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PHYSICS MCQs

Total Questions : 421 | Page 1 of 43 pages
Question 1.

The equivalent focal length of the two lenses in contact is 80 cm . If the focal length of one of the lenses is 20 cm. The power of the second lens is :


  1.    1.66 D
  2.    3.75 D
  3.    - 3.75 D
  4.    - 1.66 D
 Discuss Question
Answer: Option C. -> - 3.75 D

The focal length of the combination is given by

           `1/F = 1/f_1 + 1/f_2`

Here :  F = 80 cm = 0.8 m.,  `f_1` = 20 cm  = 0.2 m.

`:.`       `1/f_2` =` 1/F - 1/f_1`

or       `1/f_2 `= `1/0.8 - 1/0.2`

or       `1/f_2` = `(1 - 4)/(0.8) = -3/0.8 =  - 3.75`

But   `   p = 1/f`

So, power  `p_2 = 1/f_2`

Hence , `p_2` = - 3.75 D



Question 2.

If the critical angle for total internal reflection from a medium to vacuum is `30^circ` the velocity of light in the medium is


  1.    3 x `10^8` m/s
  2.    1.5x `10^8` m/s
  3.    6 x `10^8` m/s
  4.    2.4 x `10^8` m/s
 Discuss Question
Answer: Option B. -> 1.5x `10^8` m/s

From the relation , `mu = 1/sin C`

or   ` c_v/c_m = 1/sinC`

where C = critical angle = `30^circ`

`c_v` = velocity of light in vacuum  = 3 x `10^8` m/s

`c_m` = velocity of light in medium

Therefore ,  `(3 xx 10^8)/(c_m) = (1)/(sin30^circ)`

or  `c_m= 1/2 xx 3 xx 10^8`

               = 1.5 x` 10^8` m/s


Question 3.

The angle of prism is `6^circ` and its refractive index for green light is 1.5. For a green ray passes through it the deviation  will be :


  1.    `30^circ`
  2.    `25^circ`
  3.    `12^circ`
  4.    `3^circ`
 Discuss Question
Answer: Option D. -> `3^circ`

For small angled prism -   `delta m = (mu - 1)A`

where  `delta m ` = deviation of light in prism,

`mu` = refractive index = 1.5

A = angle of prism = `6^circ`

Therefore , `delta m` = (1.5 - 1) x `6^circ`

                                  = 0.5 x `6^circ =   3^circ`  



Question 4.

Two plane mirrors are inclined at an angle of` 45^circ` to each other . If an object is placed in between them , the number of images formed is :


  1.    4
  2.    5
  3.    6
  4.    7
 Discuss Question
Answer: Option D. -> 7

Number of images = `(360^circ)/(theta)-1`

                               = `(360^circ)/(45^circ) - 1`

                                = 8 - 1 = 7


Question 5.

Find the change in internal energy of the system when a system absorbs 2 kcal of heat and at the same time does 500 J of work :


  1.    7900 J
  2.    8200 J
  3.    5600 J
  4.    6400 J
 Discuss Question
Answer: Option A. -> 7900 J

According to the first law of thermodynamics

                    `Delta Q = Delta W + Delta u`

Given :         `Delta Q = 2 xx 10^3 xx 4.2 J`

                     `Delta W =  500 J`

So,               ` Delta u = 2 xx 10^3 xx 4.2 J - 500 J`

                      `            = 8400 - 500 =   7900 J`


Question 6.

The first law of thermodynamics is conserved with the conservation of  :


  1.    number of mole
  2.    energy
  3.    temperaure
  4.    number of molecules
 Discuss Question
Answer: Option B. -> energy

The first law of thermodynamics is basically the law of conservation of energy . It states that if a system absorbs heat `Delta Q` and as a result its internal energy changes by  `Delta u` and the system does a work  `Delta W`

Therefore , according to this law

        `Delta Q = Delta u + Delta W`


Question 7.

80 g of water at `30^circ C` is poured on a large block of ice at `0^circ C` .The mass of the ice that melts is :


  1.    160 g
  2.    80 g
  3.    40 g
  4.    30 g
 Discuss Question
Answer: Option D. -> 30 g

By the principle of calorimety    Heat lost = heat gain

         `Ms Delta theta `= mL

where :    m = mass of ice

M = mass of water = 80g

s = specific heat of water = 1 cal/`g^circ C`

`Delta theta` = temperature of water = `30^circ`

L = latent heat of ice = 80 cal/g

So,    80 x 1 x`30^circ` = m x 80

Hence ,         m = 30 g.  


Question 8.

Water is flowing through a horizontal pipe of non -uniform cross-section . At he extreme narrow portion of the pipe . The water will have :


  1.    maximum speed and least pressure
  2.    maximum pressure and maximum speed
  3.    both pressure and speed maximum
  4.    both pressure and speed least.
 Discuss Question
Answer: Option A. -> maximum speed and least pressure

Bernoulli's principle states that if speed of liquid at a place is maximum then at hat place pressure will be minimum Therefore at the extreme narrow portion of the pipe of non -uniform cross section, the speed is maximum , so pressure will be minimum .


Question 9.

At which of the following temperature the molecules of a gas have twice the average kinetic energy of molecules at  `20^circ C` :


  1.    `313^circ C`
  2.    `373^circ C`
  3.    `393^circ C`
  4.    `586^circ C`
 Discuss Question
Answer: Option A. -> `313^circ C`

We know that  E `prop` T

or ` E_1/E_2 = T_1/T_2`

Here    `E_1 = E`

             `E_2 = 2E`

            `T_1 = 20 + 273 =  293 K`

`:.`        `(E)/(2E) = 293/_2`

or         `T_2 = 586 K`

Hence ,    `T_2 = 596 - 273 = 313^circ C`



Question 10.

The root mean square velocity of molecules in a sample of helium is `5/7`th that of the molecules in a sample of hydrogen. If the temperature of hydrogen sample is `0^circ C` . Then the temperature of helium sample is about :


  1.    `0^circ C`
  2.    `230^circ K`
  3.    `5.6^circ K`
  4.    `100^circ C`
 Discuss Question
Answer: Option C. -> `5.6^circ K`

From the relation , `v_(rms)` =` sqrt((3R)/(m))`

or `(v_(rms)He)/(v_(rms) H_2)` = `sqrt(T_1/m_1 xx m_2/T_2)`

or   `5/7 = sqrt((T_1 xx 2)/(4 xx (0 + 273)))`

or     `(5/7)^2 = (T_1)/(2(273))`

or     `T_1 = (546 xx 25)/(49)`

                  = 278.6 K

Hence , `T_1 = 5.6^circ C`




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