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Quiz 7
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Q.1
Three uncharged capacitors of capacitance C1, C2 and C3 are connected as shown in figure to one another and to point A, B, and D at potentials VA , VB , VD . then the potential at O will be
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a)
0%
b)
0%
c)
0%
d)
Explanation
Potential drop across C1=VA - VO q1 / C1=VA - VO q1=(VA - VO)C1similarly q2=(VB - VO)C2q3=(VD - VO)C3From the law of conservation of charges q1 + q2 + q3=0 (VA - VO)C1 + (VB - VO)C2 + (VD - VO)C3=0 on rearranging turns we getAnswer: (b)
Q.2
The Gaussian surface for calculating the electric field due to a charge distribution is
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a) any closed surface around the charge distribution
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b) any surface near the charge distribution
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c)a spherical surface
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d)a symmetrical closed surface at every point of which electric field has a single fixed value
Explanation
Answer: (d)
Q.3
Three capacitors of capacitance 4µF, 6µF and 12µF are connected first in series and then in parallel. What is the ratio of equivalent capacitance in the two cases?
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a) 2:3
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b) 1:11
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c)11:1
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d)1:3
Explanation
Answer:(b)
Q.4
A parallel plate capacitor has a capacitance of 50pf in air and 105pf when immersed in oil. The dielectric constant of the oil is
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a) 50/105
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b) 1
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c) 2.1
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d) ∞
Explanation
C'=kC Answer: (c)
Q.5
When a capacitor is connected to a battery
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a)an alternating current flows in the circuit
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b) no current flows at all
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c)a current flows for some time and finally decreases to zero
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d)current keeps on increasing and reaches maximum after some time
Explanation
Answer: (c)
Q.6
When a charge q is moved once round a circle of radius R with charge Q at the centre of the circle, work done is
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a) zero
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b) Qq / 4πεoR
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c)Qq / 4πεoR2
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d)Qq /R
Explanation
Answer: (a)
Q.7
A work of 100 J is performed in carrying a charge of -5C from infinity to a particular point in an electrostatic field. The potential of this point is
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a) 100V
0%
b) 5V
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c)-20V
0%
d)20V
Explanation
Potential at point=Work/charge V=100/(-5)=-20V Answer:(c)
Q.8
Two copper spheres A and B of same size are charged to some potential. A is hollow and B is solid. Which of the two holds more charge>
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a) Solid sphere cannot hold any charge
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b) Hollow sphere can not hold any charge
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c) Both have zero charge
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d) Both have same charge
Explanation
Answer: (d)
Q.9
Potential energy of two equal negative point charges 2µC each held apart in air is
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a)2J
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b) 2eV
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c)4J
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d)0.036J
Explanation
From formula V=Kq1q2 / rV=9×109×4×10-12 / 1=0.036JAnswer: (d)
Q.10
An electron initially at rest is accelerated through a P.D. of one volt. The energy acquired by electrons is
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a) 1 J
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b) 1.6×10-19 J
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c)10-19 J
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d)1.6×10-19 erg
Explanation
Answer: (b)
Q.11
The S.I. unit of surface integral of electric field is
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a) NC-1
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b) Nm2C
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c)J/C
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d)V/m
Explanation
Answer:(c)
Q.12
A bird sitting on a high power line
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a) gets killed instantly
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b) gets a mild shock
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c) is not affected practically
