Module 3 Flashcards
What is potential difference
A potential difference always arises between two differently charged points. An electrical voltage occurs, as soon as there is a deficiency of electrons at one point and an excess of electrons in another point.
What device is used to separate charges
A generator
What are current transformers
Current transformers (CTs) are special transformers which allow the measurement of AC currents without breaking the current-carrying leads.
What is the charge of a single electron
0.1602x10 to the -18
A charged atom is said to be what
Ionised
What is a molecule
A molecule is an electrically neutral group of two or more atoms held together by chemical bond
What is a cation
A positively charged ion
What is an anion
A negatively charged ion
How are molecules held together
Covalent bonds
How are ionic compounds held together
Ionic bonds
How are intermetallic compounds held together
Metallic bonds
Generally how many electrons do good conductors have in the outer valence orbit
4
What are insulators also known as
Non conductors
During static electricity what are free electrons
Electrons in the outer layer that can be handed to the other material
What is charging by induction
A charged rod is placed near a metal bar it steals the charge
What is Coulomb’s law
Opposites attract and likes repel
What temperature must cables not exceed
60°c
Is pure water a conductor
No
Tap water is due to the amount of impurities
What is electrolysis
The decomposition of chemical compounds by passing an electrical current through a liquid
What is the conducting substance called in the electrolysis process
Electrolyte
What can be done with an oscillograph
Processes in electrical engineering can be recorded
What can be done with an oscilloscope
Processes can be visualised but not recorded
Why is the electron beam oscilloscope used today
Easy to operate, versatile and shock resistant
What is a potential difference
A potential difference arises between two differently charged points
How is voltage generated
By separating positive and negative charges
What is Electromotive force (EMF)
Also known as voltage
The difference in energy between electrical charges or potentials
What is a voltage called if it has the same polarity over time
DC
What are the two different ways of indicating measured voltage
- A pointer deflection (analogue meter)
- Numerical read out (digital meter)
What is electrical current
The flow of electrical charges in a certain direction
When measuring current with an ammeter which current must you connect positive to positive and negative to negative
DC current
AC current doesn’t matter
What is breaking action
Breaking action of a conductive material is dependant on the atomic density of the material. The greater the spacing of the ions the smaller the breaking action becomes
What’s is charge (Q) unit
Coulomb
What is conventional current flow
Current flows from the positive to negative poles
What is different in current flow in metals apposed to other materials
The electrons move because of the construction of the material and this is called electron flow from negative to positive
How is electricity produced by light
When light is shone on certain semiconductors such as silicon or germanium charge separation happens
What are photocells
Generators of electrical voltage through light
What is electron emission
The separation of electrons from a bonding metal
What is the process called for emitting electrons by heat
Thermonic emission
Voltage generation by heat is used for what?
Measuring EGT for an aircraft turbine engine
How is a voltage generated with a Piezo Chrystal
It is deformed by using pressure and is called the piezoelectric effect
What is used in dynamometers
The piezoelectric effect
What is the most important form of voltage generation in engineering
Induction
What is the operating principle in all generators
Induction
What is “capacity” of a DC primary cell
The quantity of electricity (charge in Q in ampere hours)
Can primary cells be recharged or reused
No
Can secondary cells be charged or reused
Yes, they are used in aircraft, cars, laptops
What are batteries capacity rated in
Ampere hours (Ah)
What voltage do lead acid cells have when fully charged
2v and 1.83 final discharge voltage
What is the voltage of Nickel Cadmium (NiCad) cells when fully charged
1.2v and fall to 1v when discharged
When is the electrolyte level in a Nickel cadmium battery at its lowest
After discharge
What is the combination of cells called
A battery
In a parallel circuit how do you find out the total resistance
It’s lower than the lowest individual resistance
When does a batteries IR become greater
As the battery discharges
A batteries IR is equal to what?
