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Medical physics > Electrostatics > Flashcards

Electrostatics Flashcards

1
Q

Where does electromagnetic force come from?

A

Electric charge

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2
Q

What can the electrostatic interactions be that gravitational can’t?

A

Attractive or repulsive

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3
Q

What is the elementary electric charge?

A

The electron

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4
Q

What is the equal but opposite to the electron?

A

The proton

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5
Q

What does Coulomb’s law state?

A

The electromagnetic force with which 2 charged particles attract or repel each other is directed along the line joining them

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6
Q

Formula for Coulomb’s law

A

F=K* (q1*q2)/r^2

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7
Q

SI unit for electric charge

A

Coulombs

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8
Q

What kind of quantity is electric charged in the SI system?

A

A derived quantity

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9
Q

What is a dielectric?

A

A poor electric conductor/ an insulator

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10
Q

Why is matter generally electrically neutral macroscopically?

A

Because the negative charges of the electrons are compensated by the positive of the protons

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11
Q

When is a current generated?

A

If there is an external charge and the charged particles are able to migrate (in a conductor)

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12
Q

What happens if an external charge is present but the electrons can’t migrate?

A

The atoms deform, get polarised (when in a dielectric or insulating medium)

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13
Q

What is an electric field?

A

Region of space where the force is working

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14
Q

What is an electric field that is constant over time called?

A

Electrostatic field

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15
Q

Work of electric field equation

A

W=Fs=Eq*s

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16
Q

What does it mean that the electrostatic field is conservative?

A

The work done to bring a charge from one point to another in space depends only on the initial and final position and not on the trajectory
Thus work along a closed trajectory is zero

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17
Q

Equation for electrostatic potential

A

V=U/q

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18
Q

What is the electrostatic potential difference between two points within an electric field?

A

Voltage, ^V
The ratio of work done by the force of the field when an exploratory charge moved from A to B along any trajectory

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19
Q

Volt=

A

Joule/coulomb=Watt/ampere

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20
Q

What is the electron volt (eV)?

A

The potential energy of an electron

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21
Q

What kind of quantity is the electric potential?

A

Scalar quantity

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22
Q

What does the flow of an electric field through a surface depend on? (3 things)

A

Intensity of the field
Extension of the surface (area)
Oritentation of the surface in the field

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23
Q

What does Gauss’ theory give?

A

Mediated information on a surface

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24
Q

What does Gauss theorem state

A

The total electric flux through a closed surface is proportional to the total charge enclosed by that surface

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25
Q

What does Gauss’ law make it possible to determine?

A

The distribution of charges present in a region of space

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26
Q

What is the extracellular potential assumed to be?

A

0

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27
Q

What happens with charges during depolarisation?

A

Positive charges enter the cell

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28
Q

What is a cell at rest?

A

Negative inside and positive outside
Membrane is polarised

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29
Q

What is contractions of the heart triggered by?

A

Spontaneously by the sinoatrial node

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30
Q

What triggers contraction of striated muscles?

A

Nerve impulses

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31
Q

What does an electrocardiograph record?

A

The potential difference between two electrodes

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32
Q

Where are the electrodes placed when doing an electrocardiograph?

A

One for each upper limb and one for the left ankle

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33
Q

What does the P spike show on an ECG?

A

Depolarisation of the atria

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34
Q

What does the R spike on an ECG show?

A

Ventricles depolarization

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35
Q

What does the T spike on an ECG show?

A

Repolarization of ventricles

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36
Q

What do conductors present?

A

Electric charges that are free to move

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37
Q

Which ions are free to move in liquids?

A

Both negative and positive (electrolytic solutions)

38
Q

What charges move in insulators?

A

None, neither positive or negative are free to move, no flow of current

39
Q

What happens to polarisation once the external electric field is removed?

A

It ceases

40
Q

What do semiconductors behave as?

A

Insulators at low temp
Conductors at room temp

41
Q

What is the electrical capacity of an insulated conductor?

A

The constant ratio between the total charge of the conductor and it potential

42
Q

Equation for electrical capacity

A

C=Q/V

43
Q

What is the unit of measurement of the electrical capacity?

A

Farad=coulomb/volt

44
Q

What is electric current?

A

Ordered flow of electric charges

45
Q

What is there in direct current?

A

Constant flow of charges in one direction (when the electric field is constant)

46
Q

Formula for current intensity

A

i=q/t

i: current intensity

47
Q

What is Ohm’s 1st law?

A

R=^V/i

48
Q

What is electrical resistance measured in?