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d) gets a fatal shock
Explanation
Answer: (c)
Q.13
The number of lines of force that radiate outwards from one coloumb of charge is
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a)9×109
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b) 8.85×10-10
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c)infinite
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d)1.13×1011
Explanation
from Gauss's theorem φ=∑ (q/εo φ=1 / (8.85×10-12=1.13×1011Answer: (d)
Q.14
The mutual interaction between two charged bodies is best understood in terms of
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a) the action at a distance view
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b) the field concept
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c)a material connection between the charges
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d)medium in between the charges
Explanation
Answer: (b)
Q.15
Torque on a dipole in an electric field is maximum when angle between p and E is
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a) 0°
0%
b) 90°
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c)45°
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d)180°
Explanation
Answer:(b)
Q.16
When an electric dipole is placed in a uniform electric field, it experiences
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a) a force as well as torque
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b) a torque but not force
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c) a force but not torque
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d) neither for nor torque
Explanation
Answer: (b)
Q.17
The value of a charge q at the centre of two equal and like charges Q so that the three charges are in equilibrium [ kerala P.M.T 2004]
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a)-Q/4
0%
b) +Q/4
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c)Q
0%
d)Q/2
Explanation
resultant for on Q should be zeroLet distance between two Q charges be 2r kqQ/r2 =kQQ/(2r)2 q=Q/4 since Q charges are like q should be negative ∴ q=-Q/4Answer: (a)
Q.18
A charged spherical shell does not produce an electric field at any [ kerala P.M.T. 2004]
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a) interior point
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b) outer point
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c)beyond 2 meters
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d)beyond 10 meters
Explanation
Answer: (a)
Q.19
The charge on 4µF capacitor in the given circuit is
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a)12 µF
0%
b) 24 µF
0%
c)36 µF
0%
d)32 µF
Explanation
1µF and 5µF condensers are in parallelCp=1+5=6µF Now 4µF and 6µF capacitors are in series∴ (1/Cs)=(1/4) + (1/6)=(5/12) Cs=12/5=2.4 µF Charge on each capacitor 4µF and 6µF capacitor=Cs V=2.4×10=24µC Answer:(b)
Q.20
Two equal metal balls are charged to 10 and -20 units of electricity. Then they are brought in contact with each other and then are brought in to original distance. The ratio of magnitudes of force between the two balls before and after contact is [ Kerala P.M.T 2004]
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a) 8:1
0%
b) 1:8
0%
c) 1:2
0%
d) 2:1
Explanation
Here q1=10 units and q2=-20 units On touching, charges are redistributed charge on each sphere is q=(-20+10) /2=-5 units Now F1 / F2=q12 / q=200/25=8:1 Answer: (a)
Q.21
A conductor having a cavity is given a positive charge. Then field strengths EA, EB and EC at points A ( with in cavity) at B (within conductor) and C ( outside conductor) will e [ kerala P.M.T 2004]
0%
a)EA=0, EB=0, EC=0
0%
b) EA=0, EB=0, EC ≠=0
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c)EA ≠=0, EB=0, EC≠=0
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d)EA ≠=0, EB≠=0 , EC≠=0
Explanation
Electric field inside the cavity and inside metallic conductor is zero, and out side the conductor electric field is not zeroAnswer: (b)
Q.22
The potential at point P, which is forming a corner of a square of side 93mm with charges Q1=33 nC, Q2 -51 nC, Q3=47nC located at the other three corners is nearly [ kerala P.E.T 2004]
0%
a) 16 kV
0%
b) 4 kV
0%
c)400 V
0%
d)160 V
Explanation
Sides are 93nm thus diagonal QR=93√2 Potential at P is scalar addition of potential at P due to individual charges Answer: (b)
Q.23
The plates of a parallel plate capacitor are charged up to 100V. A 2mm thick plate is inserted between the plates, then to maintain the same potential difference between capacitor plates, the distance between the capacitor plate is increased by 1.6 mm. The dielectric constant of the plate is
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a)5
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b) 1.25
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c)4
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d)2.5
Explanation
To maintain same potential capacitance in both the cases must be same thus εA/d=εA / d' Thus d=d' let t be the thickness of dielectric slabbut d'=d - t +t/k+1.6 d=d -t +t/k+1.6 t -t/k-1.6=0t( 1- (1/k) -1.6=02( 1- (1/k)=1.61 - (1/k)=0.8 1/k=1-0.8k=5Answer: (a)
Q.24
