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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
What does Gauss' law make it possible to determine?
The distribution of charges present in a region of space
26
What is the extracellular potential assumed to be?
0
27
What happens with charges during depolarisation?
Positive charges enter the cell
28
What is a cell at rest?
Negative inside and positive outside Membrane is polarised
29
What is contractions of the heart triggered by?
Spontaneously by the sinoatrial node
30
What triggers contraction of striated muscles?
Nerve impulses
31
What does an electrocardiograph record?
The potential difference between two electrodes
32
Where are the electrodes placed when doing an electrocardiograph?
One for each upper limb and one for the left ankle
33
What does the P spike show on an ECG?
Depolarisation of the atria
34
What does the R spike on an ECG show?
Ventricles depolarization
35
What does the T spike on an ECG show?
Repolarization of ventricles
36
What do conductors present?
Electric charges that are free to move
37
Which ions are free to move in liquids?
Both negative and positive (electrolytic solutions)
38
What charges move in insulators?
None, neither positive or negative are free to move, no flow of current
39
What happens to polarisation once the external electric field is removed?
It ceases
40
What do semiconductors behave as?
Insulators at low temp Conductors at room temp
41
What is the electrical capacity of an insulated conductor?
The constant ratio between the total charge of the conductor and it potential
42
Equation for electrical capacity
C=Q/V
43
What is the unit of measurement of the electrical capacity?
Farad=coulomb/volt
44
What is electric current?
Ordered flow of electric charges
45
What is there in direct current?
Constant flow of charges in one direction (when the electric field is constant)
46
Formula for current intensity
i=q/t i: current intensity
47
What is Ohm's 1st law?
R=^V/i
48
What is electrical resistance measured in?
Ohm
49
What is Ohm's 2nd law?
R=roh*(L/S) roh: resistivity (the higher the resistivity the higher the resistance)
50
What doe resistivity depend on?
Material and temperature
51
What happens to resistivity in conductive solids when temperature increases?
It increases moderately
52
Does a high resistivity make a material a good or bad conductor?
Bad
53
Equation for conductivity
Sigma=1/roh
54
What is the opposite of resistivity?
Conductivity
55
What is the electric current density?
The amount of charge that flows through a unit section of area in a conductor in the unit of time
56
What is the direction of the electric current in relation to the direction of electrons?
Opposite
57
Formula for total resistance for resistors in series
R=R1+R2+R3...
58
Formula for total current for resistors in series
i=i1=i2=i3=...
59
Formula for total potential difference for resistors in series
^V=^V1+^V2+^V3+...
60
Formula for total resistance for resistors in parallel
1/R=1/R1 + 1/R2 + 1/R3+...
61
Formula for total current for resistors in parallel
i=i1+i2+i3+...
62
Formula for total potential difference for resistors in parallel
^V=^V1=^V2=^V3=....
63
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?
Carry out work
64
What is the electromotive force?
e.m.f The work done on a unit electric charge travelling along a circuit
65
e.m.f=?
=^W/^q=(R+r)*i
66
What is the joule effect?
The thermal effect of a current
67
What happens to the kinetic energy upon collision of electrons?
Part of it is released to the lattice atoms, increasing their thermal agitation
68
W=?
^Ekin -^Ep
69
What is the intensity of a circuit of the resistance is very low? (think Ohm's law equation)
Very high
70
What will the heat produced by the joule effect be if the intensity is high?
Also high Q=R*I^2*t
71
What can a very high intensity (current) of a circuit lead to?
Sparking or melting of the conductor Short circuiting
72
What are fuses?
Sections of conductors inserted in a circuit with a low melting temperature and a small cross section
73
What is the resistance of the fuse in comparison with the other sections of the circuit?
Greater as the resistance is inversely proportional to the cross section
74
What happens to a circuit with a fuse in the event of a short circuit/ voltage overload?
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
Is the electrical resistance of the human body higher or lower when the body is wet?
Much lower
76
What causes death when the intensity/current is high?
Ventricular fibrillation Cardiac paralysis
77
What causes death when the voltage is high?
Respiratory paralysis (action on the bulbar centers)
78
What are molecules called when they are dissociated into ions in solution?
Electrolytes
79
What is a solution with electrolytes called?
Electrolyte solution
80
What 3 things does the degree of dissociation depend on?
Nature of the ions Solvent dielectric constant Temperature
81
Is dissociation reversible?
Yes, there us a dynamic equilibrium between the particles that split and those that recombine
82
When can a solution conduct electric currents?
Only when it contains electrolytes (free ions)
83
When does a solution act as an insulator?
When it does not contain electrolytes (free ions)
84
Where do the cations go in electrolysis?
To the cathode (negative electrode)
85
Where do the anions go in electrolysis?
To the anode (positive electrode)
86
What is electrolysis?
The motion of electrolytes in an electrolyte solution upon application of an electric field
87
Speed of migration in electrolysis, v=?
=(Z*e*E)/f Z: valence of ion e: E: intensity of electric field f: friction coefficient
88
What can the speed of migration also be applied to?
Proteins It is used in protein electrophoresis to separate, identify and characterise protein molecules
89
What happens to the ions when they reach the electrode in electrolysis?
They are neutralised
90
Which direction in the force field if the charge is negative?
Inward (towards the charge)
91
Which direction in the force field if the charge is positive?
Outward (away from the charge)

Medical physics (10 decks)

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