Direct Electric Current... - Lecture 10

Question 1

Define the term ELECTRIC CURRENT

Answer 1

Electric current describes the directional movement of electric charge.

Question 2

Define the term CURRENT

Answer 2

Current is the amount of charge which flows through the conductor per unit of time

I = dq / dt
measured in ampere, A

Question 3

Define the term CURRENT DENSITY

Answer 3

Current density is the current per unit cross-sectional area of the conductor

j= dI / dS

j= zenv

Question 4

What are the prerequisites for the generation of electric current?

Answer 4

1. The availability of free electric charges in the conductor
2. A voltage applied between the ends of the conductor

Question 5

Define the term ELECTROMOTIVE FORCE

Answer 5

Electromotive Force is the work of the external agent for moving a unit positive charge along the conductor.

Question 6

The conductance and the resistance of the conductor are determined by?

Answer 6

1. The length L of the conductor
2. The cross-sectional area S of the conductor
3. The material of the conductor

G= y S/L
R= p S/L

Question 7

Describe Ohm’s law

Answer 7

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 )

Question 8

Define the term CONDUCTIVITY y

Answer 8

Conductivity is the conductance of a conductor with cross-sectional are 1m^2 and length 1m.

Question 9

Define the term RESISTIVITY p

Answer 9

Resistivity is the resistance of a conductor with cross-sectional are 1m^2 and length 1m.

p= 1/y

Question 10

Define the term POLAR MOLECULES

Answer 10

The positive and negative charges are separated in spaces.

Eg: water; inorganic salts and acids…

Question 11

Describe the solution of substances with polar molecules

Answer 11

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.

Question 12

Define the term DISSOCIATION

Answer 12

Dissociation is the splitting of neutral molecules into ions.

Question 13

Define the term SOLVATION

Answer 13

The ions obtained by the dissociation of the solute are surrounded by the polar solvent molecules.

Question 14

Ion diameters

Answer 14

The solvation increases the effective diameters of irons.

The number of solvent molecules surrounding an ion depends on temperature (more intense at higher temperatures).

Question 15

Define the term RECOMBINATION (association)

Answer 15

It describes the bonding of ions with opposite charges in the solution producing neutral molecules.

Question 16

Define the term DISSOCIATION CONSTANT (alpha)

Answer 16

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

Question 17

On what depends the dissociation constant (alpha)?

Answer 17

1. Relative permittivity of the solvent
2. dipole moment of the solute molecules
3. solute concentration
4. Temperatur

Question 18

Define the term ELECTROLYTES (ionic solutions)

Answer 18

Electrolytes are substances with ionic conductance.

Charge carriers are positive and negative ions

Question 19

Define the term CHARGE MOBILITY u

Answer 19

Charge mobility u is the drift (constant) velocity of the charge per unit magnitude of the electric field.

u = V/E

Question 20

Which is the formula for conductivity of electrolytes?

Answer 20

y = (alpha) cze (u+ + u-)

Question 21

Conductivity y depends on?

Answer 21

1. Concentration (directly and through (alpha))
2. Temperature (through (alpha) and u)
3. Type of electrolyte (through (alpha), u and z)

Question 22

Describe the conductivity of biological tissue.

Answer 22

Proteins and lipids have low conductivity.
The ionic solutions have high ionic conductivity.

Eg:
blood/cerebrospinal liquid > high conductivity
bones/dry skin > low conductivity

Question 23

Define the term RHEOGRAPHY

Answer 23

Rheography is a method for investigation of the supply of blood to organs by measuring their conductivity.

Question 24

Conductivity of gases

Answer 24

A pure gas consist of neutral atoms and/or molecules. Gases do not contain free charge carriers and are dielectric.

Question 25

Charge carriers for gases can be produced by?

Answer 25

[ External Ionizing Agent ]

Charge carriers can be produced by ionization using:

• high temperature
• uv light
• x-rays; gamma rays etc.

Question 26

Define the term IONIZATION POTENTIAL

Answer 26

Ionization Potential is the energy transferred from the ionizing agent to a bonded electron to break the bond with the nucleus and free electron.

Question 27

Define the term IMPACT IONIZATION

Answer 27

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

Question 28

Ionization is gases

Answer 28

Charge carriers: electrons, positive ions, negative ions

When the external ionizing agent is constant, the concentrations of ions in the gas stays constant.

Question 29

Define the term GAS DISCHARGE

Answer 29

Gas discharge is the flow of the electric current in a gas.

The current consist of an electronic and two ionic streams.

Question 30

Non-self-sustained charge AC

Answer 30

Current flows only if the external ionizing agent is operating.
AB- ohmic region
BC- saturation region

Question 31

Self-sustained charge CD

Answer 31

Current flows and no external ionizing agent is needed.

CD- avalanche discharge region

Question 32

Define the term OHMIC REGION

Answer 32

Increasing voltage results in a proportional growth of the number of current carriers reaching the electrodes per unit of time.

Question 33

Define the term SATURATION REGION

Answer 33

All ions produced by the ionizing agent per unit of time reach the electrodes and increasing voltage does not result in higher current.

Question 34

Define the term AVALANCHE DISCHARGE REGION

Answer 34

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

Question 35

Air ions

Answer 35

Mainly N2 and O2.
Can be attached to groups of neutral molecules or other particles.
Classified by diameter.

Question 36

The effect of air ions an the human organism

Answer 36

Small negative air ions produce tonic effect (ion therapy).

All other types of air ions are considered harmful!