Conceptual Questions Based on Class 12 Physics Chapter 1 Electric Charges and Fields’
Table of Contents
Q.1. Is the mass of a body affected on charging?
Solution. Yes. Electrons have a definite mass. The mass of a body slightly increases if it gains electrons while the mass decreases if the body loses electrons.
Q.2. Can two like charges attract each other? If yes, how?
Solution. Yes. If one charge is larger than the other, the larger charge induces equal and opposite charge on the nearer end of the body with smaller charge. The opposite induced charge is larger than the small charge initially present on it.
Q.3. A positively charged rod repels a suspended object. Can we conclude that the object is positively charged?
Solution. Yes, the object is positively charged. Repulsion is the surest test of electrification.
Q.4. A comb run through one’s dry hair attracts small bits of paper. Why? What happens if the hair is wet or if it is a rainy day?
Solution. When the comb runs through dry hair, it gets charged by friction. The molecules in the paper get polarized by the charged comb, resulting in a net force of attraction. If the hair is wet, or if it is rainy day, friction between hair and the comb reduces. The comb does not get charged and thus it will not attract small bits of paper.
Q.5. An electron and a proton are kept in the same electric field. Will they experience same force and have same acceleration?
Solution. Both electron and proton will experience force of same magnitude, F = eE. Since a proton has 1836 times more mass than an electron, so its acceleration will be 1/1836 times that of the electron.
Q.6. Why do the electrostatic field lines not form closed loops?
Solution. Electrostatic field lines start from a positive charge and end on a negative charge or they fade out at infinity in case of isolated charges without forming any closed loop.
Alternatively, electrostatic field is a conservative field. The work done in moving a charge along a closed path must be zero. Hence, electrostatic field lines cannot form closed loops.
Q.7. An arbitrary surface encloses a dipole. What is the electric flux through this surface?
Solution. As the total charge of a dipole is zero, so by Gauss’s theorem, the electric flux through the closed surface is zero.
Q.8. The electric lines of force tend to contract lengthwise and expand laterally. What do they indicate?
Solution. The lengthwise contraction indicates attraction between unlike charges while lateral expansion indicates repulsion between like charges.
Q.9. Why is electric field zero inside a charged conductor?
Solution. This is because charges reside on the surface of a conductor and not inside it.
Q.10. Why should a test charge be of negligibly small magnitude?
Solution. The magnitude of the test charge must be small enough so that it does not disturb the distribution of the charges whose electric field we wish to measure otherwise the measured field will be different from the actual field.