BASIC PRINCIPLES Flashcards
(39 cards)
What are Free Electrons ?
The electrons have a negative charge and the nucleus has an equal number of positive charges
making the atom electrically neutral. The negative electron is held in its orbit by its attraction
to the positive nucleus. Electrons in outer orbits are not so strongly attracted to the positive
nucleus and may easily fly off and attach themselves to a neighbouring atom in the material.
These are called free electrons.
Explain the origin of electricity
An atom that has lost an electron becomes more positive and is called a positive ion, an atom
that has gained an electron becomes more negative and is called a negative ion. If the free
electrons can be made to move in a particular direction through the material an electric current
has been created.
-Electrons are caused to move along a piece of wire by applying a positive charge from some
source at one end and a negative charge at the other. The positive charge attracts the free
electrons and the negative charge repels them so there is a flow of electrons in one direction
through the wire from the negative terminal to the positive terminal.
There are six basic means to provide the force which causes electrons to flow:
Friction - static electricity
Chemical Action - cells and batteries (primary and secondary cells)
Magnetism - generators and alternators
Heat - thermocouples (junction of two dissimilar metals)
Light - photo electric cell
Pressure - piezo electric crystals
Define Voltage, Current and Resistance ?
-EMF is measured in units of Voltage. The number of volts is a measure of the EMF or Potential
Difference (the difference in electrical potential between the positive and negative terminal).
-The current (symbol I) in a conductor is the number of electrons passing any point in the
conductor in one second and is measured in amperes or amps (symbol A)
-The obstruction in the circuit which opposes the current flow is called resistance.
FACTORS AFFECTING THE RESISTANCE
- Type of material. eg. Silver is a better conductor than Copper
- Length. The longer the wire the greater the resistance
- Cross sectional area. The thicker the wire the smaller the resistance
- Temperature. The symbol for temperature coefficient is a (alpha.. If resistance increases with an increase of temperature, the resistor is said to have a Positive Temperature Coefficient (PTC.) If resistance decreases with an increase of temperature, the resistor is said to have a Negative Temperature Coefficient (NTC.) Resistors having these characteristics are used in aircraft systems for temperature measurement.
OHMS LAW
If the Voltage remains constant any increase in resistance will cause a decrease in current and vice-versa
(Current inversely proportional to resistance).
If the resistance remains the same any increase in voltage will cause an increase in current and
vice- versa (Current directly proportional to voltage).
–V = IR
POWER
In an electric circuit Work is done by the Voltage causing the current to flow through a resistance,
creating heat, magnetism or chemical action.
The rate at which work is done is called Power and is measured in Watts
Watts (W) = Voltage (V) x Amperes (I)
Three formulae for calculating power can be derived from the two basic formulae V=IR and
W=VxI
- Voltage unknown W =
-Resistance unknown W =
-Current unknown W =
Three formulae for calculating power can be derived from the two basic formulae V=IR and
W=VxI
- Voltage unknown W = I 2/ R
-Resistance unknown W = V x I
-Current unknown W = V2/R
What is the function of a circuit breaker ?
Each electrical circuit in an aircraft will be protected by a fuse or circuit breaker which will
prevent the maximum power rating of a component to be exceeded by breaking the circuit if
the current increases.
Differentiate between Series and Parallel connections ?
-Series connection reduces current flow and therefore power consumption, but
can be impractical because individual loads (resistances) cannot be individually
controlled. Also the failure of one resistance would mean failure of the rest of
the circuit.
-Parallel connection ensures each resistor is individually controllable and
receives the same voltage. Failure of one resistor will not affect the others. Most
aircraft loads are connected in parallel.
KIRCHOFF’S LAWS
First law
The total current flow into a point on a circuit is equal to the current flow out of that point
Second law
If all the voltage drops in a closed circuit are added together, their sum always equals the voltage applied to that closed circuit.
All effects of electricity take place because of the existence of a tiny particle called the:
a. electric.
b. proton.
c. neutron.
d. electron.
D
The nucleus of an atom is:
a. positively charged.
b. negatively charged.
c. statically charged.
d. of zero potential.
A
An atom is electrically balanced when:
a. its protons and electrons balance each other.
b. the protons outnumber the electrons.
c. the electrons outnumber the protons.
d. the electric and static charges are balanced.
A
The electrons of an atom are:
a. positively charged.
b. neutral.
c. negatively charged.
d. of zero potential.
C
A material with a deficiency of electrons becomes:
a. positively charged.
b. negatively charged.
c. isolated.
d. overheated.
A
A material with a surplus of electrons becomes:
a. positively charged.
b. negatively charged.
c. over charged.
d. saturated.
B
Heat produces an electric charge when:
a. like poles are joined.
b. a hard and soft glass is heated.
c. the junction of two unlike metals is heated.
d. hard and soft material are rubbed together.
C
Friction causes:
a. mobile electricity.
b. basic electricity.
c. static electricity.
d. wild electricity.
C
Chemical action produces electricity in:
a. a light meter.
b. a generator.
c. a primary cell.
d. starter generator.
C
A photo electric cell produces electricity when:
a. two metals are heated.
b. exposed to a light source.
c. a light source is removed.
d. exposed to the heat of the sun.
B
The difference in electric potential is measured in:
a. KVAR’s
b. watts
c. amps
d. volts
D
The units of electrical power is measured in:
a. watts
b. amperes
c. ohms
d. volts
A
Materials containing ‘free electrons’ are called:
a. insulators
b. resistors
c. collectors
d. conductors
D
