Electric charges and fields

 

Electric Charges and Fields

  
 Problems

Question1. Give some points of dissimilarity and similarity between electrostatic forces and gravitational forces.

 Solution. (a) Dissimilarity

 1. Electrostatic forces are between two charges and gravitational forces are between two masses.

 2. Electrostatic forces may be attractive or repulsive. But gravitational forces are always attractive.

3. Electrostatic force between two protons is 10^-36 times stronger than the gravitational force between them.

4. Electrostatic forces operate over distances which are not large. Gravitational forces operate over very large distances.

5. Electrostatic force depends on the medium between the two charges. Gravitational force does not depend upon the medium between the two masses.

 (b) Similarity.

1. Both the forces obey inverse square law.

 2. Both forces are proportional to the product of charges (or masses) of interacting particles.

 3. Both are central forces.

4. Both are conservative forces.

 Question2. If two objects repel one another, you know both carry either a positive charge or a negative charge. How would you determine whether these charges are positive or negative?

Solution. To determine the polarity of the charge on the two objects, we bring one of the objects near a positively charged glass rod. If the object is repelled away from the rod, it must be positively charged (as like charges repel). However, if the object is attracted to the glass rod, it must be negatively charged (as unlike charges attract).

Solution. According to Coulomb's law,

Question4. What is the cause of charging?

Solution. The cause of charging is the actual transfer of electrons from one body to the other.

Question5. An isolated conducting sphere is given a positive charge. Does its mass increase, decrease, or remain the same?

Solution. It's mass decreases slightly as it loses some electrons.

Question6. An ebonite rod is rubbed with fur or wool. What type of charges do they acquire?

Solution. The ebonite rod acquires a negative charge and fur/wool acquires an equal positive charge.

Question7. What is meant by the quantization of charge?

Solution. Charge on anybody or particle can be an integral multiple of charge on an electron (- e), i.e., q = ± ne, where 11 = 1, 2, 3,

Question8. Why does an ebonite rod get negatively charged on rubbing with fur?

Solution. This is because electrons in fur are less tightly bound than electrons in the ebonite rod.

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Question9. A glass rod rubbed with silk acquires a charge + 1.6 x 10-12 C. What is the charge on the silk?

Solution. Charge on silk is equal and opposite to charge on a glass rod, i.e.,

                 q = -1.6 x 10^-12 C.

Question10. Name any two basic properties of electric charge.

 Solution. (i) Quantization of charge.

                (ii) Conservation of charge.