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electrical fundamentals > electrical fundamentals book 1 > Flashcards

electrical fundamentals book 1 Flashcards

1
Q

how many naturally occurring elements are there?

A

92

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2
Q

what is an element?

A

an atom or a group of atoms of the same type

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3
Q

what is an atom?

A

basic unit of matter, made up of protons, neutrons and electrons

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4
Q

what are the different particles that make up an atom?

A

protons, neutrons and electrons

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5
Q

what would cause an atom to become positively charged?

A

losing one or more electrons from the outer (valence) shell through ionic bonding would cause a positive charge. these atoms are called cations

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6
Q

what would cause an atom to become negatively charged?

A

gaining one or more electrons to fill the valence shell through ionic bonding would cause a negative charge. these atoms are called anions

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7
Q

what is meant by the term ion?

A

an atom or molecule with an overall positive or negative electric charge due to the gaining or losing of one or more electrons through ionic bonding

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8
Q

through what type of material does electric current flow most easily?

A

electric current flows most easily through conductors

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9
Q

what is meant by the term valency electrons?

A

electrons that occupy the outer (valence) shell of an atom

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10
Q

what is the difference between electron and conventional current flow?

A

conventional current flow is from positive terminal to negative terminal. electron current flow is from negative terminal to positive terminal

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11
Q

how are current and charge related?

A

current is the rate of flow of charge. a current of 1A means that 1 coulomb of charge flows past a point in a circuit every second

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12
Q

what is a coulomb?

A

a quantity of electricity. defined as 6.28 x 10 to the 18 electrons per second. symbol ‘C’

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13
Q

what is potential difference and in what units is it measured?

A

potential difference is the difference between charge values, which exists at the atomic level in materials with free electrons. potential difference is measured in volts

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14
Q

what is electro motive force and in what units is it measured?

A

EMF is the ability to cause current to flow in a complete circuit. EMF is measured in volts

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15
Q

what is voltage?

A

the electrical force that moves electrons through a conductor. OR the value or quantity of the potential difference or EMF. symbol ‘V’

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16
Q

what is current?

A

current is the flow of either charge or electrons through a conductor. symbol ‘I’

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17
Q

what is the force that drives current?

A

EMF (electro motive force)

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18
Q

what is resistance?

A

resistance is the measure of a materials ability to oppose current flow when an EMF is applied to it

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19
Q

what factors govern a materials resistance and how is it calculated?

A

three factors; material length, in metres; cross section area, in metres squared; and resistivity of material, in ohm metres. this is calculated by the formula: (resistivity x length)/area

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20
Q

what is conductance?

A

a material with high resistance will have low conductance and conversely, a material with low resistance will have high conductance. the unit of conductance is the Siemen, symbol (G)

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21
Q

what are the 6 ways of generating electricity?

A

chemical action; friction; pressure; light; heat; and magnetism

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22
Q

how is electricity generated through chemical action?

A

(cell) 2 dissimilar metals placed in a liquid called an electrolyte. this is a cell and by grouping cells we produce batteries

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23
Q

how is electricity generated through friction?

A

(static electricity) when two materials rub together and the electrons from one are passed to the other, creating a static (stationary) electric charge

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24
Q

how is electricity generated through pressure?

A

(crystal controlled oscillators) certain crystalline substances, i.e quartz or rochelle salt, exhibit a piezoelectric effect which results in potential difference appearing between opposite faces of the crystal when it is mechanically deformed

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25
Q

how is electricity generated through light?

A

(photovoltaic cells) these devices utilise the energy from light source to generate electricity

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26
Q

how is electricity generated through heat?

A

(thermocouples) dissimilar metals heat up and create a complete circuit, through which current flows. this effect is called the Seebeck effect. the net EMF or the resulting current is used to measure temperature

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27
Q

name the 3 main sources of stable DC current

A

batteries; thermocouples; and photocells

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28
Q

what are the two basic types of battery

A

primary cell; and secondary cell

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29
Q

what is the difference between primary and secondary cell batteries?

A

secondary cell batteries have the ability to be recharged

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30
Q

what is the basic construction of a primary dry cell?

