electric current

define electric current

the directional movement of electric charge

define current

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

What is the unit for current?

ampere, A

define current density

current per unit cross-sectional area of the conductor

what does the threshold for electrical stimulation depend on?

current density

what is required for electric current generation?

• available of free electric charges in the conductor

• a voltage applied between the ends of the conductor

what is the electric field in the conductor created by?

external agents - chemical, mechanical, thermal, radiant

define electromotive force

the work of the external agent for moving a unit positive charge along the conductor

Describe Ohm’s law

the current flowing through a conductor is directly proportional to the applied voltage

What is the conductance and resistance of a conductor determined by?

• length of the conductor

• cross-sectional area of the conductor

• conductor material

Define conductivity

conductance of a conductor with cross-sectional area 1m2 and length 1m

Define resistivity

resistance of a conductor with cross-sectional area 1m2 and length 1m

Define polar molecules and give examples

positive and negative charges are separated in space e.g. water, inorganic salts and acids, acetone

How do polar solvent molecules and polar solute molecules interact?

positively-charged ends of polar solvent molecules are attracted to the negatively-charged ends of polar solute molecules and vice versa

Define dissociation

splitting of neutral molecules into ions

How does dissociation occur between the solute and solvent?

attraction by the solvent molecules breaks the bonds between the poles of the solute

Define solvation

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

How does solvation affect ion diameter?

solvation increases ion diameter

what does the number of solvent molecules surrounding an ion depend on and how?

temperature - thermal motion of solvent molecules becomes more intense at higher temps and no. of molecules surrounding dissolved ions reduces

Define recombination

bonding of ions with opposite charges in solution, producing neutral molecules

What is reached between dissociation and recombination in solution and what happens to ion concentrations?

• dynamic equilibrium is reached

• ion concentrations remain constant

Define dissociation constant

ratio of dissociated molecules to the total number of molecules

What does the dissociation constant depend on?

• relative permittivity of solvent

• dipole moment of solute molecules

• solute concentration

• temperature - dissociation constant increases at higher temps

Define electrolytes

substances with ionic conductance

What does the current in electrolytes consist of?

two ionic streams with opposite directions

Define charge mobility

drift velocity of the charge per unit magnitude of the electric field

What does conductivity depend on? And explain.

• concentration - when conc. is low: direct proportionality

• temp - higher temps lead to higher conductivity

• electrolyte type

Describe the composition of biological tissues

proteins, lipids, electrolytes

Do proteins and lipids have high or low conductivity?

low

Why do the ionic solutions have high ionic conductivity?

large concs of many ions

Does blood and cerebrospinal liquid have high or low conductivity?

high

Do bones and dry skin have high or low conductivity?

low

What happens when electrodes are placed on the skin?

• total electric resistance of body is mainly based on kin resistance

• the current flows along the low-resistance blood vessels and lymph pathways in the body

• electric current crosses skin through sweat gland pores

How does inflammation affect conductivity?

reduced conductivity

How does increased blood supply affect conductivity?

higher conductivity

Define rheography

method for investigation of the supply of blood to organ by measuring conductivity

What is a pure gas made of?

neutral atoms or molecules

Do gases have free charge carriers?

no

Are gases dielectrics?

yes

How can you produce charge carriers in gases?

ionisation using:

• high temps

• UV light

• X-rays, gamma rays

Define ionisation potential

energy transferred from ionising agent to a bonded electron to break the bond with the nucleus and free the nucleus

Define impact ionisation

neutral atoms can be ionised on collision with fast moving charged particles

When does impact ionisation take place?

when the voltage accelerating the ions exceeds a threshold value

State the three charge carriers in gases

• electrons ejected from atoms on ionisation

• positive ions produced by ionisation

• negative ions obtained by the attachment of electrons to neutral atoms

Define recombination

bonding of ions with opposite charges to produce neutral atoms or molecules

When do the ion concentrations stay constant?

when the external ionising agent is constant

Define gas discharge

the flow of electric current in a gas 

Define non-self-sustained discharge

current flows only if the external ionising agent is operating

Define self-sustained discharge

current flows and no external ionising agent is needed

Define ohmic region

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

Define saturation region

all ions produced by the ionising agent per unit of time reach the electrodes and increasing voltage doesn’t result on higher current

Define avalanche discharge region

impact ionisation 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

Define air ions and give a characteristic

mainly ionised nitrogen and oxygen and can be attached to groups of neutral molecules or other particles

Give the four classifications of air ions

small, intermediate, large, and ultralarge

Describe the positive effect of air ions on the human organism

small negative air ions produce tonic effect

Describe the negative effect of air ions on the human organism

• all other types of air ions are harmful

• large ions in atmosphere are air pollutants