JEE MAIN - Physics (2019 - 9th April Evening Slot)
1
A thin smooth rod of length L and mass M is
rotating freely with angular speed $$\omega $$0 about an
axis perpendicular to the rod and passing
through its center. Two beads of mass m and
negligible size are at the center of the rod
initially. The beads are free to slide along the
rod. The angular speed of the system , when
the beads reach the opposite ends of the rod,
will be :-
Answer
(C)
$${{M{\omega _0}} \over {M + 6m}}$$
2
A string 2.0 m long and fixed at its ends is
driven by a 240 Hz vibrator. The string vibrates
in its third harmonic mode. The speed of the
wave and its fundamental frequency is :-
Answer
(D)
320m/s, 80 Hz
3
In a conductor, if the number of conduction
electrons per unit volume is 8.5 × 1028 m–3 and
mean free time is 25ƒs (femto second), it's
approximate resistivity is :-
(me = 9.1 × 10–31 kg)
Answer
(A)
10–8 $$\Omega $$m
4
The parallel combination of two air filled
parallel plate capacitors of capacitance C and
nC is connected to a battery of voltage, V. When
the capacitors are fully charged, the battery is
removed and after that a dielectric material of
dielectric constant K is placed between the two
plates of the first capacitor. The new potential
difference of the combined system is :-
Answer
(C)
$${{(n+1)V} \over {K + n}}$$
5
Two materials having coefficients of thermal
conductivity '3K' and 'K' and thickness 'd' and
'3d', respectively, are joined to form a slab as
shown in the figure. The temperatures of the
outer surfaces are '$$\theta $$2' and '$$\theta $$1' respectively,
($$\theta $$2 > $$\theta $$1). The temperature at the interface is :-
A He+ ion is in its first excited state. Its
ionization energy is :-
Answer
(A)
13.60 eV
7
A wedge of mass M = 4m lies on a frictionless
plane. A particle of mass m approaches the
wedge with speed v. There is no friction
between the particle and the plane or between
the particle and the wedge. The maximum
height climbed by the particle on the wedge is
given by :-
Answer
(D)
$${{2{v^2}} \over {5g}}$$
8
A particle of mass 'm' is moving with speed '2v'
and collides with a mass '2m' moving with
speed 'v' in the same direction. After collision,
the first mass is stopped completely while the
second one splits into two particles each of
mass 'm', which move at angle 45° with respect
to the origianl direction.
The speed of each of the moving particle will
be :-
Answer
(A)
2 $$\sqrt2$$v
9
A wooden block floating in a bucket of water
has 4/5 of its volume submerged. When certain
amount of an oil is poured into the bucket, it
is found that the block is just under the oil
surface with half of its volume under water and
half in oil. The density of oil relative to that of
water is :-
Answer
(C)
0.6
10
A test particle is moving in a circular orbit in
the gravitational field produced by a mass
density $$\rho (r) = {K \over {{r^2}}}$$ . Identify the correct relation
between the radius R of the particle's orbit and
its period T
Answer
(D)
T/R is a constant
11
The position vector of a particle changes with
time according to the relation
$$\overrightarrow r (t) = 15{t^2}\widehat i + (4 - 20{t^2})\widehat j$$ What is the
magnitude of the acceleration at t = 1 ?
Answer
(A)
50
12
The specific heats, CP and CV of a gas of
diatomic molecules, A, are given (in units of
J mol–1 K–1) by 29 and 22, respectively.
