JEE Mains 2021 Answer Key (Official) for February session has been released. It is released separately for four session in February, March, April and May sessions. JEE Mains is conducted for providing admission into UG engineering (B.Tech) / architecture courses (B.Arch). These courses are offered by various IITs, NITs and IIITs and other government and non-government engineering institutions. JEE Mains answer key is released by the National Testing Agency (NTA). Through this blog, candidates can get the JEE Mains Answer Keys 2021 and also previous year answer key and question papers with all solutions.

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## JEE Mains 2021 Answer Key with Solutions- 25 February Shift 1

### Physics Answer Key JEE Mains 2021 25 Feb Shift 1

**Question 1**

Given below are two statement : one is labelled as Assertion A and the other is labelled as Reason R.

Assertion A : When a rod lying freely is heated, no thermal stress is developed in it.

Reason R : On heating the length of the rod increases.

In the light of the above statements, choose the correct answer from the options given below:

- Both A and R are true but R is NOT the correct explanation of A
- A is false but R is true
- A is true but R is false
- Both A and R are true and R is the correct explanation of A

Correct answer: 1

**Question **2

A student is performing the experiment of resonance column. The diameter of the column tube is 6 cm. The frequency of the tuning fork is 504 Hz. Speed of the sound at the given temperature is 336 m/s. The zero of the meter scale coincides with the top end of the resonance column tube. The reading of the water level in the column when the first resonance occurs is:

- 13 cm
- 16.6 cm
- 18.4 cm
- 14.8 cm

Correct answer: 4

**Question **3

Two satellites A and B of masses 200kg and 400kg are revolving round the earth at height of 600 km and 1600 km respectively. If T_{A} and T_{B} are the time periods of A and B respectively then the value of T_{B} – T_{A}:

- 1.33 × 10
^{3}s - 3.33 × 10
^{2}s - 4.24 × 10
^{3}s - 4.24 × 10
^{2}s

Correct answer: 1

**Question **4

The angular frequency of alternating current in a L-C-R circuit is 100 rad/s. The components connected are shown in the figure. Find the value of inductance of the coil and capacity of condenser.

- 0.8 H and 150 mF
- 0.8 H and 250 mF
- 1.33 H and 250 mF
- 1.33 H and 150 mF

Correct answer: 2

**Question **5

A proton, a deuteron and an a particle are moving with same momentum in a uniform magnetic field. The ratio of magnetic forces acting on them is ____ and their speed is ____ in the ratio.

- 1 : 2 : 4 and 2 : 1 : 1
- 2 : 1 : 1 and 4 : 2 : 1
- 4 : 2 : 1 and 2 : 1 : 1
- 1 : 2 : 4 and 1 : 1 : 2

Correct answer: 2

**Question **6

Given below are two statement :

Statement-I: A speech signal of 2 kHz is used to modulate a carrier signal of 1 MHz. The band width requirement for the signal is 4 kHz.

Statement-II : The side band frequencies are 1002 kHz. and 998 kHz. In the light of the above statements, choose the correct answer from the options given below:

- Statement I is true but Statement II is false
- Statement I is false but Statement II is true
- Both Statement I and Statement II are true
- Both Statement I and Statement II are false

Correct answer: 3

**Question **7

If the time period of a two meter long simple pendulum is 2s, the acceleration due to gravity at the place where pendulum is executing S.H.M. is :

- π
^{2}ms^{-2} - 9.8 ms
^{-2} - 2π
^{2}ms^{-2} - 16 ms
^{-2}

Correct answer: 3

**Question **8

The pitch of the screw gauge is 1mm and there are 100 divisions on the circular scale. When nothing is put in between the jaws, the zero of the circular scale lies 8 divisions below the reference line. When a wire is placed between the jaws, the first linear scale division is clearly visible while 72nd division on circular scale coincides with the reference line. The radius of the wire is

- 1.64 mm
- 0.82 mm
- 1.80 mm
- 0.90 mm

Correct answer: 2

**Question **9

A 5V battery is connected across the points X and Y. Assume D_{1} and D_{2} to be normal silicon diodes. Find the current supplied by the battery if the +ve terminal of the battery is connected to point X.

1. ~0.5 A

2. ~1.5 A

3. ~0.86 A

4. ~0.43 A

Correct answer: 4

**Question 1**0

An α particle and a proton are accelerated from rest by a potential difference of 200 V. After this, their de Broglie wavelengths are λ_{α} and λ_{p} respectively. The ratio λ_{α}/λ_{p} is :

- 3.8
- 8
- 7.8
- 2.8

Correct answer: 4

**Question 1**1

A diatomic gas, having C_{p }= 7R/2 and C_{v }= 5R/2, is heated at constant pressure. The ratio dU : dQ : dW :

- 5 : 7 : 3
- 5 : 7 : 2
- 3 : 7 : 2
- 3 : 5 : 2

Correct answer: 2

**Question 1**2

An engine of a train, moving with uniform acceleration, passes the signal-post with velocity u and the last compartment with velocity v. The velocity with which middle point of the train passes the signal post is:

- √((v
^{2}+u^{2})/2) - (v-u)/2
- (v+u)/2
- √((v
^{2}-u^{2})/2)

Correct answer: 1

**Question 1**3

Match List-I with List-II :

List-I | List-II |
---|---|

h (Planck’s constant) | [M L^{2} T^{–2}] |

E (kinetic energy) | [M L^{2} T^{–1}] |

V (electric potential) | [M L^{2} T^{–2}] |

P (linear momentum) | [M L^{2} I^{-1}T^{–3}] |

Choose the correct answer from the options given below :

- (a)→(iii), (b)→(iv), (c)→(ii), (d)→( i )
- (a)→(ii), (b)→(iii), (c)→(iv), (d)→( i )
- (a)→(i), (b)→(ii), (c)→(iv), (d)→( iii )
- (a)→(iii), (b)→(ii), (c)→(iv), (d)→( i )

Correct answer: 2

**Question 1**4

Magnetic fields at two points on the axis of a circular coil at a distance of 0.05m and 0.2 m from the centre are in the ratio 8 : 1. The radius of coil is ____.

