Chapter 17, Problem 1RQ

Review Question 17.1 To decide whether an object is electrically charged, we need to observe its repulsion from some other objects, not its attraction. Why is attraction insufficient?

To determine

The reason behind attraction being an insufficient condition for deciding whether an object is electrically charged or not.

Answer to Problem 1RQ

Solution:

Attraction can exist between a charged and a neutral object also, due to the charging by induction. So, attraction is not sufficient to decide whether the object is charged or not. Repulsion cannot exist because of an induced charge so repulsion is a sufficient condition.

Explanation of Solution

Introduction:

There are various methods of charging an object, such as charging by friction, charging by induction, charging by conduction, and grounding.

Charging by induction is the method in which a neutral object is placed near the charged object and since there are electrons and protons in every atom of an object, they will either attract or repel each other. So, the positive (protons) and negative (electrons) charges that make a nucleus neutral, are separated by the presence of an external charged object. So, on one side there will be negatively charged electrons and on the other side there will be positively charged protons. This is called polarization of the object. The sides near the object of known charge (assume positive) will be of the opposite charge (negative) and vice versa. Neutral objects are attracted towards the charged object irrespective of the fact whether the charge is positive or negative.

Explanation:

Suppose a positively charged object is placed near a metal piece. Since electrons are loosely bound in the atoms of metals, electrons will move towards the face that is near the positively charged object. When electron leaves the atom, the atom will get positively charged. Since the face with the negative charge is near the positively charged object, the metal piece will be attracted towards the positively charged object.

To decide whether an object is electrically charged or not, the object needs to be placed near another object of known charge. If it is repelled by the object, then the first object could be of positive or negative charge. But, if the two objects are attracted to each other, it is not sufficient to decide because a charged object can induce a charge of the opposite sign on the neutral object. It means that the attraction between the objects can exist due to an induced charge. Thus, it will not be possible to decide whether the object was charged prior to the process, or charge has been induced by the second object.

Conclusion:

Therefore, repulsion is a sufficient condition for deciding whether an object is electrically charged or not.