A

Ohm

49
Q

What is Ohm’s 2nd law?

A

R=roh*(L/S)

roh: resistivity (the higher the resistivity the higher the resistance)

50
Q

What doe resistivity depend on?

A

Material and temperature

51
Q

What happens to resistivity in conductive solids when temperature increases?

A

It increases moderately

52
Q

Does a high resistivity make a material a good or bad conductor?

A

Bad

53
Q

Equation for conductivity

A

Sigma=1/roh

54
Q

What is the opposite of resistivity?

A

Conductivity

55
Q

What is the electric current density?

A

The amount of charge that flows through a unit section of area in a conductor in the unit of time

56
Q

What is the direction of the electric current in relation to the direction of electrons?

A

Opposite

57
Q

Formula for total resistance for resistors in series

A

R=R1+R2+R3…

58
Q

Formula for total current for resistors in series

A

i=i1=i2=i3=…

59
Q

Formula for total potential difference for resistors in series

A

^V=^V1+^V2+^V3+…

60
Q

Formula for total resistance for resistors in parallel

A

1/R=1/R1 + 1/R2 + 1/R3+…

61
Q

Formula for total current for resistors in parallel

A

i=i1+i2+i3+…

62
Q

Formula for total potential difference for resistors in parallel

A

^V=^V1=^V2=^V3=….

63
Q

What does a generator/voltage source have to do to carry an electric charge from one pole to the other and create an electrostatic potential difference?

A

Carry out work

64
Q

What is the electromotive force?

A

e.m.f
The work done on a unit electric charge travelling along a circuit

65
Q

e.m.f=?

A

=^W/^q=(R+r)*i

66
Q

What is the joule effect?

A

The thermal effect of a current

67
Q

What happens to the kinetic energy upon collision of electrons?

A

Part of it is released to the lattice atoms, increasing their thermal agitation

68
Q

W=?

A

^Ekin
-^Ep

69
Q

What is the intensity of a circuit of the resistance is very low? (think Ohm’s law equation)

A

Very high

70
Q

What will the heat produced by the joule effect be if the intensity is high?

A

Also high
Q=RI^2t

71
Q

What can a very high intensity (current) of a circuit lead to?

A

Sparking or melting of the conductor
Short circuiting

72
Q

What are fuses?

A

Sections of conductors inserted in a circuit with a low melting temperature and a small cross section

73
Q

What is the resistance of the fuse in comparison with the other sections of the circuit?

A

Greater as the resistance is inversely proportional to the cross section

74
Q

What happens to a circuit with a fuse in the event of a short circuit/ voltage overload?

A

The power (and thus the joule effect) is greater in the fuse than the other sections of the circuit, the fuse melts and interrupts the current

75
Q

Is the electrical resistance of the human body higher or lower when the body is wet?

A

Much lower

76
Q

What causes death when the intensity/current is high?

A

Ventricular fibrillation
Cardiac paralysis

77
Q

What causes death when the voltage is high?

A

Respiratory paralysis (action on the bulbar centers)

78
Q

What are molecules called when they are dissociated into ions in solution?

A

Electrolytes

79
Q

What is a solution with electrolytes called?

A

Electrolyte solution

80
Q

What 3 things does the degree of dissociation depend on?

A

Nature of the ions
Solvent dielectric constant
Temperature

81
Q

Is dissociation reversible?

A

Yes, there us a dynamic equilibrium between the particles that split and those that recombine

82
Q

When can a solution conduct electric currents?

A

Only when it contains electrolytes (free ions)

83
Q

When does a solution act as an insulator?

A

When it does not contain electrolytes (free ions)

84
Q

Where do the cations go in electrolysis?

A

To the cathode (negative electrode)

85
Q

Where do the anions go in electrolysis?

A

To the anode (positive electrode)

86
Q

What is electrolysis?

A

The motion of electrolytes in an electrolyte solution upon application of an electric field

87
Q

Speed of migration in electrolysis, v=?

A

=(ZeE)/f

Z: valence of ion
e:
E: intensity of electric field
f: friction coefficient

88
Q

What can the speed of migration also be applied to?

A

Proteins
It is used in protein electrophoresis to separate, identify and characterise protein molecules

89
Q

What happens to the ions when they reach the electrode in electrolysis?

A

They are neutralised

90
Q

Which direction in the force field if the charge is negative?

A

Inward (towards the charge)

91
Q

Which direction in the force field if the charge is positive?

A

Outward (away from the charge)

Medical physics (10 decks)

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