Faqs

Electrostatics Frequently Asked Questions

By: DR

1. Why sharp edges are avoided in electrical machines? Electrical machines are specially designed to store the required amount of charge. We all know that charge density is maximum at the sharp or pointed edges being the extremely small surface area (σ = q/A). Hence, sharp edges can lead to the phenomenon known as action of points. In this phenomenon, the air or dust particles coming close to the sharp edges get similar charge and are repelled thus, blowing a current of air resulting the leakage of charge from these ends. So to avoid this unnecessary leakage of charge through sharp edges, they are strictly avoided in electrical machines. 2. Vehicles carrying inflammable fuel drag a long metallic chain along the ground while in motion, why? When the vehicles are in motion, Due to the friction between air and the body of the vehicle as well as between tyres and the ground, Charge is produced. This charge may accumulate on the body of the vehicle. If the accumulation of charge in the body of the vehicle is maximum, spark may be produced which may catch fire the vapors of the fuel. This may result in huge explosion. Thus it is important for the vehicle to stop the accumulation of charge on their body. It is done so by grounding the charge thus developed with the help of metallic chain which helps to conduct the charge to the ground. 3. Why can more charge be stored on the metal surface if it is highly polished than when its surface is rough? If the metal surface is rough, it may contain a number of pointed ends which serves as the facet for the leakage of charge by the phenomenon of action of points. In this phenomenon, the air or dust particles coming close to the sharp edges get similar charge and are repelled, thus blowing a current of air. This may result the loss of charge in the surrounding. Thus if the surface is rough it may leak charge earlier. But in the highly polished surface these pointed ends are eliminated and thus can acquire more charge than that of the rough surface. 1. Can two electric lines of force ever intersect each other? No, two lines of force can never intersect each other. If the lines of force intersect each other, two tangents can be drawn at the point of intersection giving two directions of electric field at a single point which is theoretically as well as practically impossible.

2. Is the electrostatic potential necessarily zero at the point where the electric field strength (Intensity) zero? No, it is not necessary for the electric potential to be zero at the point where the electric field intensity is zero. For example, the electric field intensity inside the charged hollow sphere is zero but not the electric potential. Electric potential remains constant throughout the inner surface. We know that the electric intensity at a point is the negative gradient of potential i.e. dV E=− . If the electric intensity is zero then dr dV 0=− dr i.e. dV = 0 On integrating, we get V= constant. This implies that electric potential is not necessarily zero at the point where the electric intensity is zero. 3. Why a man inside an insulated metallic cage does not receive electrical shock when the cage is fully charged? We know that electric field intensity in side the hollow metallic object is zero because charge resides only on the surface. We know that the electric intensity at a point is the dV negative gradient of potential i.e. E = − . dr At the point where the electric intensity is zero dV 0=− dr i.e. dV = 0 On integrating, we get V= constant. Thus potential remains constant throughout the cage i.e. there is no difference of potential between man and the cage. Since the man and the cage are at same potential the man inside the cage is not likely to receive electrical shock. 4. Why gravitational force is generally neglected while calculating the electric force between two charges? Gravitational force is too small to be considered while calculating electric force between two charged particles. To show this let us consider two particles having unit masses and carrying unit charges separated by a certain distance r. Now gravitational force between these two masses is given by Newton’s law of m1 m2 6.67 × 10 −11 gravitation i.e. Fg = G 2 . This gives Fg = N…………. (1) r r2 Electrostatic force between the two unit charges held at a distance r is given by 1 q1 q 2 9 × 10 9 F = Coulomb’s law i.e. e . This gives, Fe = N………… (2) 4πε 0 r 2 r2 Dividing equation (2) by equation (1) we get

Fe 9 × 10 9 = = ………….. Fg 6.67 × 10 −11 This shows that gravitational force is too small to be considered in comparison to the electrostatic force. Hence it is neglected while calculating the electrostatic force. 5. A comb run through one’s dry hair attracts small bit of papers, why? What do you expect if the hair is wet? A comb run through one’s dry hair acquires negative charge on it due to friction with the hair. When the negatively charged comb is brought closer to small bit of papers, it induces opposite kind of charge i.e. positive charge on the nearer end of the paper. We all are familiar with the attraction property of unlike charges. Hence the comb attracts the small bit of papers. But if the hair is wet, the comb may not be able to acquire charge in the absence of friction hence the comb does not attract the bit of paper. 6. Electrostatic machines do not perform well in humid days, why? Electrostatic machines are expected to store a desirable quantity of charge. In the humid days, the moisture present in the air acts as a conducting medium. This may help to lose the charge in the surrounding by conduction. In the humid days the moisture may also make the non conducting part of the apparatus conducting which thus may lose charge in the surrounding disabling the machine to perform well. 7. Why line of force is always normal to the conducting sphere? The line of force in the electric field is the path along which a unit positive test charge will move if it is free to do so. If a positive charge is left freely in the electric field due to the charged sphere, it will always move normally away or towards the charged sphere depending upon the nature of the charge in the sphere. Hence line of force is always normal to the conducting sphere. 8. People inside steel framed building are safe from lightning, why? Charge given to any hollow sphere always resides on the surface making the electric field inside it zero. Similarly during lightning, the charge which come to strike the steel framed building also resides on the surface making electric field inside it zero. This steel frame of the building act as electrostatic shield, allowing no charge to flow inside. We know that the electric intensity at a point is the negative gradient of potential dV i.e. E = − . If the electric intensity is zero then dr dV 0=− dr i.e. dV = 0 On integrating, we get V= constant. As there is no potential difference between inner and outer surface of the building, people remain safe during lightning.

