L Theory Flashcards
What is current?
Current is the flow of electrons past a point in one second
1Amp = 6.241x10^18 electrons per second
What are the 6 methods of producing a voltage?
- chemical
- friction
- induction
- heat
- light
- pressure
For potential difference to exist between two points a circuit must have a ?
A difference in charge.
what is potential difference measured in?
Volts(V)
what force is used to push voltage around a circuit?
Electromotive Force
To produce a voltage chemically, what must the plates be made of?
Dissimilar metals.
what are the 3 requirements for inducing a voltage?
- a conductor
- a magnetic field
- relative movement
what determines the polarity of a induced voltage?
The direction of movement between the conductor and magnetic field.
what 2 things determines the voltage produced by a thermocouple?
1.types of materials used
2.diffrence in temp at both ends
what is the name of the materials used in solar panels?
Semiconductors
what materials produce voltage when pressure is applied?
piezo quartz crystal
what is the circuit symbol for current?
I for intensity
what is current measured in?
Amps(A) or Ampere
what is the outer orbit of a atom called?
Valance shell.
what is the outer electron called?
Valance electron
what is the polarity of a electron?
negative
what is the name of the electron that is released form a atom?
free electron.
what is the name of a atom that that has released a electron?
Positive ion
what is stated in ohms law?
That a current flowing through a conductor, is directly proportional to the applied voltage across it, providing that the temperature remains constant, and is inversely proportional to the resistance.
what is the formula for ohms law?
Voltage(v)=Current(I)×Resistance(R)
what is a series circuit and its main down fall?
A circuit where two or more components are connected end to end.
downfall: one component fails the circuit is broken
how do you find the resistance of a series circuit?
add all of the resistances up
Rt = R1+R2+R3
don’t forget your units ohms(Ω)
what happens to current and voltage in a series circuit
currant stays constant and voltage changes
what is a parallel circuit?
all of the components are connected side by side said to be parallel to each other
what is the formula for total resistance in a parallel circuit.
1/Rt=1/r1+1/r2+1/r3 + …
what happens to voltage and currant in a parallel circuit?
the voltage is the same in every component but the currant is different for every component.
what is a series parallel circuit?
a circuit with components in series and parallel
what part of a series parallel circuit do we calculate first?
we start with the parallel section of the circuit so that we can then calculate the rest as a series circuit.
how are ammeters connected in a circuit?
a ammeter must be connected in series.
how are voltmeters connected in a circuit?
a voltmeter must be connected in parallel.
what is this symbol (µ) when it comes to units?
micro 1/1000000 or 0.000001
what is this symbol (m) when it comes to units?
milli 1/1000 or 0.001
what is this symbol (k) when it comes to units?
kilo 1,000
what is this symbol (M) when it comes to units?
Mega 1,000,000
what is this symbol (G) when it comes to units?
Giga 1,000,000,00
what is the formula for mechanical power?
power = (force x distance)/time
electrical power depends on what two factors?
- number of electrons being moved (current)
- the rate at which they are made to travel (voltage)
what is the formula for electrical power?
power = current x voltage
what is the unit for power?
watts(W)
how many watts are in a 1 horse power?
746 watts
what are the two types of resistance values for equipment like lamps and toasters?
cold resistance( no voltage applied)
hot resistance (voltage applied)
can be measured using GPTME fluke or MIT instrument set on the ohms scale
what does passing a current through a conductor produce?
heat and a magnetic field
why are fuses important?
to protect the supply as if the equipment is drawing more then the fuse rating it already has a issue.
what are the two different types of fuses
- cartridge(normal domestic fuse)
2.high rupturing capacity(quartz powder)
what is the potential difference of the sea?
0 volts.
what is the resistance of a human?
1000 ohms
why do we earth all equipment and what is the highest allowed resistance of a earth point?
earthing is there to protect us from electric shock as it is the easiest route at 0.1ohms.
what are the 3 main causes of earth faults?
