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Physics · 2024 · 5 marks
CBSE 2024 · Region 4 · Set 1 · Q33
(a) (i) Mention the factors on which the resonant frequency of a series LCR circuit depends. Plot a graph showing variation of impedance of a series…
(i)
Mention the factors on which the resonant frequency of a series LCR circuit depends. Plot a graph showing variation of impedance of a series LCR circuit with the frequency of the applied a.c. source.
(ii)
With the help of a suitable diagram, explain the working of a step-up transformer.
(iii)
Write two causes of energy loss in a real transformer.
(i)
With the help of a diagram, briefly explain the construction and working of ac generator.
(ii)
An electron is revolving around a proton in an orbit of radius $\displaystyle r$ with a speed $\displaystyle v$. Obtain expression for magnetic moment associated with the electron.
Marking-scheme solution
(a)(i)
Inductance
Capacitance
OR
$\displaystyle \nu_{0}=\frac{1}{2 \pi \sqrt{L C}}$
Working – when an alternating voltage is applied to the primary, the resulting current produces an alternating magnetic flux which links the secondary and induces an emf in it.
(iii)
Causes of energy loss (any two):
(1)
Flux leakage
(2)
Resistance of the windings
(3)
Hysteresis
(4)
Eddy currents
(b)
Construction – It consists of a coil placed in a magnetic field. The coil is mounted on a rotor shaft. The ends of the coil are connected to an external circuit by means of slip rings and brushes.
Working – The coil is rotated in the uniform magnetic field by some external means. The rotation of the coil causes the magnetic flux through it to change, so an emf is induced in the coil.
(ii)
The equivalent current:
$\displaystyle I=\frac{q}{t}=\frac{e}{\dfrac{2 \pi r}{v}}=\frac{e v}{2 \pi r}$
Magnetic moment of revolving electron:
$\displaystyle m=I A$
$\displaystyle =\frac{e v}{2 \pi r} \times \pi r^{2}$
$\displaystyle =\frac{1}{2} e v r$
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CBSE Class 12 Physics past-paper question from the 2024 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.