Electrical Term 1 Flashcards
What is positive power dissipation?
Component that consumes energy eg lightbulb/ areoplane
What are independent sources?
Constant supply of voltage/ current
What are independent sources represented by?
Cirlces
What are dependent sources represented by?
Diamonds
What do the first three/ four bands on a resistor tell you?
Numbers for resistance value - last of these bands is the resistance multiplier
What is the last band on a resistor tell you?
Tolerance - actual value will vary from nominal - due to manufacturing- represents max deviation
How do inductors store energy?
As a magnetic field
Forward biaising
Current flowing
Reverse biasing
Current restricted
Which way does current flow in a resistor?
From triangle base to line
What are transistors constructed from?
Semi conductor materials
What are the three transistor terminals called?
The collector, The base, the emitter
What is a transistor like?
A tap - how much current can flow
What is the saturated mode of a transistor?
When it is operated completely on or off
What is a node in a circuit?
Where two or more branches connect
What is an independent loop?
At least one branch high is not part of another independent loop
What is a mesh?
A loop that doesn’t house loops inside of it
What is the theorem of network typology?
b= l + n - 1
What is an open circuit modelled by?
Very high resistance
What resistance is there in a short circuit?
No resistance
Why might you use multiple resistors rather than one?
less expensive, lower concentration of dissipated heat,
What are planar circuits?
Can be drawn on a flat surface with no branches crossing each other
What is superposition in relation to circuits?
Find voltages/ currents as if the sources act alone - then sum these
In superposition what is a voltage source replaced with?
Internal resistance modelled as a resistor
(or short circuit if ideal)
In superposition what is a current source replaced with?
an open circuit
What are the two types of equivalent circuits?
Thenenin and Norton
What is in Thevenin’s Equivalent Circuit?
an open circuit voltage and a resistor in series modelling equivalent resistance
What is in Norton’s equivalent circuit?
a current source and equivalent resistor in parallel
What is the goal of impedance matching?
to deliver the maximum power from the cell to the load - has nothing to do with efficiency
When is impedance matching generally used?
When the load is a signal rather than power transmission (eg audio engineering)
What is the maximum power transfer theorem?
The maximum power is transferred when the load resistance equals the Thevenin or Norton resistance of the power network
What two parts of time response for an AC circuit are there?
transient response and steady state response
What is an LTI system?
time-invariant, it doesn’t matter when the change is applied the output will be the same
What two important properties do LTI systems satisfy?
Homogeneity and Superposition
What does homogeneity refer to in the context of AC circuits?
if we increase input (eg multiply by a constant) the output will change by the same amount
What does superposition refer to in the context of AC circuits?
output = sum of inputs
If the input to a LTI system is a sinewave the output will be a sinewave with the same…?
frequency
As well as resistance AC circuits have?
Reactance
Reactive elements… energy
store and release
What is impedance of an AC circuit?
combination of resistance and reactance
What is reactance?
A property that opposes a change in current
What phase shift between voltage and current occurs when reactance is present?
90 degree
With capacitive reactance the current waveform /// the voltage by 90 degrees
leads
With inductive reactance the current waveform /// the voltage by 90 degrees
lags
How do you work out impedance?
Z=R+j(Xl-Xc)
For an impedance with both inductance and capacitance at a low frequency what is the impedance like?
Large and leading as the capacitance term (1/cw) dominates
For an impedance with both inductance and capacitance at a high frequency what is the impedance like?
large and lagging (impedance term (Lw) dominates
What is the minimum impedance (with both inductance and capacitance)?
R (resistance) and Lw & 1/Cw cancel) - hence the angle is zero
What is it called when impedance is a minimum (and equal to R)?
Resonance
What is reactive power due to?
Due to the movement of energy within a circuit (between reactive components) - the energy doesnt leave the circuit
What is the name of power dissipated by a resistor?
active/real power
What is reactive power measured in?
VAr (Volt amperes reactive)
The power angle and impedance angle are the same when…
there is a single resistive load - all the components are in series
What is the reverse piezo electric effect?
a voltage is applied to a material which produces a force
What are some applications of the piezo electric effect?
spark ignitors, microphones
What is the difference between a wound field machine and a permanent magnet machine?
The stator has its own coils - you can confíela the strength of the magnetic field
What are some disadvantages of a wound field machine compared to a permanent magnet machiene?
Field has to have its own power supply, losses are associated with the resistance of the field coils
What two things are used to deal with the fact the armature coils are mounted to a rotating structure?
Commutator and brushes
Where does the energy conversion take place in a DC brushed machiene?
In the air gap between the rotor and stator
Why would an air gap in a DC brushed machine need to be small?
So it doesn’t effect the amount of magnetic flux ( air has a very low relative permeability)
Why would an air gap in a DC brushed machiene need to be large enough?
Accommodates manufacturing tolerances (doesn’t cause the rotor scraping and jamming on the Moro out)
What is a positive to cylindrical cells?
robust - able to withstand forces and pressures
What are button cells used for?
low power applications
What are prismatic cells used for?
custom designed for specific applications to protect against mechanical damage