- Dampness
- hard or brittle insulation
- accidental damage
what will two earths do in a circuit?
by pass components and lower the resistance and increase the currant leading to possible damage.
how is a magnetic force induced?
by passing a current through a conductor
what are temporary magnets
magnets that have there forces induced by a current or another magnet
what do poles of the same type do ?
they repel each other
what can be added to steel to produce a stronger magnet
nickel
cobalt
aluminium
what is the right hand grip rule?
thumb points in direction of current flow
fingers wrap round the conductor to show the magnetic field.
what is the normal supply on a boat?
3 phase 450v 60hertz
how many times does a sine wave reach peak?
twice once positive and once negative in one cycle
what does a coil of wire make?
a basic inductor.
what is self inductance?
when AC flows through a coil an alternating magnetic field is generated around the coil creating a EMF
what is mutual inductance?
when EMF is generated in a second coil
why do we use a soft iron core in transformers?
to allow the magnetic flux to reach the secondary coil as soft iron is better then air at transferring magnetic flux.
what is the purpose of a transformer?
to step voltage up or down depending on its windings.
what are the the two types of transformer?
- core type
- shell type
what will happen to the voltage when the secondary coil has twice as many windings as the primary coil?
the voltage will step up to double the input voltage in the primary coil.
do transformers work with DC?
no because DC only creates a change in magnetic field when it is turned off or on.
why do we use a laminated core rather then just a block of soft iron in transformers?
as the resistance of a block is low allowing for more Eddie currants which would counteract the magnetic flux from the induced coil.
How are input cables ladled for 3 phase?
red L1
yellow L2
blue L3
how are output cables ladled for 3 phase?
red A
yellow B
blue C
what are the six parts of a brushless AC generator?
prime mover
high frequency generator
automatic voltage regulator
AC exciter generator
rectifier assembly
main AC generator
what does generated voltage depend on?
strength of the magnetic field and the amount of the field that cut the conductors
what is the most common generator?
AC brushless generator
what does AVR stand for and why is it important?
Automatic Voltage Regulator
it is used to power the exciter stators with DC and senses the power output from the AC generator stators
why do we need the exciter parts of a AC brushless generator?
so that we can provide the generator rotor with AC for the rectifier assembly
what are the advantages of AC motors?
- less maintenance
- smaller for same power as DC
what is the most common AC motor
AC induction motor (squirrel cage induction motor)
what are the two main components of a motor?
stator and rotor
what is the polarity of a rotor compared to the stator?
opposite so that its magnetic field is attracted to the stators magnetic field causing the rotor to turn.
what two things determine the torque of a DC motor.
1.magnitude of the armature current.
2.strength of the magnetic field of the field windings
how do you remember the left hand rule.
First Finger = magnetic Field
seCond finger = direction of Current
thuMb = Movement
what is back EMF?
EMF induced that opposes the main EMF.
what are the 3 categories of DC motor?
series
shunt (parallel)
compound (mix of series and parallel
what are the characteristics of a series motor
good starting torque
poor speed control
slow with heavy loads rapid with light loads
always need a load or will be damaged
field winding in series with armature
what are the characteristics of a shunt motor
field windings in parallel with armature
many turns of thin gauge wire
poor starting torque
good regulation under load
torque develops with armature current
constant speed
what are the characteristics of a compound motor?
best of both motors
good torque Characteristics
good speed regulation
what is a geometrical neutral axis?
neutral axis between field windings
what is a magnetic neutral axis?
a axis that moves with the distortion of the field winding magnetic field.
what are interpoles used for?
they are opposite in polarity to the next pole in the rotation to offset varying armature current and assist rotation.
What holds the electrons in the atom?
Force of Attraction
why do we use centre tapped transformers?
to reduce the potential electric shock by half if the cable becomes damaged. 115v to 57.5V
how can AC induction motors be wound
Star
Delta
what is the name of the natural magnetic iron ore?
Magnetite
what are the 2 most common permanent magnets?
- bar
- horseshoe
speed at which AC changes direction is called?
Frequency
the level to which the voltage or current rises to is called ?
Amplitude
how are AC cycles per second expressed?
Hertz
what is the unit for inductance?
Henry’s
what is the frequency equation?
time (seconds) = 1/ frequency
what are the two type of transformers?
1 core
2 shell
what are the new colours for 3 phase
Brown L1 A
Black L2 B
Grey L3 C
