Subject

Physics

Class

CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsShort Answer Type

11.

Draw typical output characteristics of an n-p-n transistor in CE configuration. Show how these characteristics can be used to determine output resistance.        


The output characteristics of an n-p-n transistor in CE configuration are shown below:



Output resistance is defined as the ratio of change in collector-emitter voltage to change in collector current at a constant base current. The reciprocal of the slope of the linear part of the output characteristics represents the output resistance.

 straight r subscript straight o space equals space open parentheses fraction numerator increment straight V subscript CE over denominator increment straight I subscript straight c end fraction close parentheses subscript straight I subscript straight B end subscript

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12.

A parallel beam of light of 500 nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1 m away. It is observed that the first minimum is at a distance of 2.5 mm from the centre of the screen. Calculate the width of the slit.


Distance of nth minima from the centre of screen is given by, straight a space equals space straight n space λD divided by straight d semicolon space a is the width of the slit.

For first minimum, n=1

So, width of the slit is given by, 


straight a space equals space fraction numerator 500 space straight x space 10 to the power of negative 9 end exponent cross times 1 over denominator 2.5 space cross times 10 to the power of negative 3 end exponent end fraction space straight m

space space space space equals space 2 space cross times space 10 to the power of negative 4 end exponent space straight m

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13.

A wire AB is carrying a steady current of 12 A and is lying on the table. Another wire CD carrying 5A is held directly above AB at a height of 1 mm. Find the mass per unit length of the wire CD so that it remains suspended at its position when left free. Give the direction of the current flowing in CD with respect to that in AB.
[Take the value of g = 10 ms–2]


Current carried by wire AB= 12 A

Current carried by CD = 5 A

 i) The current carrying conductors will repel each other if the direction of current along both the conductor is same.

 ii) When current along the conductors is flowing in the same direction, then the wires will attract each other.

When CD remains suspended above AB, 

Error converting from MathML to accessible text.

where r is the distance between the wires.

 

Therefore, 

Error converting from MathML to accessible text.

The direction of current in CD should be opposite to the direction of current in AB. 

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14.

A convex lens of focal length f1 is kept in contact with a concave lens of focal length f2. Find the focal length of the combination.


Focal length of convex lens = +f1

 Now, using lens formula, we have

plus 1 over straight f subscript 1 space equals space fraction numerator 1 over denominator v apostrophe end fraction space minus space 1 over u                       ... (1) 

For concave lens of formula, focal length is (-f2)

negative 1 over straight f subscript 2 equals 1 over v space minus space fraction numerator 1 over denominator v apostrophe end fraction space                       ... (2) 

Now, adding equations (1) and (2), we have

1 over straight f subscript 1 space minus space 1 over f subscript 2 space equals space 1 over v space minus space 1 over u               ... (3) 

Now, for the combined lens using the lens formula, we have,

1 over straight f space equals space 1 over v space minus space 1 over u                        ... (4) 

Therefore, from equations (3) and (4), we have

1 over straight f space equals space 1 over f subscript 1 space minus space 1 over f subscript 2

This is the required focal length of the combination of lenses.

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15.

Define the activity of a given radioactive substance. Write its S.I. unit.          


The rate of decay of a radioactive substance is called activity of that substance. Activity is the negative of the rate of decay of the radioactive substance. 

SI unit is Becquerel (Bq.)

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16.

A wheel with 8 metallic spokes each 50 cm long is rotated with a speed of 120 rev/min in a plane normal to the horizontal component of the Earth’s magnetic field. The Earth’s magnetic field at the plane is 0.4 G and the angle of dip is 60°. Calculate the emf induced between the axle and the rim of the wheel. How will the value of emf be affected if the number of spokes were increased?


Emf induced in a rod of length ‘l’ with angular speed  in a uniform magnetic field is given by, 

Error converting from MathML to accessible text. 

For earth’s magnetic field,

Horizontal component is given by, 

straight B subscript straight H space equals space open vertical bar straight B subscript straight l space end subscript close vertical bar cos space straight delta

Vertical component is given by, 

straight B subscript straight V space equals space open vertical bar straight B subscript straight l close vertical bar space sin space straight delta 
Therefore, emf induced is given by, 

straight epsilon space equals space 1 half open vertical bar straight B subscript straight l close vertical bar space cos space straight delta space straight l squared space straight omega

space space space equals space 1 half space straight x space 0.4 straight x space 10 to the power of negative 4 end exponent space cos space 60 to the power of straight o space straight x space left parenthesis 0.5 right parenthesis squared space straight x space 2 πν space

space space space equals space 1 half straight x space 0.4 space straight x space 10 to the power of negative 4 end exponent space straight x space 1 half space straight x space left parenthesis 0.5 right parenthesis squared space straight x space 2 πν space straight x open parentheses space fraction numerator 120 space rev over denominator 60 space sec end fraction close parentheses space

space space space equals space 10 to the power of negative 5 end exponent space straight x space 0.25 space straight x space 2 space straight x space 3.14 space straight x space 2 space

space space space equals space 3.14 space straight x space 10 to the power of negative 5 end exponent space volt 

 Emf induced is not dependent on the number of spokes.

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17.

Define the current sensitivity of a galvanometer. Write its S.I. unit. Figure shows two circuits each having a galvanometer and a battery of 3 V. When the galvanometers in each arrangement do not show any deflection, obtain the ratio R1 / R2


Current sensitivity is defined as the ratio of deflection produced in the galvanometer to the current flowing through it.

 Mathematically it is given as, 

Current space Sensitivity comma space straight s subscript straight i space equals space straight theta over straight I

SI unit is radian per ampere.

No current flows through the galvanometer for balanced Wheatstone bridge.

4 over straight R subscript 1 space equals space 6 over 9 space

rightwards double arrow space R subscript 1 space equals space fraction numerator 4 space x space 9 over denominator 6 end fraction space equals 6 space capital omega
a n d space

6 over 12 space equals space R subscript 2 over 8 space

rightwards double arrow space R subscript 2 space equals fraction numerator 6 space x space 8 space over denominator 12 end fraction

therefore space R subscript 1 over R subscript 2 space equals space 6 over 4 equals 3 over 2

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18.

The graph shown in the figure represents a plot of current versus voltage for a given semiconductor.

  

Identify the region, if any, over which the semiconductor has a negative resistance.


The part of the curve BC has a negative slope because with increase in current, voltage decreases.

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19.

Write the expression for the de Broglie wavelength associated with a charged particle having charge ‘q’ and mass ‘m’, when it is accelerated by a potential V.


De-broglie wavelength for a charged particle is given by, 

                            straight lambda space equals space straight h over straight p space equals space fraction numerator straight h over denominator square root of 2 mqV end root end fraction

2291 Views

20.

A rectangular conductor LMNO is placed in a uniform magnetic field of 0.5 T. The field is directed perpendicular to the plane of the conductor. When the arm MN of length of 20 cm is moved towards left with a velocity of 10 ms–1, calculate the emf induced in the arm. Given the resistance of the arm to be 5 (assuming that other arms are of negligible resistance) find the value of the current in the arm.


Emf induced in a moving rod is given by, 

E = -Blv

Emf induced in a moving rod is given by, 

E = + Blv

   = 0.5 x 0.2 x 10

   = 1 v

Therefore, current in the rod is given by, 

I = straight epsilon over straight R space equals space 1 fifth space equals space 0.2 space A 

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