12. What is the action of Dielectric in Capacitor? Dielectrics are the insulators, which, when placed in between the plates of the capacitors, increases the capacitance of the capacitor. We know Dielectric constant of the medium is given by the ratio of Capacitance with dielectric to that without it. i.e. C K = medium C air Let us consider a charged parallel plate capacitor and suppose E0 be the value of applied electric field whose direction is from positive to negative charge. When a dielectric slab of dielectric constant K is introduced in between the plates, the molecules of the dielectric get polarized. There appears net positive and negative charge net polarization charge which also constitutes electric field in the direction opposite to that of applied field. Let it be E p. Thus net electric field inside the capacitor becomes E r=E0- E p. Again Dielectric constant of a medium is the ratio of applied electric field to that with reduced electric field due to polarization i.e. E0 K= E0 − E p Thus the value of K is always greater than 1. Thus, when a dielectric is introduced in between the capacitors, the capacitance of the capacitor in creases.

Eo

EP

Er=Eo-EP

13. Differentiate between Dielectric strength and Dielectric constant. Dielectric Strength Dielectric Constant 1. It is the maximum value of electric field 1. It is relative permitivity of the medium that can be applied to the dielectric without which is the ratio of permittivity of medium electric breakdown. ε to the permitivity of vacuum. i.e. K = . ε0 −1 2. Its SI unit is volt per meter ( Vm ). 2. Being the ratio of two like quantities, it is 3. It is vector quantity. unit less. 3. It is scalar.

14. Define one electron volt. Electron volt is defined as the amount of energy gained by an electron on accelerating through a Potential Difference of one volt. We know from the definition of potential difference, work done P.D. = Charge moved work done = charge moved × P. D. Or We have charge on 1 electron =1.6 × 10 −19 coulomb , P.D. = 1 volt Thus, 1ev = 1 volt × 1.6 × 10 -19 C 1ev = 1.6 × 10 -19 J 15. What is Equipotential Surface? Can two equipotential surfaces intersect? A surface, electrostatic potential at every point of which is same, is called equipotential surface. Two Equipotential surfaces can never intersect each other. If they intersect each other, there will two values of electric potential at a single point which is in contradiction to the definition of equipotential surface. Hence they can never intersect each other. 17. Define Capacitance. What are the factors on which the capacitance of a parallel plate capacitor depends? The capacitance of a conductor is defined as its ability to store electrical charge. The SI unit of capacitance is Farad. If a conductor is given a charge of +q then its potential rises to V volt. Then, It is found that the potential of the conductor is directly proportional to charge on it i.e. q ∝ V i.e. q = C V Thus q C= V The capacitance of a parallel plate capacitor depends upon following factors: (a) Directly proportional to the dielectric constant of the medium i.e. C ∝ K . (b) Directly proportional to the Area of the plates i.e. C ∝ A . 1 (c) Inversely proportional to the distance between the plates i.e. C ∝ . d Kε 0 A Combining a, b, and c we get, C = where ε 0 is the permitivity of vacuum. d

18. Differentiate between polar and non polar molecules.

Polar molecules 1. The centre of gravities of positive and negative charge do not coincide due to asymmetric charge distribution. 2. Polar molecules possess some finite electric dipole moment due to finite separation of positive and negative charge. 3. Example: H2 , CO2 , N2

Non- Polar molecules 1. The centre of gravity of positive and negative charge coincides due to symmetric charge distribution. 2. Polar molecules do not possess net electric dipole moment due to zero separation of positive and negative charge. 3. H2O, NH3, N2O.

19. What is the principle of electrostatic shielding? We know that electric field inside the hollow conductor is zero. If an object placed inside the hollow conductor is subjected to an electric field, the object will experience no effect of the applied field i.e. to say the object is shielded by hollow conductor. The concept of shielding is properly used in audio signal transmission. In which the cables carrying audio signals are shielded in side another hollow conductor to preserve these signals from external electric fields. 20. What happens if dielectric inside the parallel plate capacitor is replaced by metal slab? If dielectric inside the parallel plate capacitor is replaced by metal slab, the capacitance of the capacitor increases only if the slab does not touch the plates. But if the slab touches both the plates, both plates will be at same potential and the capacitance becomes zero.

21. A charged conical conductor loses its charge earlier than a similarly charged sphere, why? It is known that charge density depends upon the curvature of the surface. The charge density at the cone of the conductor becomes maximum quickly as its curvature is very small. As soon as, the strength of electric field at the cone becomes maximum, it loses charge on the surrounding by the phenomenon of action of point. But for the sphere, charge density is same every where in the surface which reaches maximum only after supplying maximum amount of charge. Thus, charged conical conductor loses its charge earlier than a similarly charged sphere. 22. Automobile ignition failure occurs in damp weather, why? Spark plug containing porcelain accumulates a film of dirt on it. The dirty surface can pick up moisture from air. In damp weather, due to captured moisture, the porcelain becomes conducting and spark can leak through it to outside instead discharging it across the gap. Thus automobile ignition becomes failure in damp weather. 23. A Charged particle is free to move in an electric field. Will it always move along an electric line of force? The charged particle not always moves along the electric lines of force. If it was initially at rest, it will move along the line of force but if it was initially released in the electric field with certain velocity making an angle with the line of force, Its resultant path will not be along line of force. More over the test charge at rest will travel along the line of force only if the line of force is St. Line. If the line of force is curved, the charge will not move along the line of force because line of force gives only the direction in which the charge is accelerated not the direction of velocity. 24. An Electric Line of Force cannot have sudden breaks why? It is known that tangent at any point in the lines of force gives the direction of electric field at that point and the direction of electric field changes from point to point. Line of force is a continuous curve, it does not have sudden breaks because the breaking of the line indicates the absence of electric field at these broken points which is not usual.