AC Flashcards
Formula for frequency to period
F = 1/T, vice versa
One whole period
What’s phase shift measured in
Degrees, max of 180 (totally out of phase)
Calculating average of AC:
What values do you need
Vpk (voltage peak): Max voltage
Vpp (voltage peak to peak): Max positive voltage to max negative voltage
Vrms (voltage route mean square): average
Formula for calculating Vrms
Whats it roughly equal to
Vrms = Vpk / √2
Vpk x 0.707
Formula for calculating wavelength
What rough values do you need to know
What short cut can you take with these
Wavelength = Speed of Light/Frequency
Speed of light = 300,000,000 metres per second
Use speed as 300 and frequency in MHz
What is reactance
What is the phase shift in an inductor
What’s the ratio of Vrms and Irms known as
Opposition to current flow from a purely inductive (XL) or capacitive (XC) circuit
90 degrees, voltage leads current (since current is resisted initially)
Inductive reactance (opposition to change in current)
What’s the graph look like for current/reactance vs frequency in an inductor?
What’s the formula for reactance
What do each part mean
As the frequency goes up, current decreases (as less time for inductor to start to let current through)
Reactance goes up (as current decreases)
XL = 2πfL
XL = Inductance Reactance in Ohms
f = frequency in Hertz
L = Inductance in Henries
How to use Ohms law in an AC circuit
What can this be used for
Important: what can they catch you out with in exam
Use Vrms and Irms instead
Calculating reactance of an inductor/capacitor
Needs RMS values - remember to multiply by 0.707 if not already
What is the phase shift in an capacitor
What’s the ratio of Vrms and Irms known as
90 degrees, current leads voltage (since when the voltage starts to decrease, the current immediately starts leaving the cap even though the voltage is still positive)
Capacitive reactance (opposition to change in voltage)
Voltage changes slower because of the charge leaving the cap and propping it up
What’s the graph look like for current/reactance vs frequency in an capacitor?
What’s the formula for reactance
What do each part mean
As the frequency goes up, the current increases (since the capacitor spends less time being full)
Reactance decreases
XC = 1 / (2πfC)
XL = Inductance Capacitance in Ohms
f = frequency in Hertz
L = Capacitance in Farads
What happens when you have resistance and reactance in the same circuit
What’s the symbol for this
You get impedance
Opposition to both energy storage and transfer within the circuit
Symbol is Z
What’s the resonant frequency of an LC circuit?
The rate at which the inductors and capacitors discharge
As frequency increases, capacitor reactance decreases and inductor reactance increases
The point at which they’re equal is the frequency of the circuit
What happens if you put an inductor and a capacitor in:
Parallel
Series
High impedance at resonant frequency, low impedance elsewhere
“Traps” a given frequency
Low impedance at resonant frequency, high impedance elsewhere
What is the quality factor of a circuit
What are the three factors which can tell us this
How do we calculate the two main ones using a graph
Ratio of bandwidth (range of frequencies the circuit will take)
No unit, just the higher the better
FC = centre frequency
FL = lower frequency
FU = upper frequency
Upper and lower are equal to where the signal strength is 50% (-3dB, x 0.707)
How to construct a low pass filter
How to construct a high pass filter
Inductor to Vout, caps to ground
At high frequencies the cap shorts to ground and inductors has high reactance
Cap to Vout, inductors to ground
At low frequencies the cap has high reactance and the inductor shorts
