# Electromagnetic Theory Questions and Answers – Electrostatic Energy

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This set of Electromagnetic Theory Questions and Answers for Experienced people focuses on “Electrostatic Energy”.

1. The electrostatic energy in an electric field does not depend on which of the following?
a) Magnitude of charges
b) Permittivity
c) Applied electric field
d) Flux lines

Explanation: The energy in an electric field directly magnitude of charges. Thus electric field and flux density are also dependent. But the applied field affects only the polarisation and it is independent of the energy in the field.

2. Calculate the energy in an electric field with flux density 6 units and field intensity of 4 units.
a) 12
b) 24
c) 36
d) 48

Explanation: The energy in an electric field is given by, W = 0.5 x D x E, where D = 6 and E = 4. We get W = 0.5 x 6 x 4 = 12 units.

3. Calculate the energy in an electric field with permittivity of 56 and field intensity of 36π(in μJ)
a) 3.16
b) 5.16
c) 7.16
d) 9.16

Explanation: The energy in an electric field is given by, W = 0.5 x D x E. Since D = εE, we get W = 0.5 x ε x E2. On substituting the data, we get 3.16 microjoule.
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4. Equipotential surface is a
a) Real surface
b) Complex surface
c) Imaginary surface
d) Not existing surface

Explanation: Equipotential surface is an imaginary surface in an electric field of a given charge distribution in which all the points on the surface are at the same electric potential.

5. The work done in moving a test charge from one point to another in an equipotential surface is zero. State True/False.
a) True
b) False

Explanation: Since the electric potential in the equipotential surface is the same, the work done will be zero.

6. When curl of a path is zero, the field is said to be conservative. State True/False.
a) True
b) False

Explanation: By Stoke’s theorem, when curl of a path becomes zero, then
∫ E.dl = 0. In other words the work done in a closed path will always be zero. Fields having this property is called conservative or lamellar fields.

7. If the electric potential is given, which of the following cannot be calculated?
a) Electrostatic energy
b) Electric field intensity
c) Electric flux density
d) Permittivity

Explanation: Using potential, we can calculate electric field directly by gradient operation. From E, the flux density D can also be calculated. Thus it is not possible to calculate energy directly from potential.

8. Superconductors exhibit which of the following properties?
a) Ferromagnetism
b) Polarisation
c) Diamagnetism
d) Ferrimagnetism

Explanation: Since superconductors have very good conductivity at low temperatures (σ->∞), they have nearly zero resistivity and exhibit perfect diamagnetism.

9. Debye is the unit used to measure
a) Permittivity
b) Electric dipole moment
c) Magnetic dipole moment
d) Susceptibility

Explanation: Debye is the standard unit for measurement of electric dipole moment. 1 Debye = 3.336 x 10-30 Coulomb-meter.

10. Ceramic materials possess which of the following properties?
a) Brittle and low dielectric constant
b) Rigid and low dielectric constant
c) Brittle and high dielectric constant
d) Rigid and high dielectric constant

Explanation: Ceramic materials are generally brittle. Since these materials are used in capacitors, they have higher dielectric constant than polymer. With respect to energy, they possess high electrostatic energy due to very high dielectric constant (W α ε).

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