AC Flashcards
what is the root mean square current of an AC
the root mean square value of an alternating current is that equivalent constant direct current that will dissipate the same average power in a given resistance load
SWAP EXACTLY FOR RMS OF Alt voltage
what does an ideal transformer mean ?
other assumptions
zero resistance coils, 100% efficient, mean power in primary coil = mean power in secondary coil
resistance of the secondary coil is negligible compared to the load resistance (if there are calculations based on load resistance in secondary coil to measure power output)
why must the primary voltage be an alternating current ?
Vp must be an alternating current that creates varying magnetic field if not the flux in the primary coil would not change (except for short time immediately after emf applied) and there would be no induced emf (secondary voltage) in the secondary coil. Constant DC voltage produces constant magnetic field which will not result in any induced emf
explain why magnetic flux density not same in both coils and what is the same one ?
magnetic flux is the same in both coils as they are linked magnetically via the soft iron core so a change in flux in secondary coil Weill be equal and opposite to the primary coil by Lenz law
magnetic flux density not the same as area is not uniform (depends on the figure). The smaller the area, the larger the magnetic flux density as the magnetic flux is constant and mag flux = BA
Magnetic flux = BpAp = BsAs
sources of energy losses and how to solve them (normal AC not transformer)
- resistance of wires produces heating- (link) (coil that have smaller number of turns carry higher currents - as seen through the inverse formula, so use thicker wires as resistance = pl/A)
- eddy currents causes heating (bulk piece of the metal is laminated)
- hysteresis produces heating
- flux leakage
softy iron easily magnetised and demagnetised can use pt 3 + increases magnetic flux density due to higher permeability so magnetic flux linkage of pri and sec coils increases
use of single diodes in half wave rectification + learn to draw (see page 13)
Rectification is the conversion of AC supply to DC
As the applied pd to the rectifier and the load is a sinusoidal wave, the divide is forward biased during one half cycle and reversed biased during the next half cycle
During the half cycle when the rectifier is forward biased, current flows round the circuit, creating a pd across R (load). This pd will have almost the same pd as the applied pd if the forward resistance of the divide is small compared to R
During the half cycle where the diode is reverse biased, little to no current flows through the divide and the pd across R is 0
This is repeated for each cycle of AC input. The current flow is unidirectional and so is the pd across R. Although it fluctuates, it never changes direction
how to reduce power losses
to reduce power loss for a given amount of power generated, electricity is transmitted at high
voltage (i.e. low current) by stepping up the a.c. voltage using a transformer.
questions to look through D5,D7,D9,D11
OK
how does laminating the core reduce energy losses due to these currents
When laminated, the core is electrically divided into smaller sections. The flux linkage in each section is reduced. This leads to smaller induced e.m.f. (ε = -dΦ/dt) and less heat loss
Also, the narrowing of the path the current takes further reduces the current as resistance is increased (R = ρl/A).
why are alternating currents advantages when transmitting electricity
Alternating currents are used because the voltage can be stepped up and down using transformers. (DC docent work cos no induced emf all explained in front)
The power loss I2R in cable due to resistance of the cable can be reduced when the voltage is stepped up so
that the current in transmission cable is small.
