Alternating Current Flashcards
Define root-mean-square values of alternating current
The root-mean-square value of an alternating current is the value of a steady d.c. that will dissipate energy at the same average rate as the a.c. in a resistive load.
Why do we use root mean square values?
For the rating of electrical appliances which uses alternating current, the average value of current or voltage is not a representative value because the magnitude of a sinusoidal current or voltage is always changing and so will its heating effect. Furthermore, the average value of a sinusoidal current or voltage is zero which misleads consumers to think that the appliance consumes no energy at all.
How does a transformer work?
The current in the primary coil will set up a magnetic field which cuts through the secondary coil due to the presence of the iron core.
Since the current in the primary coil is alternating, the magnetic flux density will also be varying.
There will be a magnetic flux linkage in the secondary coil, resulting in an induced e.m.f. in the secondary coil in accordance to Faraday’s law.
As flux is continuously increasing and decreasing, so polarity of e.m.f. is continously switching which results in the e.m.f. in the secondary coil to alternate.
Why is ideal transformer not possible?
There is heat dissipated in the iron core as there is induced current/eddy currents.
Resistance of coils results in more heat dissipated when there is current.
Some leakage of magnetic flux as not all the magnetic flux produced by the primary coil may be connected to the secondary coil.
Why is a soft iron used for the core for a transformer?
It can be easily magnetised and demagnetised by the varying magnetic field of the primary coil when connected to alternating current. This minimises the time delay of the induction between pimary and secondary coils.
The presence of an iron core maximises the flux linkage between the primary and secondary coils.
