NEB Class 12 Science Physics (NEB Released Items) Question Paper 2081 (Set C) Nepal

a) Rectifier (1 mark):

A rectifier is a device that converts alternating current (AC) into direct current (DC).

Full wave rectification using two P-N junction diodes (2 marks):

In a centre-tapped full-wave rectifier, the secondary of a transformer is centre-tapped and its two ends are connected to two P-N junction diodes $D_1$ and $D_2$, whose outputs are joined to a common load resistor $R_L$.

• During the positive half cycle of the AC input, the upper end of the secondary is positive, so diode $D_1$ is forward biased and conducts while $D_2$ is reverse biased. Current flows through $R_L$ in a fixed direction.
• During the negative half cycle, the lower end is positive, so diode $D_2$ is forward biased and conducts while $D_1$ is reverse biased. Current again flows through $R_L$ in the same direction.

Because one diode conducts in each half cycle, current flows through the load during both halves of the AC cycle, producing a unidirectional (pulsating DC) output. Hence both half cycles of the AC are rectified.

(A labelled circuit diagram should show the centre-tapped transformer, diodes $D_1$ and $D_2$, and the load resistor $R_L$.)

b) Truth table for the digital circuit (2 marks):

Using the marking scheme output (with $A'$ and $B'$ as the complements of $A$ and $B$), the truth table is:

This output corresponds to the OR-type combination $Y = A + B$ (the output is 1 whenever at least one input is 1, except row 1 where both are 0).

a) Time interval between the two sounds (3 marks):

The sound reaches the listener through two media — the steel rail (fast) and the air (slow).

Speed of sound in air:

Time through air:

Speed of sound in steel:

Time through steel:

Time interval between the two sounds:

Since the two arrival times differ by about 6 s, the person genuinely hears the sound twice (first through the rail, then through the air), confirming the claim is correct.

b) Justification that AB/CD represents wave velocity (2 marks):

• In Graph I ($y$ versus $x$ at a fixed time), the marked length AB represents the wavelength $\lambda$ (the spatial period of the wave).
• In Graph II ($y$ versus $t$ at a fixed point), the marked length CD represents the time period $T$ (the temporal period of the wave).

Therefore:

Since wave velocity $v = \dfrac{\lambda}{T}$ (equivalently $v = f\lambda$), the ratio AB:CD is exactly the wave velocity.

a) Two advantageous uses of eddy currents (2 marks):

1. Electromagnetic braking — eddy currents induced in a moving metal disc/rail oppose the motion and bring trains and other vehicles to a smooth, frictionless stop.
2. Induction heating / diathermy — eddy currents are used to heat metals in induction furnaces and for medical diathermy (deep tissue heating).

(Other valid examples: induction motors, energy meters, speedometers.)

b) i) Maximum induced current (2 marks):

The induced EMF is $\varepsilon = N\dfrac{d\Phi}{dt} = NA\dfrac{dB}{dt}$, and the induced current is

Using the maximum slope $\dfrac{dB}{dt}$ read from the graph together with $N = 240$, $A = 1.2\times10^{-4}\ \text{m}^2$ and $R = 25\ \Omega$, the maximum induced current is

c) i) Product of the currents (2 marks):

The force per unit length between two long parallel currents is

Rearranging for the product of currents:

c) ii) Direction of the force (2 marks):

When both conductors carry current in the same direction, the wires attract each other. The force on each wire points toward the other wire (the two attractive forces are directed inward, along the line joining the wires).

   I →        I →
   |  ---- F →    ← F ----  |
  wire 1            wire 2

(Parallel currents in the same direction attract; the force arrows point inward, toward each other.)