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Very Short Answer Questions (PYQ) Q. 1. Sketch the electric field lines for two point charges q 1  and q 2  for q 1  = q 2  and q 1  > q 2  separated by a distance d. [CBSE Chennai 2015]    Ans.  When the charges are equal, the neutral point N lies at the centre of the line  joining the charges. However, when the charges are unequal, the point N is closer to the smaller charge. Q. 2. A positive point charge +q is kept in the vicinity of an uncharged conducting plate. Sketch electric field lines srcinating from the point on the surface of the plate. [CBSE (AI) 2009]  Ans. Q. 3. Why do the electrostatic field lines not form closed loops? [CBSE (AI) 2014, Allahabad 2015]  Ans. Electric field lines start from positive charge and terminate at negative charge. If there is a single positive charge, the field lines start from the charge and terminate at infinity. So, the electric field lines do not form closed loops. Q. 4. 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, Reason:  This is because the charge resides only on the surface of the conductor. Q. 5. Two insulated charged copper spheres A and B of identical size have charges q  A  and  – 3q  A  respectively. When they are brought in contact with each other and then separated, what are the new charges on them? [CBSE (F) 2011]  Ans. Q. 6. Two insulated charged copper spheres A and B of identical size have charges qA and q B  respectively. A third sphere C of the same size but uncharged is brought in contact with the first and then in contact with the second and finally removed from both. What are the new charges on A and B? [CBSE (F) 2011]  Ans. Q. 7. Fig. shows three point charges +2q,  –  q and +3q. The charges +2q and  – q are enclosed within a surface ‘S’. What is the electric flux due to this configuration through the surface ‘S’?  [CBSE Delhi 2010]  Ans. Electric flux = (Net charge enclosed within the surface) Q. 8. What is the electric flux through a cube of side 1 cm which encloses an electric dipole? [CBSE Delhi 2015]  Ans.  Net electric flux is zero. Reason:   (i)  Independent to the shape and size. (ii)  Net charge of the electric dipole is zero. Q. 9. Two charges of magnitudes  –  2Q and + Q are located at points (a, 0) and (4a, 0) respectively. What is the electric flux due to these charges through a sphere of radius ‘3a’ with its centre at the srcin?  [CBSE (AI) 2013]   Ans. Electric flux, ϕ   = Concept:  Imagine a sphere of radius 3a about the srcin and observe that only charge  – 2Q is inside the sphere. Q. 10. How does the electric flux due to a point charge enclosed by a spherical Gaussian surface get affected when its radius is increased? [CBSE Delhi 2016]  Ans. Electric flux through a Gaussian surface, enclosing the charge q is φE =  This is independent of radius of Gaussian surface, so if radius is increased, the electric flux through the surface will remain unchanged. Q. 11. A charge Q µC is placed at the centre of a cube. What would be the flux through one face? [CBSE (F) 2010, (AI) 2012]  Ans. Electric flux through whole cube = Electric flux through one face = Q. 12. A charge q is placed at the centre of a cube of side l. What is the electric flux passing through two opposite faces of the cube? [CBSE (AI) 2012]  Ans.  By symmetry, the flux through each of the six faces of the cube will be same when charge q is placed at its centre. Thus, electric flux passing through two opposite faces of the cube = Q. 13. 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 electric field (i.e., θ = 0).   (ii)   In unstable equilibrium PE is maximum, so θ = π, i.e., dipole moment is antiparallel to electric field. (OIQ) Q. 1. Is the force acting between two point charges q1 and q2 kept at some distance apart in air attractive or repulsive when (i) q 1 q 2 >0 (ii) q 1 q 2 <0?  Ans. (i)  Repulsive (ii)  Attractive. Q. 2. What is the nature of electrostatic force between two point electric charges q 1  and q 2  if (a) q 1  + q 2  > 0? (b) q 1  + q 2  < 0?  Ans. (a)  If both q 1  and q 2  are positive, the electrostatic force between these will be repulsive. However, if one of these charges is positive and is greater than the other negative charge, the electrostatic force between them will be attractive. Thus, the nature of force between them can be repulsive or attractive. (b)  If both q 1  and q 2  are  – ve, the force between these will be repulsive. However, if one of them is  – ve and it is greater in magnitude than the second+ve charge, the force between them will be attractive. Thus, the nature of force between them can be repulsive or attractive. Q. 3. 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. 4. Figure shows a point charge + Q, located at a distance from the centre of a spherical metal shell. Draw the electric field lines for the given system. [CBSE Sample Paper 2016]
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