Electric Charges and Fields Important Questions

Very Short Answer Questions [1 Marks Question]

Q 1. Does the charge given to a metallic sphere depend on whether it is hollow or solid? Give reason for your answer. [CBSE Delhi 2017]

Ans. No, the charge given to a metallic sphere does not depend on whether it is hollow or solid because the charge only resides on the surface of the conductor. 

Q 2. Two insulated charged copper spheres A and B of identical size have charges qA and –3qA respectively. When they are brought in contact with each other and then separated, what are the new charges on them? [CBSE (F) 2011]

Ans. Charge on each copper sphere = 

qA – 3qA/2 = – qA

Q 3. What is the electric flux through a cube of side 1 cm which encloses an electric dipole? [CBSE Delhi 2015]

Ans. The net electric flux will be zero. It is so because:

(i) Electric Flux is independent of the shape and size of the cube.

(ii) Net charge of the electric dipole is zero. 

Q 4. What orientation of an electric dipole in a uniform electric field corresponds to its (i)stable and (ii)unstable equilibrium? [CBSE Delhi 2010] [HOTS] 

Ans. (i) In stable equilibrium the dipole moment is parallel to the direction of the electric field (i.e., θ = 0).

(ii) In unstable equilibrium PE is maximum, so θ = π, i.e; the dipole moment is antiparallel to the electric field.

Q 5. Define electric field strength. Is it a vector or a scalar quantity? 

Ans. The electric field strength at a point in an electric field is defined as the electrostatic force acting on a unit positive charge when placed at that point and its direction is along the direction of electrostatic force. Electric field strength is a vector quantity.

Q 6. A capacitor has been charged by a dc source. What is the magnitude of conduction and displacement current, when it is fully charged ? [Delhi, 2013]

Ans. Electric flux through plates of capacitor,

Q 7. What is the geometrical shape of an equipotential surface due to a single isolated charge ? [Delhi, 2013]

Ans. The equipotential surfaces of an isolated charge are concentric spherical shells and the distance between the shells increases with the decrease in electric field and vice-versa.

Q 8. Draw the pattern of electric field lines, when a point charge – Q is kept near an uncharged conducting plate. [Outside Delhi, 2019]

Ans. The positive charge will be induced on the uncharged conducting plate, kept near it. So, the lines of forces will start from metal plate and end -Q

Q 9. What kind of charge is produced while rubbing a glass rod and silk together?

Ans. While rubbing glass rod and silk together, they produces opposite charges. Whhile the glass rod have negative charge, the silk will have positive charge.

Q 10.What is the name of the experiment that establish the quantum nature of electric charge?

Ans. The experiment that establish the quantum nature of electric charge is called – Millikan’s Oil Drop Experiment.

Q 11.A body can have a charge 8.0 x 10^-20 C. Explain if the statement is true or false with logic.

Ans. This statement is false, a body can not have a charge 8.0 x 10^-20 C, as it is less that the charges of electron and proton, which detemines the core chare of any body. Hence, the minimum charge a body can have is- 1.6 x 10^-19.

Q 12. Is Newton’s third law maintained by the Coulomb’s Law?

Ans. Coulomb Law maintains Newton’s Third Law of Motion. It states “the force experienced by two charges are equal in magnitude  but mutually opposite in direction”.

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Short Answer Questions [2 Marks Question]

Q1. Define electric dipole moment. Is it a scalar or a vector quantity? What is its SI unit? [CBSE (AI) 2011, 2013, (F) 2009, 2012, 2013]

Ans.

The electric dipole moment is defined as the product of either charge and the distance between the two charges. Its direction is from negative to positive charge. 

i.e., |p| = q(2l) 

Electric dipole moment is a vector quantity. 

Its SI unit is coulomb-metre.

Q 2. Two equal balls having equal positive charge ‘q’ coulombs are suspended by two insulating strings of equal length. What would be the effect on the force when a plastic sheet is inserted between the two? [CBSE AI 2014]

Ans. The force will decrease upon inserting a plastic sheet. 

Reason: Force between two charges each ‘q’ in vacuum is 

F0=140. q2r2

On inserting a plastic sheet (a dielectric K > 1) 

Then F=140K. q2r2

 i.e., Force F=F0K

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Short Answer Questions [3 Marks Question]

Q1. (a) Draw the equipotential surfaces corresponding to a uniform electric field in the z-direction.

