Electrostatics Flashcards
Where does electromagnetic force come from?
Electric charge
What can the electrostatic interactions be that gravitational can’t?
Attractive or repulsive
What is the elementary electric charge?
The electron
What is the equal but opposite to the electron?
The proton
What does Coulomb’s law state?
The electromagnetic force with which 2 charged particles attract or repel each other is directed along the line joining them
Formula for Coulomb’s law
F=K* (q1*q2)/r^2
SI unit for electric charge
Coulombs
What kind of quantity is electric charged in the SI system?
A derived quantity
What is a dielectric?
A poor electric conductor/ an insulator
Why is matter generally electrically neutral macroscopically?
Because the negative charges of the electrons are compensated by the positive of the protons
When is a current generated?
If there is an external charge and the charged particles are able to migrate (in a conductor)
What happens if an external charge is present but the electrons can’t migrate?
The atoms deform, get polarised (when in a dielectric or insulating medium)
What is an electric field?
Region of space where the force is working
What is an electric field that is constant over time called?
Electrostatic field
Work of electric field equation
W=Fs=Eq*s
What does it mean that the electrostatic field is conservative?
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
Equation for electrostatic potential
V=U/q
What is the electrostatic potential difference between two points within an electric field?
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
Volt=
Joule/coulomb=Watt/ampere
What is the electron volt (eV)?
The potential energy of an electron
What kind of quantity is the electric potential?
Scalar quantity
What does the flow of an electric field through a surface depend on? (3 things)
Intensity of the field
Extension of the surface (area)
Oritentation of the surface in the field
What does Gauss’ theory give?
Mediated information on a surface
What does Gauss theorem state
The total electric flux through a closed surface is proportional to the total charge enclosed by that surface
What does Gauss’ law make it possible to determine?
The distribution of charges present in a region of space
What is the extracellular potential assumed to be?
0
What happens with charges during depolarisation?
Positive charges enter the cell
What is a cell at rest?
Negative inside and positive outside
Membrane is polarised
What is contractions of the heart triggered by?
Spontaneously by the sinoatrial node
What triggers contraction of striated muscles?
Nerve impulses
What does an electrocardiograph record?
The potential difference between two electrodes
Where are the electrodes placed when doing an electrocardiograph?
One for each upper limb and one for the left ankle
What does the P spike show on an ECG?
Depolarisation of the atria
What does the R spike on an ECG show?
Ventricles depolarization
What does the T spike on an ECG show?
Repolarization of ventricles
What do conductors present?
Electric charges that are free to move
Which ions are free to move in liquids?
Both negative and positive (electrolytic solutions)
What charges move in insulators?
None, neither positive or negative are free to move, no flow of current
What happens to polarisation once the external electric field is removed?
It ceases
What do semiconductors behave as?
Insulators at low temp
Conductors at room temp
What is the electrical capacity of an insulated conductor?
The constant ratio between the total charge of the conductor and it potential
Equation for electrical capacity
C=Q/V
What is the unit of measurement of the electrical capacity?
Farad=coulomb/volt
What is electric current?
Ordered flow of electric charges
What is there in direct current?
Constant flow of charges in one direction (when the electric field is constant)
Formula for current intensity
i=q/t
i: current intensity
What is Ohm’s 1st law?
R=^V/i
What is electrical resistance measured in?
Ohm
What is Ohm’s 2nd law?
R=roh*(L/S)
roh: resistivity (the higher the resistivity the higher the resistance)
What doe resistivity depend on?
Material and temperature
What happens to resistivity in conductive solids when temperature increases?
It increases moderately
Does a high resistivity make a material a good or bad conductor?
Bad
Equation for conductivity
Sigma=1/roh
What is the opposite of resistivity?
Conductivity
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
What is the direction of the electric current in relation to the direction of electrons?
Opposite
Formula for total resistance for resistors in series
R=R1+R2+R3…
Formula for total current for resistors in series
i=i1=i2=i3=…
Formula for total potential difference for resistors in series
^V=^V1+^V2+^V3+…
Formula for total resistance for resistors in parallel
1/R=1/R1 + 1/R2 + 1/R3+…
Formula for total current for resistors in parallel
i=i1+i2+i3+…
Formula for total potential difference for resistors in parallel
^V=^V1=^V2=^V3=….
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
What is the electromotive force?
e.m.f
The work done on a unit electric charge travelling along a circuit
e.m.f=?
=^W/^q=(R+r)*i
What is the joule effect?
The thermal effect of a current
What happens to the kinetic energy upon collision of electrons?
Part of it is released to the lattice atoms, increasing their thermal agitation
W=?
^Ekin
-^Ep
What is the intensity of a circuit of the resistance is very low? (think Ohm’s law equation)
Very high
What will the heat produced by the joule effect be if the intensity is high?
Also high
Q=RI^2t
What can a very high intensity (current) of a circuit lead to?
Sparking or melting of the conductor
Short circuiting
What are fuses?
Sections of conductors inserted in a circuit with a low melting temperature and a small cross section
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
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
Is the electrical resistance of the human body higher or lower when the body is wet?
Much lower
What causes death when the intensity/current is high?
Ventricular fibrillation
Cardiac paralysis
What causes death when the voltage is high?
Respiratory paralysis (action on the bulbar centers)
What are molecules called when they are dissociated into ions in solution?
Electrolytes
What is a solution with electrolytes called?
Electrolyte solution
What 3 things does the degree of dissociation depend on?
Nature of the ions
Solvent dielectric constant
Temperature
Is dissociation reversible?
Yes, there us a dynamic equilibrium between the particles that split and those that recombine
When can a solution conduct electric currents?
Only when it contains electrolytes (free ions)
When does a solution act as an insulator?
When it does not contain electrolytes (free ions)
Where do the cations go in electrolysis?
To the cathode (negative electrode)
Where do the anions go in electrolysis?
To the anode (positive electrode)
What is electrolysis?
The motion of electrolytes in an electrolyte solution upon application of an electric field
Speed of migration in electrolysis, v=?
=(ZeE)/f
Z: valence of ion
e:
E: intensity of electric field
f: friction coefficient
What can the speed of migration also be applied to?
Proteins
It is used in protein electrophoresis to separate, identify and characterise protein molecules
What happens to the ions when they reach the electrode in electrolysis?
They are neutralised
Which direction in the force field if the charge is negative?
Inward (towards the charge)
Which direction in the force field if the charge is positive?
Outward (away from the charge)
