Direct Electric Current... - Lecture 10 Flashcards
Define the term ELECTRIC CURRENT
Electric current describes the directional movement of electric charge.
Define the term CURRENT
Current is the amount of charge which flows through the conductor per unit of time
I = dq / dt
measured in ampere, A
Define the term CURRENT DENSITY
Current density is the current per unit cross-sectional area of the conductor
j= dI / dS
j= zenv
What are the prerequisites for the generation of electric current?
- The availability of free electric charges in the conductor
- A voltage applied between the ends of the conductor
Define the term ELECTROMOTIVE FORCE
Electromotive Force is the work of the external agent for moving a unit positive charge along the conductor.
The conductance and the resistance of the conductor are determined by?
- The length L of the conductor
- The cross-sectional area S of the conductor
- The material of the conductor
G= y S/L R= p S/L
Describe Ohm’s law
The current I flowing through a conductor is directly proportional to the applied voltage u
I= Gu
G= conductance of the conductor (unit: siemens; s)
R= 1/G ; resistance if the conductor (unit: ohm )
Define the term CONDUCTIVITY y
Conductivity is the conductance of a conductor with cross-sectional are 1m^2 and length 1m.
Define the term RESISTIVITY p
Resistivity is the resistance of a conductor with cross-sectional are 1m^2 and length 1m.
p= 1/y
Define the term POLAR MOLECULES
The positive and negative charges are separated in spaces.
Eg: water; inorganic salts and acids…
Describe the solution of substances with polar molecules
The positively-charged ends of the polar molecules of the solvent are attracted by the negatively-charged ends of the polar molecules of the solute and vice versa.
Define the term DISSOCIATION
Dissociation is the splitting of neutral molecules into ions.
Define the term SOLVATION
The ions obtained by the dissociation of the solute are surrounded by the polar solvent molecules.
Ion diameters
The solvation increases the effective diameters of irons.
The number of solvent molecules surrounding an ion depends on temperature (more intense at higher temperatures).
Define the term RECOMBINATION (association)
It describes the bonding of ions with opposite charges in the solution producing neutral molecules.
Define the term DISSOCIATION CONSTANT (alpha)
The dissociation constant (alpha) is the ratio of dissociated molecules to the total number of molecules.
alpha = n+/c = n-/c
c=the number of dissolved molecules per unit (concentration)
n+/- = the number of positive/negative charged ions per unit volume
On what depends the dissociation constant (alpha)?
- Relative permittivity of the solvent
- dipole moment of the solute molecules
- solute concentration
- Temperatur
Define the term ELECTROLYTES (ionic solutions)
Electrolytes are substances with ionic conductance.
Charge carriers are positive and negative ions
Define the term CHARGE MOBILITY u
Charge mobility u is the drift (constant) velocity of the charge per unit magnitude of the electric field.
u = V/E
Which is the formula for conductivity of electrolytes?
y = (alpha) cze (u+ + u-)
Conductivity y depends on?
- Concentration (directly and through (alpha))
- Temperature (through (alpha) and u)
- Type of electrolyte (through (alpha), u and z)
Describe the conductivity of biological tissue.
Proteins and lipids have low conductivity.
The ionic solutions have high ionic conductivity.
Eg:
blood/cerebrospinal liquid > high conductivity
bones/dry skin > low conductivity
Define the term RHEOGRAPHY
Rheography is a method for investigation of the supply of blood to organs by measuring their conductivity.
Conductivity of gases
A pure gas consist of neutral atoms and/or molecules. Gases do not contain free charge carriers and are dielectric.
Charge carriers for gases can be produced by?
[ External Ionizing Agent ]
Charge carriers can be produced by ionization using:
- high temperature
- uv light
- x-rays; gamma rays etc.
Define the term IONIZATION POTENTIAL
Ionization Potential is the energy transferred from the ionizing agent to a bonded electron to break the bond with the nucleus and free electron.
Define the term IMPACT IONIZATION
Neutral atoms can be ionized on collision with fast moving charges particles.
(With kinetic energy exceeding the ionization potential)
> takes place when the voltage acceleration the ions exceeds a threshold value
Ionization is gases
Charge carriers: electrons, positive ions, negative ions
When the external ionizing agent is constant, the concentrations of ions in the gas stays constant.
Define the term GAS DISCHARGE
Gas discharge is the flow of the electric current in a gas.
The current consist of an electronic and two ionic streams.
Non-self-sustained charge AC
Current flows only if the external ionizing agent is operating.
AB- ohmic region
BC- saturation region
Self-sustained charge CD
Current flows and no external ionizing agent is needed.
CD- avalanche discharge region
Define the term OHMIC REGION
Increasing voltage results in a proportional growth of the number of current carriers reaching the electrodes per unit of time.
Define the term SATURATION REGION
All ions produced by the ionizing agent per unit of time reach the electrodes and increasing voltage does not result in higher current.
Define the term AVALANCHE DISCHARGE REGION
Impact ionization takes place at these high voltages and the number of charge carriers produced in the gas per unit of time increases rapidly
- this leads to higher currents and the discharge becomes self-sustained
Air ions
Mainly N2 and O2.
Can be attached to groups of neutral molecules or other particles.
Classified by diameter.
The effect of air ions an the human organism
Small negative air ions produce tonic effect (ion therapy).
All other types of air ions are considered harmful!
