NEB Class 12 · Past paper
The official NEB Class 12 model questions for Physics, all 19 questions with solved model answers.
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Answer in brief, any four questions. (a) Two wires, one of copper and one of iron, have the same diameter and carry the same current. In which wire will the drift velocity of the free electrons be more? (b) Draw a circuit diagram of a meter bridge to determine the resistance of a wire and give the formula used. (c) Mention any two factors on which the thermo-electric emf produced in a thermocouple depends. (d) How does the conduction of electricity in a metallic conductor differ from that in an electrolyte? (e) When a charged particle moves in a magnetic field, does its kinetic energy always remain constant? Explain. (f) If the number of turns of a solenoid is doubled, keeping other factors constant, how does the self-inductance change?
(a) The drift velocity is $vd = \dfrac{I}{nAe}$, so for the same current $I$ and the same area $A$ we have $vd \propto 1/n$. Copper has a larger free-electron density than iron, so for the same current the iron wire, having the smaller
Answer in brief, any four questions. (a) A discharge tube is filled with different gases in turn under the same pressure. Will the discharge occur at the same voltage across the electrodes? (b) Draw a logic symbol and write a truth table for a two-input NAND gate. (c) An electron and a proton travel at the same speed. Which has the shorter wavelength? (d) What are the units of activity of radioactive elements? (e) Distinguish between leptons and quarks. (f) Explain the significance of Hubble's constant.
(a) No. The breakdown (striking) voltage depends on the ionisation potential of the gas, and this differs from one gas to another. So even at the same pressure, different gases need different voltages before the discharge occurs. (b) A N...
Answer in brief, any one question. (a) Distinguish between light waves and sound waves. (b) The whistle of an approaching train is shriller. Why?
(a) Light waves are transverse electromagnetic waves that need no material medium and travel at $3\times10^8\ \text{m s}^{-1}$ in vacuum, and they can be polarised. Sound waves are longitudinal mechanical waves that require a material medium and travel much more slowly (about $340\ \text{m s}^{-1}$ in air), and they cannot be polarised.
(b) This is the Doppler effect. As the train approaches, each successive compression is emitted from a point nearer the listener, so the wavefronts are crowded together and the wavelength reaching the ear is shortened. The observed frequency therefore rises,
$$f' = f\left(\frac{v}{v - v_s}\right) > f,$$
so the whistle sounds shriller (higher pitched) as the train approaches.
Answer in brief, any one question. (a) State and explain Huygens's construction. (b) Diffraction of audible sound is easily observable, why?
(a) Huygens's principle states that every point on a wavefront acts as a source of secondary spherical wavelets that spread forward at the wave speed, and the wavefront at a later instant is the forward envelope, the common tangent surfa...
Answer any three questions. (a) State and explain the principle of a potentiometer. Describe, with a circuit diagram, the use of a potentiometer to compare the emfs of two primary cells. (b) Derive an expression for the force between two parallel current-carrying conductors and hence explain why two straight conductors carrying current in the same direction attract each other. (c) Define relative permeability and susceptibility of a substance. Compare para- and ferro-magnetic materials on the basis of these quantities. (d) Derive an expression for the impedance of an ac circuit containing a resistor, inductor and capacitor in series, and obtain the resonant frequency.
(a) The principle of a potentiometer is that when a steady current flows through a uniform wire, the potential drop across any portion is proportional to its length, $V \propto \ell$. To compare two emfs, each cell is in turn balanced ag...
Answer any three questions. (a) What is a Zener diode? With a circuit diagram, explain the use of a Zener diode as a voltage regulator. (b) Describe the construction and working principle of a He-Ne laser and write its important uses. (c) Derive the relation N = N0 e^(-lambda t) for radioactive decay and obtain the relation between the disintegration constant and half life. (d) What do you mean by energy crisis? Discuss the energy consumption scenario of Nepal.
(a) A Zener diode is a heavily doped p-n junction designed to work in the reverse breakdown region, where it maintains a nearly constant voltage $VZ$ across itself even as the current through it changes. To use it as a voltage regulator,...
Answer any one question. (a) Explain why and how Laplace corrected Newton's formula for the velocity of sound in air, and discuss the effect of temperature on the velocity of sound in a gas. (b) What is intensity of sound? Prove that the intensity of sound is proportional to the square of the amplitude of vibration for a given source.
(a) Newton assumed that sound travels through air isothermally, which gives $v = \sqrt{P/\rho}$, a value about 15% lower than the measured speed. Laplace corrected this by pointing out that the compressions and rarefactions happen so rap...
Answer any one question. (a) Describe Michelson's experimental method for the measurement of the velocity of light with necessary theory. (b) What is polarization of light? Discuss polarization by reflection and prove Brewster's law.
(a) In Michelson's rotating-mirror method, light from a source is reflected off one face of an eight-sided rotating mirror, travels a large known distance $D$ to a fixed concave mirror, and returns. During the round-trip time $t = 2D/c$,...
