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Medical Physics > Magnetic Phenomena > Flashcards

Magnetic Phenomena Flashcards

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1
Q

What is magnetism used for in biology?

A
  • lungs shows effect of dust inhaling
  • heart can determine blood volume
  • liver can show iron stores
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2
Q

What is magnetism?

A

Magnetism is the force of attraction or repulsion between various substances.

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3
Q

What is magnetic field?

A

A region in the neighbourhood of a magnet, electric currents, or changing electric field, in which magnetic forces are observed.

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4
Q

How is magnetic field produced?

A

By moving charged particles or electric currents.

(Electric current can be macroscopic currents in wires, or microscopic currents associated with electrons in atomic orbits.)

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5
Q

What is a magnet? Which poles attract? Which poles repel?

A

a body possessing the property of magnetism.
- must have two poles (south and north)
- unlike poles attract
- like poles repell

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6
Q

How can a magnetic field be visualized?

A

in terms of the force line

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7
Q

How can the magnetic field be visualised?

A

shake little magnets near the magnetic field.

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8
Q

What are the main characteristics of magnetic lines?

A
  • always form closed loops
  • never cross eachother
  • 2 unlike poles=pulled together
  • magnetic lines of force pass through all material
  • always enter and leave a magnetic material at a 90*
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9
Q

What is Magnetocardiography?

A

Technique to measure the magnetic fields produced by electrical activity in the heart using extremely sensitive devices

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10
Q

What is a magnetic flux? What are its units?

A

The Magnetic flux Φ through some surface is proportional to the number of field lines passing through that surface.
The SI unit of magnetic flux is the Weber (Wb).

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11
Q

What are magnetic field lines?

A

The lines of the magnetic field indicate the direction of the field in a given region.

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12
Q

How do mmagnetic field lines look like?

A
  • originate at the north pole of a magnet
  • form loops
  • end at the south pole
    (either of the same magnet or of some other nearby magnet)
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13
Q

Where do magnetic field lines converge? Where do the lines spread? What do the lines tell?

A
  • lines converge near the poles, where the field is strong,
  • spread out as their distance from poles increases.
  • lines are spaced so that the number per unit area is proportional to the field strength in a given area.
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14
Q

What are the characteristics of magnetic lines of force?

A
  • continuous
  • always form closed loops.
  • never cross one another.
  • parallel magnetic lines of force travelling in the same direction repel one another
  • tend to shorten themselves
  • pass through all materials, both magnetic and nonmagnetic.
  • always enter or leave a magnetic material at a right angle to the surface.
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15
Q

What are the two different vectors used to represent a magnetic field?

A

1) the magnetic flux density, or MAGNETIC INDUCTION, B (the total magnetic field including also the contribution made by the magnetic properties of the materials in the field);
2) the MAGNETIC FIELD STRENGTH, or magnetic field intensity, H(the magnetic field produced by the flow of current).

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16
Q

What is the magnetic flux density? What is its symbol? How to calculate it?

A

The Magnetic flux density (B) is defined as the amount of magnetic flux Φ in an area taken perpendicular to the magnetic flux’s direction.

B= ΦxA
Φ - angle between the magnetic field lines and the normal to area A.

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17
Q

Define magnetic induction.

A

the force per unit length per unit current acting on a current-carrying conductor at a right angle to the field lines.

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18
Q

What does magnetic field depend on?

A
  • charge (q)
  • velocity (v)
  • distance (r)
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19
Q

What are the SI units of Magnetic induction B?

A
  • Tesla [ T ];
  • Gauss (1 Tesla =10,000 Gauss).
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20
Q

What is magnetic field intensity H? What is it used for? What are its SI units?

A

The magnetic field intensity is used to characterise the strength of the magnetic field due to the external sources.

SI units: A/m

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21
Q

How can one isolate a single magnetic pole?

A

IMPOSSIBLE.
regardless how many times a magnet is broken, one of the faces will be the north pole and the other one will be the south pole.

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22
Q

How is magnetic field produced?

A

The orbital motion of an electron creates tiny atomic current loops, which produce a magnetic field.

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23
Q

State the main laws for magnetic field of steady current.

A
  1. Gauss’s law for magnetism
  2. Biot-Savart Law
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24
Q

Describe Gauss’s law for magnetism.

A
  • Gauss’s law states that the MAGNETIC FIELD B HAS DIVERGENCE EQUAL TO ZERO.
    = magnetic monopoles do not exist.

The law in this form states that for each volume element in space, there are exactly the same numbers of “magnetic field lines” entering and exiting the volume.

