Physics MCQs for NEET — Practice Questions with Answers

Experiment 6.3 description states, 'It is observed that the galvanometer shows a momentary deflection when the tapping key K is pressed. The pointer in the galvanometer returns to zero immediately. If the key is held pressed continuously, there is no deflection in the galvanometer.' This supports Faraday's law that emf is induced only by a change in magnetic flux. When the key is pressed, current changes from zero to maximum, causing a change in flux. When held pressed, current and thus flux are constant, so no emf.

The definition of magnetic flux is given as $\Phi_B = B \cdot A = BA \cos \theta$, where $\theta$ is the angle between the magnetic field vector B and the area vector A.

Magnetic flux is given by $\Phi_B = BA \cos \theta$. Even if B and A are constant, changing the angle $\theta$ (orientation of the coil) will change $\cos \theta$ and thus change the magnetic flux, inducing an emf. The NCERT states, 'one method to induce an emf or current in a loop is through a change in the loop’s orientation or a change in its effective area.'

Faraday's experiments (6.1, 6.2, 6.3) consistently showed that induced current/emf arises from 'relative motion between a magnet and a coil' (or two coils) or from a 'change in the current (and resulting magnetic field) in a nearby coil'. The core principle is that the magnetic flux 'associated with coil C1' must change. Hence, relative motion creating changing flux or the field itself changing is necessary.

The context states: 'From the V-I characteristic of a junction diode we see that it allows current to pass only when it is forward biased. So if an alternating voltage is applied across a diode the current flows only in that part of the cycle when the diode is forward biased. This property is used to rectify alternating voltages and the circuit used for this purpose is called a rectifier.' This directly explains why the diode's unidirectional conduction property is crucial for rectification.

The NCERT text says: 'If an alternating voltage is applied across a diode in series with a load, a pulsating voltage will appear across the load only during the half cycles of the ac input during which the diode is forward biased. Such rectifier circuit, as shown in Fig. 14.18, is called a half-wave rectifier.' This confirms that the output is a pulsating DC voltage present only during half-cycles.

The context states: 'When A is negative, the diode is reverse-biased and it does not conduct. The reverse saturation current of a diode is negligible and can be considered equal to zero for practical purposes.' This means during the negative half-cycle, the diode is reverse-biased, and virtually no current flows through the load.

The NCERT text clearly mentions: 'The circuit using two diodes, shown in Fig. 14.19(a), gives output rectified voltage corresponding to both the positive as well as negative half of the ac cycle. Hence, it is known as full-wave rectifier.'

The context explains about full-wave rectifiers: 'So for a full-wave rectifier the secondary of the transformer is provided with a centre tapping and so it is called centre-tap transformer.' While a transformer is often used in half-wave rectifiers for voltage scaling, a center-tapped transformer is a defining feature of the specific full-wave rectifier configuration discussed.

The NCERT states: 'As can be seen from Fig.14.19(c) the voltage rectified by each diode is only half the total secondary voltage.' If $V_{peak}$ is the total secondary voltage, then each diode rectifies $V_{peak}/2$.