Physics - Online Test
Q1. Two parallel plate capacitors of capacitances C and 2C are connected in parallel and charged to a potential difference V by a battery. The battery is then disconnected and the space between the plates of capacitor C is completely filled with a material of dielectric constant K = 3. The potential difference across the capacitors now becomes
Answer : Option D
Explaination / Solution:
The charges on the capacitors after being charged to a potential V are.After being filled with a material of dielectric K=3 the capacitor which initially had a capacitance C has now the capacitance KC=3C. The common potential
Q2. A disk of radius a/4 having a uniformly distributed charge 6C is placed in the x-y plane with its centre at (−a/2, 0, 0). A rod of length a carrying a uniformly distributed charge 8C is placed on the x-axis from x = a/4 to x = 5a/4. Two point charges −7C and 3C are placed at (a/4, −a/4, 0) and (−3a/4, 3a/4, 0), respectively. Consider a cubical surface formed by six surfaces . The electric flux through this cubical surface is
Answer : Option C
Explaination / Solution:
Electric flux through a surface is given by Where q= net charge enclosed inside the surface Since half the disc lies inside the surface and one fourth of rod lies inside the surface, point charge lies inside the surface and point charge 3C lies outside the surface So the net charge enclosed by the surface is So
Q3. In the network shown in the figure, C1 = 6 and C = 9 . The equivalent capacitance between points P and Q is
Answer : Option B
Explaination / Solution:
The circuit is reduced as follows: The capacitances are in micro farads.
The capacitance between P and Q is 3μF
The capacitance between P and Q is 3μF
Q4. Under the influence of the coulomb field of charge +Q, a charge −q is moving around it in an elliptical orbit. Find out the correct statement(s).
Answer : Option A
Explaination / Solution:
Since the charge –q is moving in elliptical orbit so to make its motion stable the total angular momentum of the charge is constant since it experience a centripetal force from the charge +Q so it follow the motion as the motion of earth around sun.
Q5. Three capacitors, each of capacitance C = 3 mF, are connected as shown in the figure. The equivalent capacitance between points P and S is
Answer : Option A
Explaination / Solution:
If P is at positive potential, then Q is at negative potential and R is at positive potential. The system therefore reduces to 3 capacitors in parallel. C= 9μF
If P is at positive potential, then Q is at negative potential and R is at positive potential. The system therefore reduces to 3 capacitors in parallel. C= 9μF
Q6. Consider a system of three charges placed at points A, B and C, respectively, as shown in the figure. Take O to be the centre of the circle of radius R and angle CAB =
Answer : Option C
Explaination / Solution:
The electric field due to charges at A and B are equal and opposite, So at O the electric field is due to C only, which has a magnitude The potential energy of the system is not zero Force between B and C
Q7. An oil drop of 12 excess electrons is held stationary under a constant electric field of in Millikan’s oil drop experiment. The density of the oil is 1.26 g cm-3. Estimate the radius of the drop. (g = 9.81 m ; e = 1.60 × 10-19 C).
Answer : Option D
Explaination / Solution:
Density of oil, ρ = 1.26 gm/ = 1.26 × kg/ Acceleration due to gravity, g = 9.81 m Charge on an electron, e = 1.6 × C Radius of the oil drop = r Force (F) due to electric field E is equal to the weight of the oil drop (W) F = W Eq = mg Ene=mg Where,q = Net charge on the oil drop = ne m = Mass of the oil drop= Volume of the oil drop × Density of oil So 2.55×(12×1.6×)= 1.26X × So r = 9.82 × mm Therefore, the radius of the oil drop is 9.82 × mm.
Q8. A parallel plate air filled capacitor shown in the Fig. (a) has a capacitance of 2 . When it is half filled with a dielectric of dielectric constant k = 3 as shown in Fig. (b), its capacitance becomes
Answer : Option D
Explaination / Solution:
The capacitance of the first capacitorThe second capacitor is considered to be made of two capacitors C1( air filled) and C2( dielectric) connected in series.
The equivalent capacitance
The equivalent capacitance
Q9. A point charge q = -8.0 nC is located at the origin. Magnitude of the electric field vector at the field point x = 0.949m y = -1.643m is
Answer : Option C
Explaination / Solution:
Given x= 0.949m and y= -1.643 m So
Q10. A parallel plate air filled capacitor shown in Fig. (a) has a capacitance of 2 . When it is half filled with a dielectric of dielectric constant k= 3 as shown in Fig. (b), its capacitance becomes
Answer : Option B
Explaination / Solution:
The capacitance of the first capacitor The second capacitor is considered to be made of two capacitors ( air filled) and ( dielectric) connected in parallel.
