NEB Class 12 Science Physics Question Paper 2077 Nepal

In a metal, charge is carried by a very large number of free (delocalized) electrons that drift easily under an applied field, giving high conductivity. In an electrolyte, charge is carried by ions which are far fewer in number and much more massive than electrons. The ions move slowly through the solution, experiencing strong viscous resistance and frequent collisions with solvent molecules, so their drift speed (mobility) is small. Hence the number density of charge carriers and their mobility are both much lower, making the conductivity of an electrolyte very low compared to that of a metal.

Advantages of A.C. over D.C.:

1. A.C. voltage can be easily stepped up or stepped down using a transformer, which is not possible directly with D.C.
2. Because it can be stepped up to high voltage, A.C. can be transmitted over long distances with much smaller power loss ($I^2R$ loss) in the lines.
3. A.C. is cheaper and easier to generate (using simple A.C. generators/alternators) than D.C.
4. A.C. machines (motors, generators) are simpler, more rugged and require less maintenance.
5. A.C. can be readily converted into D.C. (by rectification) when needed.

In the photoelectric effect, a photon of energy $h\nu$ is absorbed by a metal and its energy is transferred to an electron, ejecting it (light energy → kinetic energy of electron). In the production of X-rays, the reverse happens: fast-moving electrons strike a target and are suddenly decelerated, and their kinetic energy is converted into electromagnetic radiation (X-ray photons), i.e. kinetic energy of electron → photon energy. Since one process converts photon energy into electron kinetic energy and the other converts electron kinetic energy into photon energy, X-ray production is the inverse of the photoelectric effect.

Threshold of hearing: It is the minimum sound intensity that an average human ear can just detect (hear) at a frequency of about 1 kHz. Its standard value is $I_0 = 10^{-12}\,\text{W/m}^2$.

One bel: The intensity level of a sound is said to be one bel when the ratio of its intensity $I$ to the reference (threshold) intensity $I_0$ is 10, i.e. $\log_{10}\!\left(\dfrac{I}{I_0}\right) = 1$. In general the intensity level in bel is $\beta = \log_{10}\!\left(\dfrac{I}{I_0}\right)$.

No, polarization is not possible for longitudinal waves. Polarization is the restriction of the vibrations of a wave to a single plane, and it is a property exhibited only by transverse waves, in which the particle vibrations are perpendicular to the direction of propagation (so they can be confined to one plane). In a longitudinal wave the particles vibrate along the direction of propagation; there is only one possible direction of vibration and no perpendicular plane to restrict, so polarization cannot occur. Hence polarization is impossible for longitudinal waves (e.g. sound waves cannot be polarized).

Why an ammeter has very low resistance: An ammeter is connected in series in a circuit to measure current. If its resistance were large it would significantly increase the total resistance of the circuit and reduce the current to be measured, giving a wrong reading. To measure the current accurately without altering it, the ammeter must have a very low resistance (ideally zero).

Converting a galvanometer into an ammeter: A galvanometer is converted into an ammeter by connecting a small resistance called a shunt ($S$) in parallel with it. The shunt provides a low-resistance path so that most of the current passes through it and only a small fixed fraction $I_g$ (the full-scale deflection current) passes through the galvanometer.

If $G$ is the galvanometer resistance, $I_g$ its full-scale current and $I$ the total current to be measured, the shunt is

The combination has a very small effective resistance and can measure large currents.

He-Ne laser is a continuous, four-level gas laser.

Construction: A narrow glass discharge tube contains a mixture of helium and neon in the ratio of about 10:1 at low pressure. Two electrodes maintain a high-voltage electric discharge, and the tube is closed by two mirrors (one fully reflecting, one partially reflecting) forming the optical resonant cavity.

Working:

1. The electric discharge accelerates electrons, which collide with He atoms and raise them to metastable excited states (about 20.61 eV and 19.81 eV).
2. These excited He atoms collide with ground-state Ne atoms and transfer their energy (resonant energy transfer) because Ne has energy levels almost equal to those of excited He. This raises Ne atoms to higher energy levels and produces a population inversion in neon.
3. A photon emitted by a Ne atom stimulates other excited Ne atoms to emit identical photons (stimulated emission), producing coherent light. The main laser transition gives red light of wavelength 632.8 nm.
4. The mirrors reflect the photons back and forth, amplifying the beam; part of the beam emerges through the partially reflecting mirror as an intense, coherent, monochromatic laser beam.

Doppler effect: It is the apparent change in the frequency (or pitch) of a wave observed when there is relative motion between the source of the wave and the observer.

Let the true frequency of the stationary source be $f$, the speed of sound be $v$ and the speed of the observer be $v_o$.

Observer moving towards the source: The apparent frequency is

so it increases.

Observer moving away from the source: The apparent frequency is

so it decreases.

Change in frequency between the two cases:

Michelson's rotating-mirror method:

Arrangement: Light from an intense source falls on one face of an eight-sided (octagonal) rotating mirror $R$. The reflected beam travels a large, accurately measured distance $D$ to a distant fixed concave mirror $M$ (on a far-off mountain), which returns the light back to the rotating mirror, and then to the observer's eye through a telescope.

Working:

1. When the mirror $R$ is stationary, the returning light is reflected from the same face and the image is seen.
2. When $R$ is rotated and its speed is gradually increased, the image disappears and reappears. The image is seen clearly again when, during the time the light travels to $M$ and back (distance $2D$), the mirror has turned by exactly one-eighth of a revolution, so that the next face occupies the position of the previous one.
3. If $n$ is the number of revolutions per second of the mirror at this condition, the time for one-eighth revolution is

4. In this time the light travels $2D$, so the velocity of light is

By measuring $D$ and $n$ accurately, the velocity of light $c$ is determined.