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Question
Answer the following question.
Draw the diagram of a device that is used to decrease high ac voltage into a low ac voltage and state its working principle. Write four sources of energy loss in this device.
Solution
A transformer is a device that is used to either increase or decrease the ac voltage level. In order to decrease the high ac voltage level into a low ac voltage level we need a step-down transformer, whose diagram is as follows:
Working Principle:
A transformer works on the principle of electromagnetic induction. Alternating current in the primary coil produces a changing magnetic flux due to this an induced current is set up in the secondary coil. Losses in a transformer: Copper loss - The windings of the transformer have finite resistance due to which some energy is lost in the form of heat. It can be diminished using thick copper wires.
Iron loss - Loss is in the bulk of iron core due to the induced eddy currents in the iron core. It is minimized by using a thin laminated core.
Hysteresis loss - Alternating magnetizing and demagnetizing of the iron core causes the loss of energy in the form of heat. It is minimized using a special alloy of the iron core with silicon that has low hysteresis loss.
Magnetic loss - All the magnetic flux due to the primary coil does not pass through the secondary coil. So there is some leakage of flux. This loss can be minimized by winding primary over the secondary coil.
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(i) An a.c. source of emf ε = 200 sin omegat is connected to a resistor of 50 Ω . calculate :
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(2) Root mean square (rms) value of emf
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Answer the following question.
In a series LCR circuit connected across an ac source of variable frequency, obtain the expression for its impedance and draw a plot showing its variation with frequency of the ac source.
The resonant frequency of a RF oscillator is 1 MHz and its bandwidth is 10 kHz. The quality factor will be :
For an LCR circuit driven at frequency ω, the equation reads
`L (di)/(dt) + Ri + q/C = v_i = v_m` sin ωt
- Multiply the equation by i and simplify where possible.
- Interpret each term physically.
- Cast the equation in the form of a conservation of energy statement.
- Integrate the equation over one cycle to find that the phase difference between v and i must be acute.
Select the most appropriate option with regard to resonance in a series LCR circuit.