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25
Q

Describe Biot-Savart’s Law.

A

The Biot-Savart law states how the VALUE OF THE MAGNETIC FIELD vat a specific point in space from one short segment of current-carrying conductor DEPENDS ON EACH FACTOR THAT INFLUENCES THE FIELD.

a) the magnetic induction B created by electric current, is proportional to the current strength I;
b) magnetic induction B depends on form and magnitude of conductor carrying electric current
c) magnetic induction B in any point of the field depends on the point P position in respect of the conductor.

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26
Q

What factors is magnetic field dependent on?

A
  1. The magnitude and direction of the current;
  2. The geometry of the configuration, (the charges flow in a straight line, in one or more circles, etc.);
  3. The medium which surrounds the current, (vacuum, air, wood, iron, etc).
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27
Q

What do magnetic fields form around a long wire?

A

The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire.

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28
Q

Explain the right-hand rule.

A

hold a pencil (acting like it is a wire)

thumb = direction of the current
fingers = encircle the wire in the direction of the B vector.

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29
Q

What is Amperes law? When is amperes law correct?

A

Ampere showed that the flux density B at any point near the conductor is directly proportional to the current I and inversely proportional to the distance ‘r’ from the conductor.
This law is true for steady current only.

30
Q

Explain the right-hand rule to show the force on a single moving charge.

A

thumb = direction of velocity,
fingers = magnetic field direction,
palm direction = direction of the force on charge.

31
Q

What is the equation for a force on a single moving charge?

A

A general form of the force equation is:
F=B·q·v·sinθ

where is the angle between the velocity v and magnetic induction B.
If v = 0 and θ = 0, then F = 0.

32
Q

How to calculate electric force?

A

F = qE

q- charge of the particle
e- strength of electric field

33
Q

What is LORENTZ FORCE? What does it not affect?

A

The Lorentz force is the force on a point charge due to electromagnetic fields.

Lorenz force does not affect the velocity of moving charge, but does change direction of motion

34
Q

What are the magnetic properties of materials?

A

1) magnetic moment (magnetic field created by the moving charge in an atom.

35
Q

How to calculate the magnetic moment?

A

when Z electrons move around the nucleus, magnetic moment is vectorial sum of all electrons magnetic moments.

36
Q

In which 3 ways can magnetic materials be described as?

A
  • Magnetization M;
  • Magnetic susceptibility χ;
  • Magnetic permeability μ;
37
Q

What is magnetization? How is it expressed?

A

The magnetization M represents how strongly a region of material is magnetized.

The magnetization of material is expressed in term of net magnetic dipole moments pm in the material.

38
Q

Explain magnetization.

A
  • If the material is not magnetized the orientation of atoms’ magnetic moment is chaotic.
    magnetization = 0 = chaos
  • In magnetic field magnetization is bigger as 0 because magnetic moments in magnetic field are less or more oriented.
39
Q

What are permanent magnetic materials composed of?

A

composed of small regions or “domains” each of which exhibit a net magnetic moment

40
Q

What does an unmagnetized magnet possess?

A

An unmagnetized magnet will possess domains that are randomly oriented with respect to each other, providing a net magnetic moment of zero.

41
Q

What is magnetic susceptibility?

A

Magnetic susceptibility is a measure of the magnetization of a material in response to an applied magnetic field. Or, it property that determines how easily a specimen can be magnetized

Magnetic susceptibility specifies how much the relative permeability differes from one.

42
Q

Explain magnetic permeability.

A
  • Magnetic permeability is internal ability to form magnetic field or ability to be magnetized.
  • depends upon the nature of medium where the point is placed
43
Q

What are the different types of permeability?

A
  • Absolute permeability μ;
  • Permeability of free space μ0 (4π∙10-7 Henry/meter); * Relative permeability μr
44
Q

What is the units of magnetic permeability. Briefly explain their meanings.

A

If μ >1 then material is attracted to the field.

If μ < 1, then material is repelled by the field.

45
Q

How can materials be classified? (response to externally applied magnetic fields)

A

1.Diamagnetic, μ<1
(weakly repelled from a strong magnet)

2.Paramagnetic, μ>1
(weakly attracted to a magnet)

3.Ferromagnetic μ»1.
(ferromagnetism is found only in solids)

46
Q

Explain Diamagnetic materials, giving examples.

A
  • water, wood, gold, mercury, bismuth, tissues of human body
  • weak form of magnetism
  • arises when an external field is applied.
  • arises due to a change in the orbital motion of electrons on application of a magnetic field.
  • no magnetic dipoles in the absence of a magnetic field and when a magnetic field is applied the dipole moments are aligned opposite to field direction.
  • relative magnetic permeability is less than 1;
  • Magnetic susceptibility is less than 0, (therefore diamagnetics are repelled by magnetic fields).
47
Q

Explain paramagnetic materials. State examples.

