# Short notes Electrostatic | ESE, GATE & SSC JE

Contents

### Q. 1. A metallic sphere is charged negatively. Will its mass increase, decrease or remain the same?

Ans. When the sphere is negatively charged, it means electrons have been added to it. Since electrons have finite mass, the mass of the negatively charged sphere will increase.

### Q. 2. Electrostatic experiments cannot be conducted successfully on humid days. Explain.

Ans. The humid air becomes conducting. Therefore, the static charge on the apparatus leaks off into the air. For this reason, electrostatic experiments do not work well on humid days.

### Q. 3. What are the most significant differences between electrostatic forces and gravitational forces?

Ans. (i) Gravitational forces are always attractive but electrostatic forces may be attractive or repulsive depending upon the signs of the charges.
(ii) The gravitational constant (G) is independent of the nature of the medium. However. electrical constant k (= 1/4 π ε0 εr) depends upon the nature of the medium.
(iii) Electrostatic forces are extremely large as compared to gravitational forces. For example, the electrostatic force of attraction between an electron and a proton is about 1039 times stronger than the gravitational force between them.

### Q. 4. What similarities do electrostatic .forces have to gravitational forces?

Ans. (i) Both obey inverse square law.
(ii) Both are central forces, i.e., forces act along the lines joining the centers of the bodies.
(iii) Both are conservative forces. i.e., work done by them does not depend upon the path followed.
(iv) Both involve the property of the interacting particles—the mass in one case and the charge in the other.

### Q. 5. Any conducting object connected to the earth is said to be grounded. Explain.

Ans. Because the earth is a very big source of electrons or sink. Its potential remains const whether electrons are given to it or removed from it. For this reason, the electric potential of the earth is assumed zero.

### Q.6 Vehicles carrying inflammable materials usually haw chains that hang down and di on the ground. Why?

Ans. When a vehicle is in motion, its tyres rub against the road and get charged due to friction. Further, due to friction or air, the body of the vehicle also gets charged. The accumulated charge becomes excessive, sparking may occur and the inflammable mater may catch lire. Since the chain ropes are touching the ground, the charge leaks to t earth. Hence, the danger or tire is avoided.

### Q. 7. You cannot disturb the electrical neutrality of ordinary matter very much. Explain.

Ans. Under ordinary conditions, a body is electrically neutral, i.e., it has the same amounts of positive charge and negative charge. When a body is charged (positively or negatively this electrical neutrality is disturbed. Suppose you are charging a body positively by removing electrons from it. The positive charge on the body tends to pull the negative charge (i.e., electrons) hack. As the body gets more and more positive charge, the electrostatic force tending to pull the negative charge hack also increases. Therefore we cannot place a large charge (positive or negative) on a body.

### Q. 8. If Q1 Q2 < 0 what can one say about the nature of dice between the charges?

Ans. If Q1 Q2< 0, it means the product of the magnitude of the charges is negative. In other words, these are unlike charges, i.e., one charge is positive and the other charge is negative. Hence, the electrostatic force between them is attractive.

### Q. 9. Why is electric current and not charge taken as the Andamental quantity in SI units?

Ans. For practical reasons, electric current and no charge has been taken as the fundamental quantity, although one is derivable from the other ( q = It). The important consideration which led to the selection of current as the fundamental quantity is that it serves as the link between electric, magnetic and mechanical quantities and can be readily measured.

### Q. 10. Although ordinary rubber is insulator the rubber tyres of aircraft city made slightly conducting. Why?

Ans. During the take-off and landing, the friction between tyres and the run-way causes electrification of tyres. If the tyres are non-conducting, the excessive charge will accumulate on the tyres which may cause sparking. If the material of the tyres is slightly conducting. the accumulated charge can flow to earth, thus eliminating any danger of fire.

### Q. 11. Does the electric potential increase or decrease along an electric line of force?

Ans. The electric field intensity acts in the direction of the electric line of force. We know that E = — dr, dr. This means that in the direction of electric field intensity, the potential decreases. It is the same thing as potential decreases along an electric line of force.

### Q.12. Can Two equipotential surfaces intersect each other?

Ans. No. The intersection of two equipotential surfaces means that at the point of intersection. there are two values of potential which is impossible. Alternatively, electric lines of force are normal to equipotential surfaces. If two equipotential surfaces intersect each other. it means that the electric field at the point of intersection has two directions which is not possible.

### Q. 13. Electric field intensity in a given region is zero. Can we conclude that the electric potential must be zero?

Ans. Not necessarily. E = dV/dr or 0 = — dV/dr. Since dr cannot be zero, dV = 0. This means that electric potential is constant. The constant value can be zero or non-zero.

### Q. 14. Two nearby points are at the same potential. What is the intensity of the electric field in this region?

Ans. E = — dV/dr where dV is changing in the potential for displacement dr. Since there is no change in potential, dV/dr = 0. Hence, electric intensity in the region is zero.

### Q. 15. How do we represent the electric potential field geometrically?

Ans. An electric field is represented geometrically by electric lines of force. However, the electric potential field is represented geometrically by equipotential surfaces. Note that in an electric field, there is a unique value of the potential at every point. Therefore, there exists an electric potential field.