JEE Main 2020 exam - Physics ( 7th January first shift)
Q1. Which of the following gives a reversible operation?
For reversible operation, NOT gate is used. If an input is then output=. The following circuit represents the NOT gate
Q2. A 60 HP electric motor lifts an elevator having a maximum total load capacity of 1000 kg. If the frictional force on the elevator is 4000N , then the speed of the elevator at full load is close to: (1 HP=746 W, g=10 ms-2).
The elevator moves upward hence frictional force (F) opposes their motion downward.
so, net force act on elevator =mg+F
Fnet=(1000 x 10+4000)=14000 N
using power =Fnet x speed
Q3. An LCR circuit behaves like a damped harmonic oscillator. Comparing it with a physical spring-mass damped oscillator having dampling constant 'b', then the correct equivalence would be:-
For the spring-mass damping system, the governing equation is given by
For an LCR damped oscillator, the equation is
Comparing 1 and 2
So the answer is option 1
Q4. As shown in the figure, a bob of mass m is tied by a massless string whose other end portion is wound on a flywheel (disc) of radius r and mass m. When released from rest, the bob starts falling vertically. When is has covered a distance h, the angular speed of the wheel will be:-
Q5. Three-point particles of masses 1.0 kg, 1.5 kg and 2.5 kg are placed at three corners of a right triangle of sides 4.0 cm, 3.0 cm, and 5.0 cm as shown. The centre of mass of the system is at a point:
let m1=1 kg, m2=1.5 kg and m3=2.5 kg
x1=0, x2=3, x3=0 and y1=0, y2=0, y3=4
Let point A be origin and mass m1=1.0 kg be at origin.
so centre of mass of the system is at (0.9,2).
Q6. A parallel plate capacitor has plates of area A separated by distance d with a dielectric which has a dielectric constant that varies as
where x is the distance measured from one of the plates.
If then the total capacitance of the system is best given by expression by
If K filled between the plates -
Q7. The time period of revolution of electron in its ground state orbit in a hydrogen atom is . The frequency of the electron in its first excited state (in s-1) is:-
Velocity of the nth orbit
radius of the nth orbit
Q8. The current I1 (in A) flowing through resistor in the following circuit is:-
From equation (1) and (2)
Q9. A satellite of mass m is launched vertically upwards with an initial speed u from the surface of the earth. After it reaches a height of R(R=radius of r=earth), it ejects a rocket of mass so that subsequently the satellite moves in a circular orbit. The kinetic energy of the rocket is (G=gravitational constant; M= mass of the earth):
Before the rocket rejection
Apply energy conservation
After the rocket rejection
Apply momentum conservation
So Kinetic energy of the rocket
Q10. A long solenoid of radius R carries a time(t) dependent current . A ring of radius 2R is placed coaxially near its middle. During the time interval , the induced current (IR) and the induced EMF (VR) in the ring changes as:-
Now considering solenoid as ideal solenoid extended up to infinite and ring as its centre
Since induced emf is changing so current will also be changing
So, since direction of emf is changing so direction of current is also changing.
And it will be zero at t=0.5 sec
Q11. The radius of gyration of a uniform rod of length l, about an axis passing through a point away from the centre of the rod and perpendicular to it, is:-
Moment of inertia of rod about an axis perpendicular through COM
Radius of Gyration
Q12. Two moles of an ideal gas with are mixed 3 moles of another ideal gas with . The value of for the mixture is:-
For ideal gas:-
For first case:-
For second case:-
Q13. A liter of dry air at STP expands adiabatically to a volume of 3l. If , then work done by air is [take air to be an ideal gas]:-
At STP, Pressure = P1 = 100000 Pascal
Volume = V1 = 1 liter (As given in question)
Q14. If the magnetic field in a plane electromagnetic wave is given by ; then what will be the expression for the electric field?
wave is propagating in -x direction, i.e., in - i direction.
the direction of EMW wave is in the direction of .
Since B is in j direction and EMW is in -i direction. Therefore E is in (k) direction.
Q15. A polarizer-analyzer set is adjusted such that the intensity of light coming out of the analyzer is just 10% of the original intensity. Assuming that the polarizer-analyzer set does not absorb any light, the angle by which the analyzer needs to be rotated further to reduce the output intensity to be zero is:
output intensity is given by
Initial output intensity=10% of I_0
Final output intensity=O
means new angle is
the angle by which the analyser needs to be rotated further is
Q16. Speed of transverse wave of a straight wire (mass=6.0 g, length=60 cm and area of cross-section=1.0 mm2) is 90 ms-1. If the young's modulus of wire is , the extension of wire over its natural length is:
The linear mass density
Q17. Two infinite planes each with uniform surface charge density + are kept in each such a way that the angle between them is 300. The electric field in the region shown between them is given by:
Assuming sheet to be non conducting
Where y and x are corresponding unit vectors
Q18. If we used a magnification of 375 from a compound microscope of tube length 150mm and an objective of focal length 5mm, the focal length of the eyepiece should be close to :
The magnification is given by
Q19. Visible height of wavelength 600010-8 cm falls normally on a single slit and produces a diffraction pattern. It is found that the second diffraction min. is at 60 from the central max. If the first minimum is produced at , the is close to,
For first minima,
Q20. Consider a coil of wire carrying current I, forming a magnetic dipole. The magnetic flux through an infinite plane that contains the circular coil and excluding the circular coil area is given by i. The magnetic flux through the area given by o. Which of the following is correct?
a. i = -
b. i >
c. i <
d. i =
Flux going right comes back to the left (forms closed loop)
Flux inside the coil come bach through outside
Q21 A particle (m=1 kg) slides down a frictionless block(AOC) starting from rest at a point A (height 2 m). After reaching C, the particle continues to move freely in air as a projectile. When it reaches its highest point P (height 1 m), the kinetic energy of the particle (in J) is (take g=10 ms-2):-
Loss of potential energy= Gain in kinetic energy
Q22. A carnot engine separates between two reservoirs of temperature 900 K and 300 K. The engine performing 1200 J of work per cycle. The heat energy (in J) delivered by the engine to the low temperature reservoir in a cycle is:-
given, W=1200 J
Q23. A loop ABCDEFA of straight edges has six corner points A(0,0,0), B(5,0,0),C(5,5,0), D(0,5,0), E(0,5,5) and F(0,0,5). The magnetic field in this region is .The quantity of flux through the loop ABCDEFA (in Wb) is:-
Total area vectot=ara of ABCD+area of DEFA=
Total Magnetic flux=
Q24. A beam of electromagnetic radiation of intensity is comprised of wavelength . It falls normally on a metal (work function =2 eV) of surface area 1 cm2. If one in 103 photons ejects an electron, total number of electrons ejected in 1s is 10x (hc=1240 eVnm, 1 eV=1.6 x 10-19J), then x is :-
Energy in 1 second =
Q25. A non isotropic solid metal cube has coefficients of linear expansion as along the x-axis and along the y-axis anf z-axis. If the coefficient of volume expansion of the solid is , then the value of C is:-
Since, it is cube, so all it's side is equal. Let the length of cube be L.
Let the coefficient of volume expansion be
After increase in temperature:-
Increase in volume of cube=
and we can also write it as:- \
from equation 1 and 2 we get:-