A
  • cancellation of magnetic moments between electron pairs is incomplete
    *magnetic moments exist without any external magnetic field.
  • magnetic moments are randomly aligned * no net magnetization without any external field.
  • When a magnetic field is applied all the dipole moments are aligned in the direction of the field.

*total magnetization drops to zero when the applied field is removed

  • The fraction of the magnetic moments is proportional to the field strength;
  • relative magnetic permeability >1;
  • positive magnetic susceptibility.
48
Q

Explain ferromagnetic materials. Giving examples.

A
  • iron, nickel, cobalt and their alloys
  • the internal magnetic field is much greater than the applied external magnetic field;
  • remain magnetized after the external field is removed;
  • permeability isn’t fixed
  • depends on the temperature and strength of an external magnetic field.
49
Q

What is a magnetic domain?

A

a region within a magnetic material which has uniform magnetization.

50
Q

What are the four quantum numbers that specify the complete and unique quantum state of a single electron in an atom?

A
  1. the principal quantum number, n,
  2. the orbital quantum number, l,
  3. the magnetic quantum number, m,
  4. the spin quantum number, s
51
Q

What does the Pauli exclusion principle state?

A

Two electrons belonging to the same atom CANNOT have the same four quantum numbers.

52
Q

What does the principal quantum number (n) describe?

A

the size of the orbital.

  • n=1, 2, 3,….
53
Q

What does the orbital quantum number (l) describe?

A

the shape of the orbital.

54
Q

What does the magnetic quantum number (m) describe?

A

the orientation in space of a particular orbital.

  • determines the energy shift of an atomic orbital due to an external magnetic field.
55
Q

What is the electron spin magnetic dipole momentum?

A

electron magnetic dipole moment can have only two directions.

56
Q

What is the Bohr magneton?

A

a physical constant of magnetic moment of electrons.

57
Q

What is the Landé g-factor?

A

a multiplicative term appearing in the expression for the energy levels of an atom in a weak magnetic field.

58
Q

Explain magnetic spin.

A

An electron has two possible orientations with different energies,
- spin up, (aligned WITH the magnetic field), - spin down, aligned AGAINST it.

All other orientations are forbidden.

59
Q

What is the ZEEMANN EFFECT?

A

The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field.

60
Q

What do unpaired electrons do?

A

An unpaired electron can move between the two energy levels by absorbing and emitting electromagnetic radiation of energy

61
Q

What is the Electron paramagnetic resonance?

A

a technique for studying chemical species that have one or more unpaired electrons.

62
Q

What happens when a person increases an external magnetic field?

A

the gap between the energy states is widened until it matches the energy of the microwaves.

63
Q

What does MRI allow for?

A
  • a variety of tissue characteristics,
  • blood flow and distribution,
  • several physiologic
    and metabolic functions.
64
Q

What does NMR allow for? What does it stand for?

A

NMR is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation.

  • Nuclear magnetic resonance
65
Q

What are the rules for determining the net spin of a nucleus?

A
  • # neutrons = # protons then the nucleus has NO spin.
  • # neutrons + # protons is odd, then the nucleus has a half-integer spin (i.e. 1⁄2, 3/2, 5/2)
  • # neutrons + # protons are both odd, then the nucleus has an integer spin (i.e. 1,2,3)
66
Q

What are the basic principles of NMR?

A
  • proton behaves as a small magnet with a magnetic moment that has both magnitude and direction.
  • no net magnetization without an external field
67
Q

What does the frequency of precision depend on in NMR?

A

depends upon its gyromagnetic ratio γ and the strength of the magnetic field B

68
Q

The stronger the magnetic field, the what the precession frequency.

A

The stronger the magnetic field, the higher the precession frequency.

69
Q

Which energy state are protons usually on? Which direction will it point in?

A
  • Most of protons will be on the low energy state
  • point in the direction of magnetic field
70
Q

What is “relaxation”? What are the different types?

A
  • The nucleus can return to a position of alignment by transferring its excess energy to other nuclei or the general structure of the material.

T1 Relaxation - restoring longitudinal magnetization

T2 relaxation- loss of phase coherence (spin-spin relaxation)
- shorter

71
Q

What is the MR signal affected by?

A
  • the presence of chemical bonds,
  • paramagnetic ions,
  • the rate of flow.

Medical Physics (8 decks)

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