Answer any two numerical questions. (a) A heating coil is to be made from nichrome wire operating on a 12 V supply with power 25 W. Calculate the length of nichrome wire needed if its cross-sectional area is 10 mm^2 and the resistivity of nichrome is 10^-6 ohm m. (b) A solenoid is to produce a magnetic field of 0.027 T at its centre. It has radius 1.4 cm, length 40 cm and carries a maximum current of 12 A. Find the minimum number of turns and the total length of wire required. (c) A conductor of length 2 m moves at an angle of 45 degrees to a uniform magnetic field of 0.2 T with a velocity of 5 m/s. Calculate the induced emf.
(a) The resistance follows from the power rating, since $R = \dfrac{V^2}{P}$: $$ \begin{aligned} R &= \frac{12^2}{25} \ &= \frac{144}{25} \ &= 5.76\ \Omega. \end{aligned} $$ With $A = 10\ \text{mm}^2 = 1\times10^{-5}\ \text{m}^2$,
Answer any two numerical questions. (a) An electron is accelerated through a potential difference of 20 kV and then circulates at right angles to a uniform magnetic field of 0.1 T. What is the radius of its path? (e/m = 1.8x10^11 C/kg) (b) Light of wavelength 600 nm falls on a photosensitive plate of work function 1.9 eV. Find the kinetic energy of the emitted photoelectrons and the stopping potential. (h = 6.62x10^-34 Js, c = 3x10^8 m/s, e = 1.6x10^-19 C) (c) The energy liberated in the fission of a single uranium-235 atom is 3.2x10^-11 J. Calculate the power produced by the fission of 2 g of uranium per day. (Avogadro constant = 6.023x10^23 /mol)
(a) The speed the electron gains from the accelerating potential difference comes from $eV = \tfrac{1}{2}mv^2$, so $$ \begin{aligned} v &= \sqrt{2\frac{e}{m}V} \ &= \sqrt{2(1.8\times10^{11})(2\times10^{4})} \ &= \sqrt{7.2\times10^{15}}...
A steel wire of length 40 cm and diameter 0.025 cm vibrates transversely in resonance with a tube, open at both ends and of effective length 60 cm, when each is sounding its fundamental note. Find the tension in the wire if the velocity of sound in air is 340 m/s and the density of steel is 7800 kg/m^3.
The wire has length $L = 0.40\ \text{m}$ and diameter $d = 0.025\ \text{cm} = 2.5\times10^{-4}\ \text{m}$, while the open-open tube has effective length $Lt = 0.60\ \text{m}$; the velocity of sound in air is
In a double-slit experiment the slits are 0.3 mm apart and interference is observed on a screen 90 cm from the slits. The second dark band is 0.3 cm from the central bright fringe. Calculate the wavelength of light used.
The slits are $d = 0.3\ \text{mm} = 3\times10^{-4}\ \text{m}$ apart, the screen is $D = 90\ \text{cm} = 0.90\ \text{m}$ away, and the second dark band lies at $y = 0.3\ \text{cm} = 3\times10^{-3}\ \text{m}$ from the central bright fringe...
Differentiate between centre of gravity and centre of mass. Mention the condition of stable equilibrium.
The centre of mass is the point at which the whole mass of a body may be taken to be concentrated when describing its translational motion, and its position depends only on how the mass is distributed. The centre of gravity is the point through which the resultant weight of the body acts. In a uniform gravitational field the two points coincide; they differ only for a very large body over which $g$ varies from point to point, and in that case the centre of gravity shifts towards the region where the field is stronger.
| Centre of mass | Centre of gravity |
|---|---|
| Depends on mass distribution | Depends on weight distribution |
| Independent of $g$ | Defined only where gravity acts |
| Exists even in zero gravity | Undefined in zero gravity |
For stable equilibrium, the centre of gravity should be as low as possible and the vertical line through it must pass within the base of the body. Then a small displacement raises the centre of gravity, and the resulting restoring torque brings the body back to its original position.
How are musical sound and noise distinguished from each other?
A musical sound is a pleasant sound produced by regular, periodic vibrations: its waveform repeats, it has a definite pitch, and it is free of sudden changes in loudness. Noise, by contrast, is an unpleasant sound produced by irregular, ...
What is the basic difference between the first law and the second law of thermodynamics? Write the significance of the second law of thermodynamics.
The first law is the principle of conservation of energy applied to heat, $dQ = dU + dW$: energy can be neither created nor destroyed, only converted from one form to another. It says nothing, however, about the direction in which a process can occur. The second law deals precisely with that direction and with the quality of energy conversion: heat flows spontaneously from a hotter to a colder body, and no engine can convert heat completely into work in a cycle.
The significance of the second law is that:
What are the defects of vision? How are they remedied?
The common defects of vision are the following:
In each case a suitable corrective lens shifts the image back onto the retina.
Deduce an expression for the energy stored in a capacitor. Can we give any desired charge to a capacitor? Explain.
To charge a capacitor of capacitance $C$, work has to be done in pushing charge onto the plates against the potential that is already building up. When the charge on the plates is $q$ the potential difference is $q/C$, so moving a furthe...
State and explain Ohm's law.
Ohm's law states that, at constant temperature (and under unchanged physical conditions), the current $I$ flowing through a conductor is directly proportional to the potential difference $V$ across its ends, $$V \propto I \quad\Rightarro...