If one penetrates a uniformly charged spherical shell, the electric field strength E
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a) increases
0%
b) decreases
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c)remains same as surface
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d)is zero at all points
Explanation
Answer: (d)
Q.25
Two small spheres, each carrying a charge q, placed r meters apart, repel each other with a force F. If one of the spheres is taken around the other one in a circular path of radius r, the work done will be
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a) F × r
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b) F × 2πr
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c)F / 2πr
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d)zero
Explanation
Answer:(d)
Q.26
An electric dipole consists of two opposite charges each of magnitude 1.0µC separated by a distance of 2.0cm. The dipole is placed in an external field of 1.0×105 N/C. The max. torque on the dipole is
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a) 0.2×10-3 N-m
0%
b) 1.0×10-3 N-m
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c) 2.0×10-3 N-m
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d) 4.0×10-3 N-m
Explanation
p=q(2a) τ=pEsinθ Answer: (c)
Q.27
A positively charged sphere hangs from a silk thread. We put a positive test charge qo at a point and measure F/qo then it can be predicted that the electric field strength E is
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a) > F/qo
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b)=F/qo
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c)< F/qo
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d)can not be estimated
Explanation
Here, the electric field due to test charge +qo opposes the field of positive charge being measured. therefore, the measured value is less than the actual value (F/qo)Answer: (a)
Q.28
A hollow metallic sphere of radius 10cm is given a charge of 3.2×10-9 C. The electric intensity at a point 4cm from the centre is
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a) 9×10-9 V
0%
b) 288 V
0%
c)2.88 V
0%
d)zero
Explanation
Answer: (d)
Q.29
The electric potential at the surface of an atomic nucleus ( Z=50) of radius 9.0×10-15 m is
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a) 80V
0%
b) 8 ×106V
0%
c)9 Volts
0%
d)9 ×105 volts
Explanation
given number of protons Z=50. Charge on nucleus Q=50×1.6×10-19 C use formula V=kQ/r Answer:(b)
Q.30
Hollow metal sphere of radius 5 cm is charged such that the potential on its surface is 10V. The potential at a distance of 2cm from the centre of the sphere is
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a) zero
0%
b) 10 V
0%
c) 4 V
0%
d) 10/3 V
Explanation
Potential inside the hollow sphere=potential at its surface Answer: (b)
Q.31
Particle A has charge +Q and particle B has charge +4Q with each of them having the same mass. When allowed to fall from rest through same electrical potential difference, ratio of their speed VA / VB will become
0%
a)2:1
0%
b) 1:2
0%
c)1:4
0%
d)4:1
Explanation
Potential of A=qV and potential energy of B=4qV Thus ratio of kinetic energy=ratio of squares of velocities as mass of both is same vA / vB=1/2Answer: (b)
Q.32
Which of the following is a volt
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a) erg per cm
0%
b) joule per coulomb
0%
c) erg per ampere
0%
d) newton/(coulomb×metre2)
Explanation
Answer: (b)
Q.33
The capacitance of an isolated conducting sphere of radius R is proportional to
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a) R2
0%
b)1 / R2
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c)1 /R
0%
d)R
Explanation
Answer:(d)
Q.34
A condenser is connected across another charged condenser. The energy in the two condenser will
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a) Be equal to the energy in the initial condenser
0%
b) Be less than that in the initial condenser
0%
c) Be more than that in the initial condenser
0%
d) BE more or less depending on the relative capacitances of the two condenser
Explanation
Answer: (b)
Q.35
Work done in moving a unit positive charge through a distance of x meter on an equipotential surface is [ Panjab C.E.T 1997]
0%
a)x joule
0%
b) (1/x) joule
0%
c)zero
0%
d)x2 joule
Explanation
Answer: (c)
Q.36
If a positive charge is shifted from lower potential to region of higher potential region, the electric potential energy
0%
a) decreases
0%
b) increases
0%
c)remains same
0%
d)may increase / decrease
Explanation
Answer: (b)
Q.37
If the flux of electric field through a closed surface is zero
0%
a) electric field must be zero everywhere on the surface
0%
b) electric field may be zero everywhere on the surface
0%
c)charge in the vicinity of the surface must be zero
0%
d)charge inside the surface must be zero
Explanation
Answer:(b, d )
Q.38
The electric potential V at any point x, y, z ( all in meters) in space is given by V=4x2 volt. The electric field at the point (1m, 0, 2m) in volt/meter is
0%
a) 8 V/m along negative X -axis
0%
b) 8 V/m along positive X -axis
0%
c) 16 V/m along negative X axis
0%
d) 16 V/m along positive Z axis
