EE340 Electromagnetic Theory: Fields in Matter

Questions and Answers

What does the magnetization M represent in a material?

The magnetic dipole moment per unit area

The magnetic dipole moment per unit mass

The strength of the electric field in the material

The magnetic dipole moment per unit volume (correct)

Which of the following describes a homogenous medium?

Properties that vary with frequency

Properties that vary with position

Properties that do not depend on position (correct)

Properties that change with the applied field

What characterizes an isotropic medium?

Parameters dependent on direction of the applied field

Parameters dependent on frequency

Parameters not dependent on direction of the applied field (correct)

Parameters that change with the applied magnetic field

Which equation correctly represents the relationship between magnetic field B and magnetic field strength H?

$B = u H$ Signup and view all the answers

In which medium do the constitutive parameters depend on frequency?

Dispersive medium Signup and view all the answers

What is the conductivity range for insulators?

< 10-4 S/m Signup and view all the answers

Which statement accurately describes conduction current?

It follows Ohm’s law. Signup and view all the answers

What characterizes a perfect conductor?

It cannot contain an electrostatic field. Signup and view all the answers

Which material has the highest conductivity value?

Perfect conductors Signup and view all the answers

What is the typical conductivity range for semiconductors?

100 - 10-4 S/m Signup and view all the answers

Which of the following statements about convection current is true?

It flows through a medium in the form of electron beams. Signup and view all the answers

What happens to free charges in an isolated conductor when an external field is applied?

They accumulate on the surface and form an induced surface charge. Signup and view all the answers

What is the required condition for conduction current to occur?

Presence of a conductor. Signup and view all the answers

What happens to the electric field inside a conductor when it is maintained at constant potential difference?

An electric field must exist inside to sustain the flow of current. Signup and view all the answers

In dielectrics, what occurs to the polarization vector after an external field is switched off?

It undergoes exponential decay due to thermalization. Signup and view all the answers

What does the equation $D_t = \epsilon_0 E_t + P_t$ illustrate in the context of dielectrics?

It indicates how electric field, polarization, and permittivity are related. Signup and view all the answers

What is meant by 'equipotential medium' in the context of conductors?

A conductor where the electric potential is the same across all points. Signup and view all the answers

What is the significance of the equation $P_t = P_0 e^{-\frac{t}{\tau}}$ in relation to dielectrics?

It describes the time-dependent decay of polarization. Signup and view all the answers

Which of the following best describes magnetization in materials?

It results from the motion of electrons around a nucleus as well as their spins. Signup and view all the answers

When is the electric field within a conductor equal to zero?

If the conductor is equipotential throughout. Signup and view all the answers

What role does thermal motion play in the polarization of dielectrics?

It randomizes the dipole moments, keeping their average at zero. Signup and view all the answers

Study Notes

Classification of Materials

Materials classified based on electrical properties: conductors, insulators (dielectrics), and semiconductors.
Conductivity measured in Siemens per meter (S/m).
Typical conductivity values:
- Conductors: ~10^7 S/m
- Semiconductors: 100 to 10^-4 S/m
- Insulators: <10^-4 S/m

Types of Current

Conduction Current
- Requires a conductor and flows due to free electrons.
- Follows Ohm’s law, expressed as J = σE.
Convection Current
- Does not require conductors; flows through mediums as electron beams.
- Does not follow Ohm’s law.

Isolated Conductors

A perfect conductor (σ = ∞) cannot contain an electrostatic field; electric field inside is zero (E = 0).
Free charges accumulate on the surface, creating an induced surface charge that cancels any applied external electric field.
Conductors are equipotential; electric potential is uniform throughout.

Conductors Maintained at Constant Potential

When a conductor has a potential difference (V) at its ends, an electric field exists inside, necessary to sustain current flow.
The relationship follows Ohm’s law.

Polarization in Dielectrics

Molecules can exhibit permanent electric moments even without an applied electric field.
Average dipole moment is zero due to random thermal motion.
When the external field is switched off, polarization vector decays exponentially, characterized by:
- P(t) = P0 e^(-t/Trelaxation).
The electric displacement field D is defined as:
- D(t) = ε0 E(t) + P(t) and can also be expressed in terms of the permittivity ε(t) of the material.

Magnetization in Materials

Electrons exhibit both orbital motion around the nucleus and spin, producing internal magnetic fields.
The magnetic moment m is defined as the equivalent current loop.
Magnetization (M) is measured in amperes per meter, representing magnetic dipole moment per unit volume.

Constitutive Relations

Three fundamental relations describe properties of media where electric (E) and magnetic (B) fields exist:
- D = εE
- B = μH
- J = σE
Relevant parameters (ε, μ, σ) characterize the medium's properties.
Classifications of media:
- Linear: Constitutive parameters are independent of the applied field.
- Homogeneous: Parameters are consistent throughout the medium.
- Dispersive: Parameters vary with frequency.
- Isotropic: Parameters are direction-independent; otherwise, materials are anisotropic, with parameters represented in tensor form.

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Description

This quiz covers the classification of materials based on their electrical properties in the context of electromagnetism. Students will learn about conductivity measurements and the distinctions between conductors and insulators. Prepare to explore how these classifications impact electromagnetic theory.