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Electromagnetic Theory > Definitions > Flashcards

Definitions Flashcards

1
Q

What do moving electric charges produce?

A

currents and magnetic fields

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2
Q

What does free space mean?

A

no charge and no current density.

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3
Q

What happens if there is a big enough electric field?

A

A big enough electric field will ionise the atom, pulling one or more electrons away. If this happens, the atom will become a conductor.

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4
Q

what is the Curl?

A

the tendency of something to rotate about a point.

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5
Q

What is the divergence?

A

How much a vector spreads out from some point.

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6
Q

What does Coulomb’s law describe?

A

The force on a test charge due to a single point charge which is at a rest distance away.

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7
Q

Describe Gauss’s law.

A

Gauss’s law quantifies the flux through any closed surface.

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8
Q

How does potential and dipole moment vary with r?

A

The potential falls off as 1/r^2 compared to the dipole which falls of as 1/r.

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9
Q

What does it mean if the universe has no sources or sinks?

A

There are no magnetic monopoles.

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10
Q

What is the divergence of a curl?

A

The divergence of a curl must be zero.

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11
Q

What are sources and sinks?

A

positive charges act as sources and negative charges act as sinks.

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12
Q

What is the flux through any closed surface?

A

a measure of the total charge inside that surface.

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13
Q

What is the reference point in electrostatics?

A

the reference point is got by setting the zero of potential to be zero at infinity.

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14
Q

What is an electric dipole?

A

A pair of charges of opposite sign separated by a small distance d.

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15
Q

Describe the torque in an electric field.

A

The torque works to align the dipole moment parallel to the electric field.

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16
Q

What does dielectric mean?

A

All charges are attached to specific atoms but they can move about a little.

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17
Q

What do subscript f and b represent?

A

free and bound

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18
Q

What is electric displacement?

A

A combination of the electric field plus the polarisation.

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19
Q

What is the electric susceptibility?

A

The degree of polarisation of a dielectric material in response to an applied field.

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20
Q

Describe the normal component of the electric field.

A

There is a discontinuity in the normal component of the electric field

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21
Q

Describe the parallel component of the electric field.

A

The parallel component of the electric field is continuous because there is no net work done in taking a charge around a closed loop.

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22
Q

Describe the normal components of electric displacement?

A

the normal components are continuous so long as there is no free charge.

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23
Q

Describe the parallel components of electric displacement?

A

the parallel components are discontinuous.

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24
Q

What is the electric field inside a conductor.

A

The electric field is zero inside a conductor.

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25
Q

What are conductors?

A

Conductors are materials where charge can move about freely.

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26
Q

What is the electric field just outside a conductor?

A

The electric field just outside a conductor is perpendicular to its surface as not every point on a conductor must be at the same potential.

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27
Q

What is the effect of filling a capacitor with dielectric material and relative permittivity ϵr?

A

The effect of the dielectric is to reduce electric field (E) and hence also potential (V).

28
Q

What is the direction of magnetic force with respect to net displacement of charge?

A

Magnetic force is always perpendicular to the net displacement of charge.

29
Q

What does the continuity equation describe?

A

A statement of charge conservation. Conservation is local

30
Q

What does the Biot-Savart law describe?

A

It describes the magnetic field arising due to a steady line current.

31
Q

What is the magnetic vector potential?

A

The magnetic vector potential is the potential energy per unit element of current.

32
Q

What are ferromagnets?

A

Ferromagnets retain their magnetisation even after removal of an external magnetic field.

33
Q

Describe magnetic torque.

A

Magnetic torque acts to align the dipole parallel to the field and so accounts for the phenomenon of paramagnetism.

34
Q

Describe the effects of paramagnetism.

A

The effects of paramagnetism are small.

35
Q

Describe diamagnetism with respect to paramagnetism.

A

Diamagnetism is typically much weaker than paramagnetism and mainly observed in atoms with even numbers of electrons.

36
Q

Define magnetisation.

A

The magnetic dipole moment per unit volume. Where Amperian currents give rise to magnetisation.