Another gas of diatomic molecules, B, has the
corresponding values 30 and 21. If they are
treated as ideal gases, then :-
Answer
(B)
A has a vibrational mode but B has none
13
Four point charges –q, +q, +q and –q are placed
on y-axis at y = –2d, y = –d, y = +d and
y = +2d, respectively. The magnitude of the
electric field E at a point on the x-axis at
x = D, with D >> d, will behave as :-
Answer
(C)
$$E \propto {1 \over D^4}$$
14
A moving coil galvanometer has a coil with
175 turns and area 1 cm2. It uses a torsion band
of torsion constant 10–6 N-m/rad. The coil is
placed in a maganetic field B parallel to its
plane. The coil deflects by 1° for a current of
1 mA. The value of B (in Tesla) is
approximately :-
Answer
(D)
10–3
15
50 W/m2 energy density of sunlight is normally
incident on the surface of a solar panel. Some
part of incident energy (25%) is reflected from
the surface and the rest is absorbed. The force
exerted on 1m2 surface area will be close to
(c = 3 × 108 m/s) :-
Answer
(A)
20 × 10–8 N
16
The area of a square is 5.29 cm2. The area of
7 such squares taking into account the
significant figures is :-
Answer
(A)
37.0 cm2
17
The logic gate equivalent to the given logic
circuit is :-
Answer
(D)
OR
18
Moment of inertia of a body about a given axis
is 1.5 kg m2. Initially the body is at rest. In order
to produce a rotational kinetic energy of
1200 J, the angular accleration of 20 rad/s2
must be applied about the axis for a
duration of :-
Answer
(D)
2 s
19
A particle 'P' is formed due to a completely
inelastic collision of particles 'x' and 'y' having
de-Broglie wavelengths '$$\lambda $$x' and '$$\lambda $$y'
respectively. If x and y were moving in opposite
directions, then the de-Broglie wavelength of
'P' is :-
A convex lens of focal length 20 cm produces
images of the same magnification 2 when an
object is kept at two distances x1 and x2
(x1 > x2) from the lens. The ratio of x1 and x2
is :-
Answer
(A)
3 : 1
21
Diameter of the objective lens of a telescope is
250 cm. For light of wavelength 600nm.
coming from a distant object, the limit of
resolution of the telescope is close to :-
Answer
(A)
3.0 × 10–7 rad
22
A massless spring (k = 800 N/m), attached with
a mass (500 g) is completely immersed in 1 kg
of water. The spring is stretched by 2 cm and
released so that it starts vibrating. What would
be the order of magnitude of the change in the
temperature of water when the vibrations stop
completely ? (Assume that the water container
and spring receive negligible heat and specific
heat of mass = 400 J/kg K, specific heat of
water = 4184 J/kg K)
Answer
(C)
10–5K
23
A very long solenoid of radius R is carrying
current I(t) = kte–at(k > 0), as a function of time
(t $$ \ge $$ 0). counter clockwise current is taken to be
positive. A circular conducting coil of radius
2R is placed in the equatorial plane of the
solenoid and concentric with the solenoid. The
current induced in the outer coil is correctly
depicted, as a function of time, by :-
Answer
(A)
24
The position of a particle as a function of time
t, is given by x(t) = at + bt2 – ct3
where a, b and c are constants. When the
particle attains zero acceleration, then its
velocity will be :
Answer
(C)
$$a + {{{b^2}} \over {3c}}$$
25
A thin convex lens L (refractive index = 1.5)
is placed on a plane mirror M. When a pin is
placed at A, such that OA = 18 cm, its real
inverted image is formed at A itself, as shown
in figure. When a liquid of refractive index μ1
is put between the lens and the mirror, The pin
has to be moved to A', such that OA' = 27 cm,
to get its inverted real image at A' itself. The
value of μ1 will be :-
Answer
(D)
4 / 3
26
The resistance of a galvanometer is 50 ohm and
the maximum current which can be passed
through it is 0.002 A. What resistance must be
connected to it in order to convert it into an
ammeter of range 0 – 0.5 A ?
Answer
(B)
0.2 ohm
27
A metal wire of resistance 3 $$\Omega $$ is elongated to
make a uniform wire of double its previous
length. This new wire is now bent and the ends
joined to make a circle. If two points on this
circle make an angle 60° at the centre, the
equivalent resistance between these two points
will be :-
Answer
(D)
5 / 3 $$\Omega $$
28
Two coils 'P' and 'Q' are separated by some
distance. When a current of 3 A flows through
coil 'P', a magnetic flux of 10–3 Wb passes
through 'Q'. No current is passed through 'Q'.
When no current passes through 'P' and a
current of 2 A passes through 'Q', the flux
through 'P' is :-