List the properties of alpha, beta and gamma rays.
Alpha ($\alpha$) rays are helium nuclei ($^{4}_{2}\text{He}$) carrying a charge of $+2e$; they are relatively heavy and slow. They have the greatest ionising power but the least penetrating power, being stopped by a sheet of paper or a few cm of air, and they are deflected by electric and magnetic fields.
Beta ($\beta$) rays are fast-moving electrons carrying a charge of $-e$ and are much lighter than alpha particles. They have moderate ionising power and moderate penetrating power, being stopped by a few mm of aluminium, and they are deflected more strongly than alpha rays and in the opposite direction.
Gamma ($\gamma$) rays are high-energy electromagnetic radiation with no charge and no mass. They have the least ionising power but the greatest penetrating power, needing thick lead or concrete to stop them, and they are not deflected by electric or magnetic fields, travelling at the speed of light.
The order of ionising power is $\alpha > \beta > \gamma$, while the order of penetrating power is $\gamma > \beta > \alpha$.
(a) The drift velocity is , so for the same current and the same area we have . Copper has a larger free-electron density than iron, so for the same current the iron wire, having the smaller
(a) Light waves are transverse electromagnetic waves that need no material medium and travel at in vacuum, and they can be polarised. Sound waves are longitudinal mechanical waves that require a material medium and travel much more slowly (about in air), and they cannot be polarised.
(b) This is the Doppler effect. As the train approaches, each successive compression is emitted from a point nearer the listener, so the wavefronts are crowded together and the wavelength reaching the ear is shortened. The observed frequency therefore rises,
so the whistle sounds shriller (higher pitched) as the train approaches.
(a) The principle of a potentiometer is that when a steady current flows through a uniform wire, the potential drop across any portion is proportional to its length, . To compare two emfs, each cell is in turn balanced ag...
(a) A Zener diode is a heavily doped p-n junction designed to work in the reverse breakdown region, where it maintains a nearly constant voltage across itself even as the current through it changes. To use it as a voltage regulator,...
(a) Newton assumed that sound travels through air isothermally, which gives , a value about 15% lower than the measured speed. Laplace corrected this by pointing out that the compressions and rarefactions happen so rap...
(a) In Michelson's rotating-mirror method, light from a source is reflected off one face of an eight-sided rotating mirror, travels a large known distance to a fixed concave mirror, and returns. During the round-trip time ,...
(a) The resistance follows from the power rating, since : With ,
(a) The speed the electron gains from the accelerating potential difference comes from , so $$ \begin{aligned} v &= \sqrt{2\frac{e}{m}V} \ &= \sqrt{2(1.8\times10^{11})(2\times10^{4})} \ &= \sqrt{7.2\times10^{15}}...
The wire has length and diameter , while the open-open tube has effective length ; the velocity of sound in air is
The slits are apart, the screen is away, and the second dark band lies at from the central bright fringe...
The centre of mass is the point at which the whole mass of a body may be taken to be concentrated when describing its translational motion, and its position depends only on how the mass is distributed. The centre of gravity is the point through which the resultant weight of the body acts. In a uniform gravitational field the two points coincide; they differ only for a very large body over which varies from point to point, and in that case the centre of gravity shifts towards the region where the field is stronger.
| Centre of mass | Centre of gravity |
|---|---|
| Depends on mass distribution | Depends on weight distribution |
| Independent of | Defined only where gravity acts |
| Exists even in zero gravity | Undefined in zero gravity |
For stable equilibrium, the centre of gravity should be as low as possible and the vertical line through it must pass within the base of the body. Then a small displacement raises the centre of gravity, and the resulting restoring torque brings the body back to its original position.
The first law is the principle of conservation of energy applied to heat, : energy can be neither created nor destroyed, only converted from one form to another. It says nothing, however, about the direction in which a process can occur. The second law deals precisely with that direction and with the quality of energy conversion: heat flows spontaneously from a hotter to a colder body, and no engine can convert heat completely into work in a cycle.
The significance of the second law is that:
To charge a capacitor of capacitance , work has to be done in pushing charge onto the plates against the potential that is already building up. When the charge on the plates is the potential difference is , so moving a furthe...
Ohm's law states that, at constant temperature (and under unchanged physical conditions), the current flowing through a conductor is directly proportional to the potential difference across its ends, $$V \propto I \quad\Rightarro...
Alpha () rays are helium nuclei () carrying a charge of ; they are relatively heavy and slow. They have the greatest ionising power but the least penetrating power, being stopped by a sheet of paper or a few cm of air, and they are deflected by electric and magnetic fields.
Beta () rays are fast-moving electrons carrying a charge of and are much lighter than alpha particles. They have moderate ionising power and moderate penetrating power, being stopped by a few mm of aluminium, and they are deflected more strongly than alpha rays and in the opposite direction.
Gamma () rays are high-energy electromagnetic radiation with no charge and no mass. They have the least ionising power but the greatest penetrating power, needing thick lead or concrete to stop them, and they are not deflected by electric or magnetic fields, travelling at the speed of light.
The order of ionising power is , while the order of penetrating power is .