Electric Charges Fields And Gauss Law(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

ELECTRIC CHARGES FIELDS AND GAUSS LAW MCQs

Total Questions : 30 | Page 2 of 3 pages

Question 11. An electric dipole is kept in non-uniform electric field. It experiences

A force and a torque
A force but not a torque
A torque but not a force
Neither a force nor a torque
If assertion is false but reason is true.

Discuss Question

Answer: Option A. -> A force and a torque
:
A
As the dipole will feel two forces which are although opposite but not equal.
Therefore, A net force will be there and as these forces act at different points of a body. A torque is also there.

Question 12. Two electric dipoles of moment P and 64 P are placed in opposite direction on a line at a distance of 25 cm. The electric field will be zero at point between the dipoles whose distance from the dipole of moment P is

5 cm
259cm
10 cm
413cm
If assertion is false but reason is true.

Discuss Question

Answer: Option A. -> 5 cm
:
A
Suppose neutral point N lies at a distance x from dipole of moment

ρ

or at a distance

X_{2}

from
dipole of 64

ρ .

Two Electric Dipoles Of Moment P And 64 P Are Placed In Oppo...

\Rightarrow \frac{1}{4 π ϵ_{0}} \frac{2 p}{x^{3}} = \frac{1}{4 π ϵ_{0}} . \frac{2 (64 P)}{(25 - x)^{3}}

\Rightarrow \frac{1}{x^{3}} = \frac{64}{(25 - x)^{3}} \Rightarrow x = 5 c m .

Question 13. When a glass rod is rubbed with silk, it

Gains electrons from silk
Gives electrons to silk
Gains protons from silk
Gives protons to silk

Discuss Question

Answer: Option B. -> Gives electrons to silk
:
B
On rubbing glass rod with silk, excess electron transferred from glass to silk. So glass rod becomes positive and silk becomes negative.

Question 14. An electric dipole is placed along the x - axis at the origin O . A point P is at a distance of 20cm from this origin such that OP makes an angle

\frac{π}{3}

with the x-axis. If the electric field at P makes an angle θ with the x-axis, the value of

θ

would be

π3
π3+tan−1(√32)
2π3
tan−1(√32)
If assertion is false but reason is true.

Discuss Question

Answer: Option B. -> π3+tan−1(√32)
:
B

An Electric Dipole Is Placed Along The X - axis At The Ori...

θ = \frac{π}{3} + α

where

t a n α = \frac{1}{2} t a n \frac{π}{3}

\Rightarrow α = t a n^{- 1} \sqrt{\frac{3}{2}}

So,

θ = \frac{π}{3} + t a n^{- 1} \sqrt{\frac{3}{2}}

Question 15. The electric field due to a charge at a distance of 3 m from it is 500 N/coulomb. The magnitude of the charge is

[\frac{1}{4 π ϵ_{0}} = 9 \times 10^{9} \frac{N - m^{2}}{c o u l o m b^{2}}]

2.5 micro-coulomb
2.0 micro-coulomb
1.0 micro-coulomb
0.5 micro-coulomb

Discuss Question

Answer: Option D. -> 0.5 micro-coulomb
:
D

E = 9 \times 10^{9} \times \frac{Q}{r^{2}} \Rightarrow 500 = 9 \times 10^{9} \times \frac{Q}{(3)^{2}} \Rightarrow Q = 0.5 μ C

Question 16. Assertion: The coulomb force is the dominating force in the universe.
Reason :The coulomb force is weaker than the gravitational force.

If both assertion and reason are true and the reason is the correct explanation of the assertion.
If both assertion and reason are true but reason is not the correct explanation of the assertion.
If assertion is true but reason is false.
If the assertion and reason both are false.
If assertion is false but reason is true.

Discuss Question

Answer: Option D. -> If the assertion and reason both are false.
:
D
Gravitational force is the dominating force in nature and not coulomb's force. Gravitational force is the weakest force. Also, Coulomb's force>> gravitational force.

Question 17. An electron enters with its velocity in the direction of the uniform electric lines of force. Consider lines of force to be straight. Then

The path of the electron will be a circle
The path of the electron will be a parabola
The magnitude of velocity of the electron will first decrease and then increase
The magnitude of velocity of the electron will first increase and then decrease

Discuss Question

Answer: Option C. -> The magnitude of velocity of the electron will first decrease and then increase
:
C
Aselectric field applies the force on electron in the direction opposite to it’s motion, the velocity of the electron will decrease.

Question 18. The wrong statement about electric lines of force is

These originate from positive charge and end on negative charge
They do not intersect each other at a point
They have the same form for a point charge and a sphere
They have physical existence
If assertion is false but reason is true.

Discuss Question

Answer: Option D. -> They have physical existence
:
D
Thereis no proof that they exist, these are just imaginary lines.

Question 19. A solid metallic sphere has a charge + 3Q. concentric with this sphere is a conducting spherical shell having charge –Q. The radius of the sphere is a and that of the spherical shell is b. (b > a). What is the electric field at a distance R (a < R < b) from the centre.

Q2πϵ0R
3Q2πϵ0R
3Q4πϵ0R2
4Q2πϵ0R2
If assertion is false but reason is true.

Discuss Question

Answer: Option C. -> 3Q4πϵ0R2
:
C
We only need to consider the E contribution due to sphere.
Shell gives zero contribution to E at any point inside it.
Use direct formula.

E = \frac{1}{4 π ϵ_{0}} \frac{3 Q}{R^{2}}

E = \frac{3 Q}{4 π ϵ_{0} R^{2}}

Question 20. Two identical conductors of copper and aluminium are placed in an identical electric fields. The magnitude of induced charge in the aluminium will be