- 0.2 m
- 0.1 m
- 0.15 m
- 1.0 m

Correct answer: 2

**Question 1**5

A solid sphere of radius R gravitationally attracts a particle placed at 3R form its centre with a force F_{1}. Now a spherical cavity of radius R/2 is made in the sphere (as shown in figure) and the force becomes F_{2}. The value of F_{1}:F_{2} is:

- 25:36
- 36:25
- 50:41
- 41:50

Correct answer: 3

**Question 1**6

Two radioactive substances X and Y originally have N_{1} and N_{2} nuclei respectively.Half life of X is half of the half life of Y. After three half lives of Y,number of nuclei of both are equal. The ratio N_{1}/N_{2} will be equal to:

- 1/8
- 3/1
- 8/1
- 1/3

Correct answer: 3

**Question 1**7

In an octagon ABCDEFGH of equal side, what is sum of

- -16i-24j+32k
- 16i+24j-32k
- 16i+24j+32k
- 16i-24j+32k

Correct answer: 2

**Question 1**8

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R. Assertion A:

Assertion A: The escape velocities of planet A and B are same. But A and B are of unequal mass.

Reason R: The product of their mass and radius must be same，M_{1}R_{1}=M_{2}R_{2}

In the light of the above statements, choose the most appropriate answer from the options given below:

- Both A and R are correct but R is NOT the correct explanation of A
- A is correct but R is not correct
- Both A and Rare correct and R is the correct explanation of A
- A is not correct but R is correct

Correct answer: 2

**Question 1**9

The current (i) at time t=0 and t=∞ respectively for the given circuit is:

- 18E/55, 5E/18
- 10E/33, 5E/18
- 5E/18, 18E/55
- 5E/18, 10E/33

Correct answer: 4

**Question **20

Two coherent light sources having intensity in the ration 2x produce an interference pattern.

The ratio (I_{max}-I_{min})/(I_{max}+I_{min}) will be

- 2√(2x) / (x+1)
- √(2x) / (2x+1)
- √(2x) / (x+1)
- 2√(2x) / (2x+1)

Correct answer: 4

**Question **21

A transmitting station releases waves of wavelength 960 m. A capacitor of 2.56 mF is used in the resonant circuit. The self inductance of coil necessary for resonance is ____ × 10–8 H.

Correct answer: 10

**Question **22

The electric field in a region is given by

E=(0.6E_{0} i + 0.8E_{0} j) N/C

The ratio of flux of reported field through the rectangular surface of area 0.2 m^{2} (parallel to y – z plane) to that of the surface of area 0.3 m^{2} (parallel to x – z plane) is a : b, where a = _____.

[Here i, j and k are unit vectors along x, y and z-axes respectively]

Correct answer: 1.00

**Question **23

In a certain thermodynamic process, the pressure of a gas depends on its volume as kV3. The work done when the temperature changes from 100°C to 300°C will be ___ nR, where n denotes number of moles of a gas.

Correct answer: 50.0

**Question **24

A small bob tied at one end of a thin string of length 1m is describing a vertical circle so that the maximum and minimum tension in the string are in the ratio 5 : 1. The velocity of the bob at the height position is _____ m/s. (Take g = 10 m/s^{2})

Correct answer: 5.00

**Question **25

In the given circuit of potentiometer, the potential difference E across AB (10m length) is larger than E_{1} and E_{2} as well. For key K _{1 }( closed), the jockey is adjusted to touch the wire at point J_{1} so that there is no deflection in the galvanometer. Now the first battery (E_{1}) is replaced by second battery (E_{2}) for working by making K_{1} open and K_{2 } closed. The galvanometer gives then null deflection at J_{2}.

The value of is **E _{1}**/E

_{2}is a/b, where a = ___.

Correct answer: 1.00

**Question **26

The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is _____ cm.

Correct answer: 15.00

**Question **27

512 identical drops of mercury are charged to a potential of 2V each. The drops are joined to form a single drop. The potential of this drop is ____ V.

Correct answer: 128.00

**Question **28

A coil of inductance 2H having negligible resistance is connected to a source of supply whose voltage is given by V = 3t volt. ( where t is in second). If the voltage is applied when t = 0, then the energy stored in the coil after 4s is _____ J.

Correct answer: 144.00

**Question **29

A monoatomic gas of mass 4.0 u is kept in an insulated container. Container is moving with velocity 30 m/s. If container is suddenly stopped then change in temperature of the gas (R=gas constant) is x/3R.value of x is

Correct answer: 3600

**Question **30

The potential energy (U) of a diatomic molecule is a function dependent on r (interatomic distance) as U= α/r^{10} – β/r^{5} -3 where α and β are positive constants. The equilibrium distance between two atoms will be (2α/**β**)^{a/b}, where a is

Correct answer: 1.00

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