A

wax or plastic seal; carbon rod (positive terminal); zinc container (negative terminal); depolariser (manganese dioxide); and ammonium chloride electrolyte (salammoniac)

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31
Q

why are certain primary cells referred to as being dry?

A

to prevent leaking, the electrolyte is mixed with wheat flour to create a thick paste

32
Q

what are the two types of secondary cells?

A

lead acid; and alkaline

33
Q

what is the basic construction of a lead acid battery?

A

resilient plastic container; positive and negative internal plates made of lead; plate separators made of porous synthetic material; electrolyte (sulphuric acid and water solution); and lead terminals for connection

34
Q

regarding the number of positive plates inside a lead acid cell, how many more negative plates will there be?

A

1 more negative plate per cell, interlocking

35
Q

what is the specific gravity of the electrolyte in a fully charged lead acid battery?

A

1.270

36
Q

in what order are sulphuric acid and water always added together to create battery acid?

A

acid is always added to water. the reverse is extremely dangerous

37
Q

what is the voltage of a lead acid battery; immediately after charge; an hour after charge; during discharge; and at fully discharged?

A

immediately after charge, 2.5 volts; an hour after charge, 2.2 volts; during discharge, 2 volts; and at fully discharged, 1.8 volts

38
Q

what is capacity in terms of batteries?

A

the quantity of electricity that can be taken from a fully charged cell at a specified discharge rate measured in amps, before the cells nominal voltage of 2 volts drops to a defined love of 1.8 volts

39
Q

what happens to the positive plate (cathode), negative plate (anode) and electrolyte of a lead acid battery during the process of producing a voltage (discharging)?

A

positive plate PbO2 (lead dioxide) - PbSO4 (lead sulphate); negative plate Pb (lead) - PbSO4 (lead sulphate); electrolyte H2SO4 (strong battery acid) - H2SO4 (weak battery acid)

40
Q

how can you determine the amount of charge in a lead acid battery?

A

the specific gravity of the electrolyte can be measured using a hydrometer because of the linear increase/decrease in value

41
Q

what electrolyte is used in alkaline batteries?

A

potassium hydroxide (specific gravity of 1.24-1.3)

42
Q

what is meant by semi-sealed with reference to alkaline batteries?

A

these cells can be charged at a high rate, which causes the electrolyte to bubble and heat up which causes thermal meltdown if not dealt with. they are arranged in steel containers and fitted with safety valves to prevent this

43
Q

what is meant by semi-open with reference to alkaline batteries?

A

similar to semi-sealed but allowed to deliberately gas to atmosphere to aid temperature control. recharge time is shorter than semi-sealed but requires “topping-up” of electrolyte more often

44
Q

what is the most common type of alkaline battery used on aircraft?

A

nickel/cadmium

45
Q

what happens to the positive plate (cathode), negative plate (anode) and electrolyte of a nickel/cadmium battery during the process of producing a voltage (discharging)?

A

positive plate Ni2O2, Ni3 and O3 (nickel oxides) - Ni(OH)2 (nick hydroxide); negative plate Cd (cadmium) - Cd(OH)2 (cadmium hydroxide); electrolyte KOH (potassium hydroxide) - KOH (potassium hydroxide)
note: no change to electrolyte

46
Q

how do you ascertain the charge state of an alkaline battery?

A

temperature of the cell can be monitored to determine charge state

47
Q

how are cells arranged to obtain high voltage output?

A

in series. in this arrangement, voltage from each cell is added together. e.g. 6 x 2 volt cells will provide an EMF of 12 volts. disadvantage is the internal resistance of each cell will also be added, creating high resistance

48
Q

how are cells arranged to obtain improved current capacity?

A

in parallel. in this arrangement, current from each cell is added together. internal resistance greatly reduced

49
Q

what are the advantages of arranging cells in a battery in a series/parallel configuration?

A

provides high voltage; high current; and lower internal resistance. this is the most common way of connecting battery cells

50
Q

what is the principle of operation of a thermocouple?