Explanation
Thus 8V/m along negative X-axis Answer: (a)
Q.39
A block of mass carrying a positive charge q is placed on a smooth horizontal table, which ends in a vertical wall situated at a distance d from block. An electric field E is switched on towards right. Assuming elastic collisions, find the time period of resultant oscillation
0%
a)
0%
b)
0%
c)
0%
d)
Explanation
two times the time taken by the block to collide on wall is the periodic time of oscillationAcceleration of the block a=qE/m d=(1/2)at2d=(1/2) (qE/m) t2 Thus 2t=√(8md/qE) Answer: (b)
Q.40
An electron having a charge -e located at A in the presence of a point charge +Q located at B is moved to a point C so that ABC is an equilateral . The work done in this process is
0%
a) Q / 4πεoAC
0%
b) Qe / 4πεoAC
0%
c)-Qe / 4πεoAC
0%
d)zero
Explanation
Electron is moving from A to C. A and C are equidistant from B and hence the potential at A due to charge +Q=potential at C due to the same charge. Since there is no potential difference between A and C no work is done in moving charge fro A to CAnswer: (d)
Q.41
An infinite number of charges each equal to q are placed along x-axis at x=1, x=2, x=4, x=8 and so on . the resultant potential at x=0 will be
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a) q / 2πεo
0%
b) q/4πεo
0%
c)q/8πεo
0%
d)q/εo
Explanation
Answer:(a)
Q.42
A solid conducting sphere of charge Q is surrounded by an uncharged concentric conducting spherical shell. The potential difference between the sphere and the shell is V. If the shell is now given a charge of -3Q, the new potential difference between them will be
0%
a) V
0%
b) 2V
0%
c) 4V
0%
d) -2V
Explanation
The potential difference depends only on the charge on the inner sphere Answer: (a)
Q.43
The effective capacitance between A and B is ( each capacitor is of 1µF)
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a)(15/2) µF
0%
b) (17/3)µF
0%
c)(13/8)µF
0%
d)(19/8)µF
Explanation
Circuit can be redrawn as Answer: (c)
Q.44
Two charged metal spheres of capacitance C1 and C2 are brought in contact and then separated. The final charges Q1 and Q2 on them will satisfy the condition
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a) Q1C1=Q2C2
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b) Q1C2 > Q2C1
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c)Q1C2 < Q2C2
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d)Q1C2=Q2C1
Explanation
both the spheres will save same potentials Q1 / C1=Q2 / C2 Answer: (d)
Q.45
How many electrons must be added to spherical conductor of radius 10cm to produce a field of 2×10-3 N/C just above the surface
0%
a) 1.39×104
0%
b) 1.6×105
0%
c)3×104
0%
d)9.1×104
Explanation
The entire charge on the spherical conductor behaves as if the entire charge is concentrated at the centre of the sphere, with respect to an external pointE=KQ/r2=K×ne/r2 Answer:(a)
Q.46
How should 5 capacitors each of value 1µF be connected so as to produce a total capacitance (3/7)µF?
0%
a) Two capacitors in parallel and the combination in series with other three capacitors
0%
b) Three capacitors in a parallel and the combination in series with other two capacitors
0%
c) Four capacitors in parallel and the combination in series with fifth capacitor
0%
d) All capacitor in parallel
Explanation
Answer: (b)
Q.47
A neutron, a proton and an electron are placed in uniform electric field
0%
a)The forces acting on them will be equal
0%
b) Their accelerations may be equal
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c)Magnitude of acceleration may be equal
0%
d)Magnitude of acceleration will be different
Explanation
Charge on Neutron=0, Force=0mass of proton and electron is different hence acceleration differentAnswer: (d)
Q.48
A particle of mass m and charge q is fastened to one end of a string of length l. The other end of the spring is fixed to the point O. The whole system lies on a frictionless horizontal plane. Initially, the mass is at rest at A. A uniform electric field in the direction shown is then switched on. Then
0%
a) the speed of the particle when it reaches B is
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b) the speed of the particle when it reaches B is
0%
c)the tension in the string when the particle reaches B is qE
0%
d)the tension in the string when the particle reaches B is zero
Explanation
Work done=qE(l -lcosθ) Using work energy theorem Work done=ΔK.E.qE(l-lcosθ)=(1/2) mv2v=√(qEl/m) Answer: (b)
Q.49
The force between two point charges in vacuum is 15N, if brass plate is introduced between the two charges, then force between them will
0%
a) Become zero
0%
b) Remains same
0%
c)Becomes 30N
0%
d)Becomes 60N
Explanation
Answer: (a)
Q.50
A ball of mass 100 gm and having a charge 4.9×10-5C is released from rest in vertical region where a horizontal electric field 2×104 N/C exists. The resultant force acting on the ball is
0%
a) 1.4 N in the direction of g
0%
b) 1.4 N making an angle of 45° with the horizontal
0%
c) 0.98 N in the direction of electric field
0%
d) 1.96 N along vertical
Explanation
angle between Electric and gravitational force is 90° Answer: (b)
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