The difference between the open circuit voltage (OCV) and the closed circuit voltage (CCV)
What materials make up thermocouples
Chromel and alumel
What temp range do thermocouples normally measure
-200 to 2300°c
What are thermocouples normally used for
To measure EGT of jet engines
What is OHMs law
The relationship between Current, Voltage and Resistance
What is Kirchhoffs first law
At any point in an electrical circuit, the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point
What are the most commonly used resistors
Fixed value resistors
What is the most common axial lead resistor
The carbon film
What is the carbon film resistor used for
Applications where a very good tolerance and temperature coefficient are not necessary
If you have a resistor with a wire 3 times longer than normal will the resistance be higher or lower or the same
3 times higher
What is resistance measured in
Ohms
When would a potentiometer be used
Where there needs to be a variation of voltage
When would a rheostat be used
In circuits where a variation in current is required
What are the two types of mechanically varied resistors that are in common use
Potentiometer and Rheostat
What very sensitive meter is used in a Wheatstone bridge
A galvanometer
What are voltage dependant resistors
The resistance value changes according to the applied voltage
What is a Wheatstone bridge
A type of electrical scale
What does a Wheatstone bridge consist of
It compares the resistance of two resistors and consists of two series connectors in parallel
What very sensitive meter is used inside a Wheatstone bridge
A galvanometer
Where are preset rheostats commonly mounted
Printed circuit boards and are normally preset at manufacturing
What is power
Power is the rate at which work is performed
What is the standard unit of electrical power
The Watt
What is the measure of charge
The coulomb
What is charge denoted by
Q
What is current measured in
Ampere
What is current denoted by
I
What is Hertz
For AC power, the amount of times per second the frequency goes back and forth
What is potential difference measured in
Volts
What is resistance measured in
Ohms
What is power measured in
Watts
What is energy measured in
Joules
What type of energy is created when current flows through a resistor
Heat
What do capacitors normally consist of
Two metal plates or foils that are insulated from each other
The charge that can be held by a capacitor depends on what
The plate area, distance between plates and the applied voltage
What is the unit of capacitance
The Farad
What increases the electric field inside a capacitor
The dialectic used
How does a capacitor hold a charge
A charge is applied between two plates which creates an electric field. The amount of charge it holds is directly proportional to the voltage applied
What capacitors can be used with AC and DC circuits
Paper or film capacitors
Where would old variable capacitors be used
Old fashioned radios
When adding a new uncharged capacitor into a circuit what would be used to prevent the current being too high
A resistor will be used
What are capacitance time constants
The higher the capacitance the longer the charging time will be. The higher the resistance the longer the charging time will be
When a capacitor charges how much does it get charged by in its first cycle
63% and will keep being topped up by 63% until by the 5th time it will be fully charged
How would you visually inspect a capacitor
Looking for bulging of the capacitor and electrolyte leakage
Up to 40% of defective capacitors suffer from what
Excessive leakage current
What defect do 25% of capacitors suffer from
Dielectric absorption
What are the lines of magnetic force called produced by a magnet
Flux
What materials are magnets normally made of
Steel or alloys of steel
All magnets have areas where their magnetism is particularly high. What is this area called
The poles
In the middle of a magnet there is an area with no attractive force, what is it called
The neutral zone
How can a metal be demagnetised
Heating above the curie temperature
Mechanical force
What is degaussing
Slowly moved away from a strong alternating magnetic field
What is a disadvantage of demagnetisation by heating beyond the curie temperature
If the temp falls below magmatism can re occur
What is magnetomotive force denoted by
The Greek letter theta
How can the magnetic field of a current carrying coil be multiplied
By inserting an iron core into the cavity
What are magnetically hard materials
Materials that are hard to magnetise or demagnetise
What is “remanence” when talking about magnetic items
When a material is removed from a coil it is still slightly magnetic and that is called remanence
What is retentivity
The ability of a material to be abe to retain residual magnetic field
What is the main purpose of a filter
They suppress certain frequencies out of a certain frequency composition and let certain other frequencies pass undamped
What’s special about CR filters
They are commonly used because of their compact design and lower price
Are LR filters used often or not often
They are rarely used
What is the series connection of a high pass filter and a low pass filter
A band pass filter
What is the parallel connection of a high pass filter and a low pass filter
Band stop filter