37
Q

What is Kb?

A

The bound surface current.

38
Q

What is H?

A

H the auxiliary field is analogous to the electric displacement. It allows us to re-express amperes law in terms of free current alone.

39
Q

Describe how magnetic susceptibility varies.

A

Magnetic susceptibility is the bulk response of the material. It is positive for paramagnets and negative for diamagnets.

40
Q

Describe B with respect to surface charge.

A

B suffers a discontinuity in surface charge.

41
Q

Describe M and H as sources and sinks.

A

M is a sink and H is its source

42
Q

how does surface current act in terms of sources and sinks?

A

surface current Kb acts as a source of magnetic field B.

43
Q

What does faraday’s law state?

A

Faraday’s law states a changing magnetic field induces an electric field.

44
Q

What does E describe in Faraday’s law?

A

It is an induced and circulating field brought about because of time-dependent changes in the magnetic field.

45
Q

What is emf?

A

electromotive force is the line integral of force per unit charge around a circuit.

46
Q

When does motional emf arise?

A

Motential emf arises when you move a wire - a current carrier- through a magnetic field.

47
Q

What are the two distinct types of electric field?

A

1) those attributable directly to electric charge
2) those induced by the changing magnetic fields

48
Q

What does Ampere-Maxwell’s law state?

A

Ampere-Maxwell’s law states a changing electric field induces a magnetic field.

49
Q

What did Maxwell call the extra term he introduced?

A

Displacement current.

50
Q

What can we say generally about boundary conditions?

A

E, B, D and H are continuous at any boundary between two different media.

51
Q

When is a reflective wave in phase with the incident wave?

A

when v2 > v1 and n2 < n1

52
Q

When is a reflective wave out of phase with the incident wave?

A

when v2 < v1 and n2 > n1

53
Q

What is phase-matching?

A

How we go about maximising energy transfer through a boundary.

54
Q

What is the difference between potential and EMF?

A

The potential is a scalar field and EMF is not a field but rather given by a closed path.

55
Q

How do we determine the direction of the induced current?

A

The direction can be determined from Lenz’s law.

56
Q

What are the three ways to induce a magnetic field?

A

1) change the magnitude of the magnetic field with respect to time.

2) change the orientation of the loop with respect to a magnetic field

3) change the area of the loop

57
Q

What does Poynting’s theorem tell you about?

A

The energy flux density.

58
Q

What is the electric field?

A

The electric field is a vector field surrounding an electric charge that describes the direction and magnitude of the electrostatic force per unit charge experienced by a small positive test charge.

59
Q

What is the electric potential?

A

The electric potential is the net energy per unit charge required to move a positive test charge from a reference point to a point within an electric field.

60
Q

Describe the phenomenon of hysteresis.

A

When current is turned off the H-field will fall to zero. However, in general the iron will retain at least part of its magnetisation. This effect is called hysteresis

61
Q

What happens when the electric field pulls the atom apart?

A

When the atom is pulled apart the mutual attraction between the nucleus and cloud pulls the atom back together. Polarising the atom and inducing a dipole moment.

62
Q

Describe Polarisation.

A

If a piece of material is polarised it acquires a dipole moment per unit volume.

63
Q

Describe a capacitor.

A

A capacitor is two conductors separated by an insulating gap.

64
Q

When is a wave linearly polarised?

A

When the E vector aligns with the x-axis and the B vector aligns with the y-axis. Such that the wave is linearly polarised along the x-axis.

65
Q

What does Poynting’s theorem state?

A

The work done per unit time on the charges by the electromagnetic force is equal to the decrease in energy stored in the field less the energy that flowed out through the bounding surface.

66
Q

What is the skin depth?

A

The length over which the amplitude of the wave is attenuated by a factor of e = 2.718.

67
Q

Describe the relationship between the conduction current and displacement current.

A

If Jc&raquo_space; Jd then it is a good conductor. If Jc &laquo_space;Jd then it is a good dielectric.

Electromagnetic Theory (2 decks)

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