Electromagnetic Theory - Unit 2 - Question Bank


1.State coulombs law.

Coulombs law states that the force between any two point charges is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them. It is directed along the line joining the two charges.

F=Q1Q2/ 4 πεr2

2.State Gauss law for electric fields

The total electric flux passing through any closed surface is equal to the total charge enclosed by that surface.

3.Define electric flux.

The lines of electric force is electric flux.

4.Define electric flux density.

Electric flux density is defined as electric flux per unit area.

5.Define electric field intensity.

Electric field intensity is defined as the electric force per unit positive charge.

E =F/ Q =Q/4 πεr2 V/m

6.Name few applications of Gauss law in electrostatics.

Gauss law is applied to find the electric field intensity from a closed surface.

      e.g. Electric field can be determined for shell, two concentric shell or cylinders

7.State Gauss law for magnetic field.

The total magnetic flux passing through any closed surface is equal to zero.

B.ds =0

8.Define potential difference.

Potential difference is defined as the work done in moving a unit positive charge from one point to another point in an electric field.

9.Define potential.

Potential at any point is defined as the work done in moving a unit positive charge from infinity to that point in an electric field.

V=Q / 4 πεr

10.Give the expression for electric field intensity due to a single shell of charge

E = Q / 4 πεr 2

11.Give the expression for potential between two spherical shells

V= 1/ 4 πε(Q1/a – Q2/b)

12.Give the relationship between potential gradiant and electric field.

E= - ▼V

13.What is electrostatic force?

The force between any two particles due to existing charges is known as electrostatic force, repulsive for like and attractive for unlike.

14.What are dielectrics?

Dielectrics are materials that may not conduct electricity through it but on applying electric field induced charges are produced on its faces .The valence electron in atoms of a dielectric are tightly bound to their nucleus.

15.What is a capacitor?

A capacitor is an electrical device composed of two conductors which are separated through a dielectric medium and which can store equal and opposite charges ,independent of whether other conductors in the system are charged or not.

16.Define dielectric strength.

The dielectric strength of a dielectric is defined as the maximum value of electric field that can b applied to the dielectric without its electric breakdown.

17.What meaning would you give to the capacitance of a single conductor?

A single conductor also possess capacitance. It is a capacitor whose one plate is at infinity.

18.Why water has much greater dielectric constant than mica.?

Water has a much greater dielectric constant than mica .because water ha a permanent dipole moment, while mica does not have.

19.What is a point charge?

Point charge is one whose maximum dimension is very small in comparison with any other length.

20.Define linear charge density.

It is the charge per unit length.

21 Define surface charge density.

It is the charge per surface area.

22.Write down the expression for capacitance between two parallel plates.

C=εA / d

23.What is meant by displacement current?

Displacement current is nothing but the current flowing through capacitor.

J= D / t

24.Write the boundary conditions at the interface between two perfect dielectrics.

i)The tangential component of electric field is continuous


ii)The normal component of electric flux density is continuous


25.Write poisson’s and laplace ’s equations.

Poisson ‘s eqn:

▼2V= - ρv / ε

Laplace’ s eqn:

▼2V= 0

26.What are the significant physical differences between Poisson ‘s and laplace ‘s equations.

Poisson ‘s and laplace ‘s equations are useful for determining the electrostatic potential V in regions whose boundaries are known. When the region of interest contains charges poissons equation can be used to find the potential. When the region is free from charge laplace equation is used to find the potential.


1. Discuss the properties and boundary conditions of dielectric materials.

2. Give and derive the expression for capacitance of coaxial cables with single and two dielectrics.

3. Write down the uniqueness theorem and explain.

4. Derive the expression for capacitance of a two-wire line.

5. Write the expression for Laplace and Poisson’s equation and derive it for various coordinate systems.

6. Deduce an expression for the joint capacitance of two capacitors, C1 and C2, (i) in series and (ii) in parallel. If C1 = 100 microfarad and C2 = 50 microfarad, calculate a) the joint capacitance and b) the total energy stored with a steady applied potential difference of 1000V.

7. In the case of a two concentric spherical shell capacitor, the radii of the two spheres differ by 4 cm, and the capacitance of the spherical conductor is 53.33 Pico farad. If the outer sphere is earthed, calculate the radius, assuming air as dielectric.

8. Obtain the boundary conditions on the interface of a dielectric and a conductor.

9. State and explain Uniqueness theorem.

10. Current density is given by J = (1/r) e-t ar A/m2. At t = 1s, calculate total outward current in a cylinder if r = 5m and also find the velocity with which the J moves at arbitrary radius ‘r’ (‘r’ = radius of cylinder).