# Physics Questions and Answers – Coulomb’s Law

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Coulomb’s Law”.

1. Coulomb’s Law is valid for ______
a) Only point charge
b) For both point charge and distributed charge
c) Only distributed charges
d) Neither distributed nor point charge

Explanation: Coulomb’s Law explains the force between two point charges only. A distributed charge can be considered as the sum of infinite point charges and thus the force between two distributed charge systems can be explained.

2. Coulomb’s Law is valid for any distance between the particles.
a) False
b) True

Explanation: Coulomb’s Law is not valid for inter-atomic distance i.e. distance less than 10-15 m. Besides, if the distance between the two bodies is less than the size of any of the bodies, this law is also invalid. We can use this law only in all other cases.

3. Which one of the following is similar between electrostatic force and gravitational force?
a) Force can be attractive or repulsive
b) The force depends on the medium between the bodies
c) Both the forces are strong forces
d) Force is inversely proportional to the distance between the bodies

Explanation: Gravitational force cannot be repulsive and it is a very weak force. Gravitational force does not depend on the medium. But both the forces are inversely proportional to the distance between them.
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4. Two 1 Coulomb charges are kept at 1m distance in air medium. Force of attraction or repulsion between them will be ________
a) 9*109 N
b) 1 dyne
c) 1 N
d) 3*103 N

Explanation: According to Coulomb’s Law, F=$$\frac {1}{4\pi \varepsilon_o }*\frac {q_1q_2}{r^2}$$. And $$\frac {1}{4\pi \varepsilon_o}$$ = 9*109N in the SI system. In this case, q1=q2=1C and r=1m. Now substituting the values we get F=9*109N. Similarly, the electric field at a distance of 1m from a 1C charge is 9*109N/C.

5. Let B be the midpoint of AC. Two point charges Q are placed at A and C. What should be the value of charge placed at B so that the system remains at equilibrium?
a) –Q/2
b) –Q/4
c) +Q/2
d) +Q/4

Explanation: Whatever the value of q be, it will undergo equal force on both sides, hence q will be steady.
Now, for stability of Q, net force on Q must be zero.
$$\frac {1}{4\pi\epsilon_o} \, \frac {Q^2}{d^2} + \frac {1}{4\pi\epsilon_o} \, \frac {Q.q}{(\frac {d}{2})^2}$$ = 0
⇒ $$\frac {Q^2}{d^2} = \frac {-Q q}{\frac {d^2}{4}}$$
⇒ Q = -49 ⇒ q = –$$\frac {Q}{4}$$

6. 1 emu = __________ C
a) 10
b) 3*109
c) 4.8*10-10
d) 0.1

Explanation: 1 emu charge is equal to 10-coulomb charge while 1 coulomb is equal to 3*109 esu charge. Emu and C both are the units of charge and hence their conversion formula is necessary for many numerical problems.

7. If the force between two charges is 9N, what will be the force if the distance between them is doubled and both the charges are increased to √2 times?
a) 9N
b) 4.5N
c) 3N
d) 3.75N

Explanation: The force will be 9*$$\frac {\sqrt2*\sqrt2}{2^2}$$ N=4.5N because the force is proportional to the product of the two charges and inversely proportional to the square of the distance between them. Similarly, if the charges are the same but the distance is doubled, the force between them will be $$\frac {1}{4}$$ times the previous value.

8. Two charges q1, q2 exert some amount of force on each other. What will happen to the force on q1 if another charge q3 is brought close to them?
a) The force will increase
b) The force will decrease
c) The force remains the same
d) The force may increase or decrease depending on whether q3 is positive or negative

Explanation: If both the charges q1 and q2 are positive then they will repel each other. Now if the third charge q3 is also positive, the repulsive force will increase. But if q3 is negative, the repulsive force will be reduced by some amount due to the attractive force between q1 and q3.

9. Two negative charges are kept at a certain distance in the air medium. What will happen if a dielectric slab is inserted between them?
a) The slab will get heated
b) Current will flow through the slab
c) Two charges will attract each other
d) The net force between the charges will be reduced

Explanation: Force between two charges is inversely proportional to the relative permittivity of the medium between them. A dielectric slab has relative permittivity more than that of air. So the force between the charges decreases. No current or heating will be noticed.

10. According to Coulomb’s Law force between two charges is k*q1*$$\frac {q_2}{r^2}$$. What will happen if one of the charges is zero?
a) No attraction or repulsion
b) Always attraction
c) Always repulsion
d) Cannot predict the nature of the force

Explanation: Though it seems from the formula that the force must be zero, an opposite charge is induced on the uncharged body and then the two opposite charges will attract each other. This also proves that induction occurs before attraction.

11. What will be permittivity of a medium which has dielectric constant 5.4?
a) 4.78*10-11 C2N-1m-2
b) 8.85*10-10C2N-1m-2
c) 4.5*10-10C2N-1m-2
d) 3.2*10-11C2N-1m-2

Explanation: εo=8.854*10-12C2N-1m-2. Therefore the permittivity will be 5.4*8.854*10-12C2N-1m-2 = 4.78*10-11C2N-1m-2. We know that permittivity of a medium is equal to its relative permittivity multiplied to the absolute permittivity of air. Air has relative permittivity =1.

12. The dimension of εo _______
a) [M-1L-3T4I2]
b) [M-1L-3T4I4]
c) [M-1L-3T2I2]
d) [M1L-3T4I2]

Explanation: εo is the absolute permittivity of air medium and has value 8.854*10-12C2N-1m-2. Its dimension is [M-1L-3T4I2]. The permittivity of a medium is the product of this term with the dielectric constant or relative permittivity of that medium.

13. What is the C.G.S. unit of charge?
a) Stat Coulomb
b) Coulomb
c) EMU
d) Pascal

Explanation: The C.G.S. unit of charge is Stat Coulomb or ESU but in the SI system the unit is Coulomb. EMU is another unit of charge in the SI system. Pascal is the unit of pressure.

Sanfoundry Global Education & Learning Series – Physics – Class 12.

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