Points to note:
1. To draw a ac wave , you need the peak value to draw the wave accurately. The r.m.s value cannot help as it is somewhere below the peak value. The frequency converted to period allows you to mark the x axis. If f = 50 Hz , then mark period as 0.02s.
2. The diagram for step up or step down transformer allows you to view the information clearly so as not to mix up the secondary and primary values. The power loss is in the wire of resistance 30 ohms. It is safest to use current passing through the 30 ohm wire to calculate Ploss = I2R.
3. The AC voltage is step down and then rectified, hence the waveform is half rectified as only one diode is used. The peak voltage after step down is 81 V. The r.m.s value of half rectified wave form is peak value /2 ( refer to lect notes as it needs proving) .The value of r.m.s acorss AB is 81/2 = 41 V. The power dissipated is (41)2 divide by 20 = 84 W.
The power with time graph need to follow the voltage time graph. Hence the peak is (81)2 divide by 20 . It only reaches this value when the voltage hit the peak value accordingly. Note the shape has a turning point at the x axis.
