 



In the figure, N pole is receding away coil (1), so in coil (1), the nearer faces will act as S-pole and in coil (2) the nearer face will also act as S-pole to oppose the approach of magnet towards coil (2), so current in coils will flow clockwise as seen from the side of magnet. The direction of current will be from P to Q in coil (1) and from C to D in coil (2).

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4.

A light bulb is rated 100 W for 220 V ac supply of 50 Hz. Calculate

(a) the resistance of the bulb;

(b) the rms current through the bulb

 

OR

 

An alternating voltage given by V = 140 sin 314 t is connected across a pure resistor of 50 W. Find

(i) the frequency of the source.

(ii) the rms current through the resistor. 

straight a right parenthesis thin space straight R space equals space straight V squared subscript rms over straight P space equals space fraction numerator 220 space straight x space 220 over denominator 100 end fraction space equals space 48 space straight capital omega

straight b right parenthesis thin space straight I subscript rms space equals space straight P over straight V subscript rms straight x 100 over 220 space equals space 0.45 space straight A  


                                                             OR


straight i space right parenthesis thin space straight V space equals space 140 space sin space 314 space straight t

because space straight V space equals space straight V subscript straight o space sin space straight omega space straight t

Hence comma space straight omega space equals space 314 space space space

rightwards double arrow space 2 πν space equals space 314 space

rightwards double arrow space straight nu space equals space 50 space Hz

ii right parenthesis
straight V subscript rms space equals space fraction numerator straight V subscript straight o over denominator square root of 2 end fraction space equals space fraction numerator 140 over denominator square root of 2 end fraction space equals space 98.9 space straight V

straight I space equals space straight V subscript rms over straight R equals fraction numerator 98.9 over denominator 50 end fraction space equals space 1.97 space straight A


 
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5.

Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker?


Resistance of a material is given by, 

Error converting from MathML to accessible text.
Resistance for copper wire and manganin wire is given by, 

Error converting from MathML to accessible text. 

That is, manganin wire is thicker.

3421 Views

6.

State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distances done with the use of transformers?  


The principle of transformer is based upon the principle of mutual induction which states that due to continuous change in the current in the primary coil an emf gets induced across the secondary coil. At the power generating station, the step up transformers step up the output voltage which reduces the current through the cables and hence reduce resistive power loss. Then, at the consumer end, a step down transformer steps down the voltage. Therefore, the large scale transmission of electric energy over long distances is done with the uses of transformers.

 

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7.

What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?     


Both electric and magnetic field vectors are perpendicular to each other and perpendicular to the direction of propagation.

 

618 Views

8.

A test charge ‘q’ is moved without acceleration from A to C along the path from A to B and then from B to C in electric field E as shown in the figure. (i) Calculate the potential difference between A and C. (ii) At which point (of the two) is the electric potential more and why? 



i) Potential difference between A and C

straight V subscript straight C space minus space straight V subscript straight A space equals space minus integral subscript straight A superscript straight C straight E with rightwards harpoon with barb upwards on top space. space dl with rightwards harpoon with barb upwards on top space cos space 180 to the power of straight o space equals space straight E space straight x space 4 space equals space 4 straight E

ii) Direction of electric field is in higher to lower potential. So, Vc > VA

2803 Views

9.

A capacitor of capacitance of ‘C’ is being charged by connecting it across a dc source along with an ammeter. Will the ammeter show a momentary deflection during the process of charging? If so, how would you explain this momentary deflection and the resulting continuity of current in the circuit? Write the expression for the current inside the capacitor.


When capacitor is charged by a dc source, ammeter will show a momentary deflection because of the presence of displacement current.

Conduction current is equal to the displacement current. 

i.e.,                 straight I subscript straight C straight space equals straight space straight I subscript straight D straight space equals straight space fraction numerator straight epsilon subscript straight o dφ subscript straight E over denominator dt end fraction

Resulting continuity of current becomes zero because,  is maximum when, capacitor is fully charged.

1310 Views

10.

Mention the two characteristic properties of the material suitable for making core of a transformer. 


Two characteristic properties are:

i) Low hysteresis loss

ii)  Low coercivity.

1377 Views

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