Alternating Currents Flashcards
Angular frequency, w = _____.
2pi f
Root mean square (r.m.s) of an alternating current Irms is defined as _____.
The value of a.c. That is equal to the steady direct current which would dissipate beat at the same average rate in a given resistor.
Pave = I^2R = Irms^2R
Irms = I0/(2^1/2) only applies for _____
Sinusoidal alternating current
P mean= ?
P0/2 = (I0/2^0.5)^2R = I0^2R/2
Name the steps in the graphical method to determine root-mean-square of current.
- Square the alternating current
- Take the mean value of the squared alternating current over the duration of 1 period
- Square root the value of the mean square alternating current.
The time-variation of a sinusoid current or voltage can be represented by the equation _____.
x = x0sinwt
What does a simple transformer consist of?
Primary coil, secondary coil and laminated soft iron core
How does a transformer work?
- Primary coil is connected to an alternating current, which causes a changing magnetic flux
- This creates a changing magnetic flux linkage in the core.
- An changing emf is induced in the core and thus the core has a changing magnetic flux with the secondary coil
- An alternating emf is induced in the secondary coil
5.
What is the function of the soft iron core?
To concentrate magnetic flux to reduce flux losses. It ensures that all magnetic flux is confined to the core so all the flux passing through one coil passes through the other.
In an ideal transformer, how are the voltages in the coils related to the number of turns? Hence derive the relationship between number of turns and current.
Np/Ns = Vp/Vs = Is/Ip
The values of I and V in a transformer are _____ values.
Root mean square
What is an ideal transformer?
Power input in primary coil = power output in secondary coil
Why are the transformer’s input and output voltages not in phase?
Faraday’s law: induced output voltage is proportional and thus varies in phase with the change in flux linkage in the iron core.
The flux linkage in the core varies in phase with the input voltage. However, the rate of change of flux linkage is not in phase with the flux linkage itself. Thus the input and output voltages are not in phase.
Lenz’s law: the induced voltage in the secondary coil will produce effects to oppose the alternating flux linkage (in the iron core which varies in phase with the input voltage) that had induced it (it is not in phase with something it is trying to oppose)
What is rectification and why is it practical do rectify an alternating current?
It is the process of converting an alternating current source into a direct current supply. This is needed because many applications only use direct current.