(b) Derive an expression for the electric potential at any point along the axial line of an electric dipole. [Delhi, 2019]

Ans. 

Q2. A spherical conducting shell of inner radius r1 and outer radius r2 has a charge Q.

(a) A charge q is placed at the center of the shell. Find out the surface charge density on the inner and outer surfaces of the shell.

(b) Is the electric field inside a cavity (with no charge) zero; independent of the fact whether the shell is spherical or not? Explain. [Outside Delhi, 2019]

Ans.

Q 3.Two balls have same kind of charge attracts each other. Explain.

Ans. Two balls with same charge can indeed attract each other. If one of the ball has greater charge than the other, this situaation will take place. Here is the logic behind this-

• if two charged balls are facing each other, they started reflecting opposite kinds of charges. to face each other. Just like the magnetic fields. 
• The balls will have same kind of charges , not the same amount of charges, hence, both tyhe balls will start producing opposite king of charges. this will make them attracted to each other. 

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Long Answer Questions [5 Marks Question]

Q1. a) Use Gauss’ law to derive the expression for the electric field (E) due to a straight uniformly charged infinite line of charge density λ C/m.

(b) Draw a graph to show the variation of E with perpendicular distance r from the line of charge.

(c) Find the work done in bringing a charge q from perpendicular distance r1to r2( r2> r1). [CBSE, 2018]

Ans. a) Electric field E due to a straight uniformly charged infinite line of charge density λ : 

Consider a cylindrical Gaussian surface of radius r and length l coaxial with line charge. The cylindrical Gaussian surface may be divided into three parts :

(i) curved surface S1

(ii) flat surface S2 and

(iii) flat surface S3.

By symmetry, the electric field has the same magnitude E at each point of curved surface Sj and is directed radially outward. We consider small elements of surfaces S1, S2 and S3.

b) Graph showing variation of E with perpendicular distance from line of charge : The electric field is inversely proportional to distance V from line of charge.

Q2. a) Derive an expression for the electric field at any point on the equatorial line of an electric dipole.

(b) The identical point charges, q each, are kept 2 m apart in air. A third point charge Q of unknown magnitude and sign is placed on the line joining the charges such that the system remains in equilibrium. Find the position and nature of Q. [Delhi, 2019]

Ans. (a) Consider an electric dipole of charges -q and +q separated by a distance 2a and placed in a free space. Let P be a point on the equatorial line of the dipole at a distance r from the centre of a dipole.

This is the required expression.

(b) Let the two charges of + q each placed at point A and B at a distance 2 m apart in air.

Suppose, the third charge Q (unknown magnitude and charge) is placed at a point O, on the line joining the other two charges, such that OA= x and OB 2-x.

For the system to be in equilibrium, net force on each 3 charges must be zero.

If we assume that charge Q placed at O is positive, the force on it at O may be zero. But the force on charge q at point A or B will not be zero. It is because the forces on a charge q due to the other two charges will act in the same direction. If charge Q is negative, then the forces on q due to the other two charges will act in opposite directions.

Hence, Q will be negative in nature. For charge (-Q) to be in equilibrium Force on charge (-q) due to charge (+q) at point A should be equal and opposite to charge (+Q) at B

Therefore, for the system to be in equilibrium a charge – Q is placed at a mid point between the two charges of + q each.

Also Have a Look at the PYQs for References:

• A solid sphere of radius R has a charge Q distributed in…. (BITSAT 2015)
• The surface charge density (in Cm⁻² )… (DUET 2009)
• A spherical conductor of radius 10cm has a charge of … (NEET 2020)
• An electron falls from rest through a vertical distance h in a uniform and….(NEET 2018)
• A charge Q is enclosed by a Gaussian spherical surface…. (NEET 2011)
• Suppose the charge of a proton and an electron differ slightly...(NEET 2017)
• A hollow metal sphere of radius R is uniformly charged….(NEET 2019)
• A square surface of side L meter in the plane… (NEET 2010)
• A toy car with charge q moves on a frictionless horizontal plane….(NEET 2018)
• An electric dipole is placed at an angle of 30^∘ with an electric field intensity…. (NEET 2016)
• An electric dipole of dipole moment p is aligned parallel to a… (NEET 2013) 
• Which of the following does not show electrical conductance?
• A particle of mass mm and charge q is placed at rest in… (KEAM)
• A plane square sheet of charge of side 0.5m0.5m has uniform surface charge density….(KEAM)
• A point dipole with dipole moment… (KEAM)

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