A

2 wires of dissimilar metals joined near the measurement point. the output is a small voltage measured between the two wires known as thermoelectricity

51
Q

what are the advantages of thermocouples?

A

verry rugged; immune to shock and vibration; useful over a wide range of temperatures; simple to manufacture; require no excitation power; no self heating; and can be made very small

52
Q

what are the disadvantages of thermocouples?

A

produces a low output signal which is non-linear; requires sensitive and stable measuring device that is able to reference junction compensation and linearisation; low signal level requires high level of care when installing to prevent potential noise sources

53
Q

name the four ways thermocouples are constructed

A

sealed and isolated from sheath; sealed and grounded to sheath; exposed bead; and exposed fast response

54
Q

list the four types of thermocouple in order of ascending (longest first) response time

A

sealed and isolated from sheath (75 seconds); sealed and grounded to sheath (40 seconds); exposed bead 15 seconds); and exposed fast response (2 seconds, can be reduced to 10-100ms with fine gauge wire)

55
Q

outline the basic structure of a photovoltaic (solar) cell

A

top: encapsulate; contact grid; antireflective (AR) coating; n-type silicon; p-type silicon: bottom

56
Q

what is the material used in the construction of a photovoltaic cell?

A

silicon

57
Q

what is ohms law?

A

the volt is the potential difference between 2 points in a circuit having resistance (R) of 1 ohm between them and carrying a current of 1 ampere, providing temperature remains constant

basically V=IR

58
Q

how do we measure voltage?

A

using a voltmeter, measured in volts

59
Q

how do we measure resistance?

A

using an ohmmeter, measured in ohms

60
Q

how do we measure current?

A

using an ammeter, measured in amps

61
Q

what are two common effects of current flow?

A

heat is generated. the higher the current, the hotter it gets; electromagnetism. a small magnetic field is created, the higher the current, the stronger the magnetic field

62
Q

how do you calculate total resistance (RT) in a series circuit?

A

RT = R1 + R2 + R3

63
Q

how do you calculate resistance in a parallel circuit?

A

1/RT = 1/R1 + 1/R2 + 1/R3

64
Q

what is kirchoffs voltage law?

A

the voltage drops in a series circuit will always equal the voltage applied

65
Q

what is kirchoffs current law?

A

the algebraic sum of the currents at a junction will always be zero

66
Q

what happens to a resistors value if you double the cross sectional area?

A

resistance = (resistivity x length)/cross sectional area

therefore, would reduce in value

67
Q

what is the difference between a positive and a negative temperature coefficient?

A

a positive temperature coefficient will indicate an increase in resistance with an increase in temperature. a negative temperature coefficient will indicate a decrease in resistance with an increase in temperature

68
Q

what is specific resistance?

A

the resistance offered by a cube of material at 0 degrees celsius

69
Q

what do the different bands on resistors identify?

A

4 band (1st digit, 2nd digit, multiplier, tolerance), 5 band (1st digit, 2nd digit, 3rd digit, multiplier, tolerance) and 6 band (1st digit, 2nd digit, 3rd digit, multiplier, tolerance and temp coefficient)

70
Q

why are there preferred values of resistors?

A

to reduce manufacturing costs. preferred values can be combined to create an equivalent value network.

71
Q

how does a wheatstone bridge calculate an unknown resistance?

A

the voltmeter will read 0 when both arms of the wheatstone bridge are balanced, i.e. the resistance of both arms is equal, causing 0 pd between the arms

72
Q

what types of fixed resistor are there?

A

3 types. wire wound; composition; and deposited film

73
Q

what are the properties of a wire wound resistor?

A

low ohmic (resistance) value and high wattage rating. their size depends on the heat to be dissipated and therefore the power loading. they are normally used for the control of large currents due to their excellent heat dissipation abilities

74
Q

what are the properties of a composition resistor?

A

cover the range <10 ohms to >20 mega ohms. low power dissipation and so are used in low current circuits

75
Q

what are the properties of a deposited film resistor?

A

cover the range 10 ohms to 1 mega ohm. power range of 0.125 watts to 1 watt

electrical fundamentals (8 decks)

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