The Class 12 Physics Board Exam Question Solution for the year 2079 outside the Bagmati region is more than just a set of answers; it's a stepping stone to your success. With this solution, you can approach your board examination with greater confidence and readiness. Remember, this journey is about growth and learning. So, make the most of this resource, believe in yourself, and stride confidently toward acing your Physics Board Exam. Good luck!
GROUP A [11×1=11]
Rewrite the correct options of each question in your answer sheet
1. In rotational motion, the quantity that imparts angular acceleration is:
A) Force
B) Torque
C) Moment of Inertia
D) Angular Momentum
Answer: Torque
2. Two identical springs are arranged with a block as in figure. The oscillation frequency of the mass is f. if one spring is removed, the frequency of the oscillation will be
A) f
B) 2f
C) √2f
C) √2f
D) f√2
Answer: f√2
3. A liquid does not wet the surface of a solid if angle of contact is,
A) 90 ̊
B) less than 90 ̊
C) greater than 90 ̊
D) 0 ̊
Answer: greater than 90 ̊
4. Identify the wrong statement
A) For isothermal process, ∆T=0
B) For isochoric process, ∆V=0
C) For isobaric process, ∆P=0
D) For cyclic process, ∆W=0
Answer: For cyclic process, ∆W=0
5. The maximum efficiency of an engine operating between 30 ̊C and 300 ̊C is
A) 4.71%
B) 47%
C) 90%
D) 9%
Answer: 47%
6. In which frequency range the infrasonic wave lies?
A) (10-20) Hz
B) (30-40) Hz
C) (20-30) Hz
D) (50-60) Hz
Answer: (10-20) Hz
7. In Fraunhofer diffraction, the incident wavefront should be
A) Elliptical
A) Elliptical
B) plane
C) spherical
D) cylindrical
Answer: plane
8. If specific resistance of a potentiometer wire is 10−7Ωm, current flowing through it is 0.1A and cross-sectional area of wire is 10−6m2, then potential gradient will be
A) 10−2V/m
A) 10−2V/m
B) 10−4V/m
C) 10−6V/m
D) 10−8V/m
Answer:10−2V/m
9. A coil having N number of turns and cross-sectional area A carries a current I. the quantity NIA is
A) Magnetic flux
B) magnetic field
C) magnetic susceptibility
C) magnetic susceptibility
D) magnetic moment
Answer: magnetic moment
10.At resonance, in series LCR circuit, which relation does not hold good
A) ω =1/√LC
B) Lω =1/Cω
C) Cω =1/Lω
D) ω =1/LC
B) Lω =1/Cω
C) Cω =1/Lω
D) ω =1/LC
Answer: 1/LC
11.Which of the following is correct?
A) E² = P²C
B) E² = P²C²
C) E² = PC²
D) E² = P²/C²
Answer: E² = P²C²
GROUP B [8×5=40]
12.The angular speed is inversely proportional to the moment of inertia, that is given by principle of conservation of energy. [1+2+2]
a. In a flywheel, most of the mass is concentrated at the rim? Explain why.
Answer:In a flywheel, most of the mass is concentrated at the rim because the moment of inertia is directly proportional to the distance of the mass from the axis of rotation. The mass at the rim is farther from the axis of rotation and hence contributes more to the moment of inertia than the mass near the center. This allows the flywheel to store more rotational energy and maintain its angular momentum for a longer period of time.
b. The angular velocity of the earth around the sun increases when it comes closer to the sun. why?
Answer: The angular velocity of the earth around the sun increases when it comes closer to the sun because of the conservation of angular momentum. As the earth moves closer to the sun, the gravitational force between the two objects increases, which results in an increase in the torque acting on the earth. Since angular momentum is conserved, an increase in torque leads to an increase in angular velocity.
c. If the earth were to shrink suddenly, what would happen to the length f the day?
c. If the earth were to shrink suddenly, what would happen to the length f the day?
Answer:If the earth were to shrink suddenly, the moment of inertia of the earth would decrease. Since the angular momentum of the earth is conserved, a decrease in the moment of inertia would result in an increase in angular velocity. This means that the length of the day would decrease since the earth would be rotating faster on its axis. However, the magnitude of this effect would be negligible since the decrease in the moment of inertia would be small compared to the total moment of inertia of the earth.
13.Simple harmonic motion is defined from periodic functions like sine or cosine functions.
a. State the basic equation of motion for a body executing simple harmonic motion. [1]
Answer: The basic equation of motion for a body executing simple harmonic motion is given by: x(t) = A cos(ωt + φ)
where x(t) is the displacement of the object from its equilibrium position at time t, A is the amplitude of the motion, ω is the angular frequency of the motion, and φ is the phase angle.
b. Find expression for velocity and acceleration of a describing SHM. [2]Answer:The velocity and acceleration of a body executing simple harmonic motion can be found by taking the time derivatives of the displacement equation. The velocity is given by:
v(t) = dx/dt = -Aω sin(ωt + φ)
and the acceleration is given by:
a(t) = d^2x/dt^2 = -Aω^2 cos(ωt + φ)
where v(t) is the velocity of the object at time t, and a(t) is the acceleration of the object at time t.
c. The tip of tuning fork goes through 550 complete vibrations in 1 sec. find the angular frequency and time period of the motion. [1+1=2]
c. The tip of tuning fork goes through 550 complete vibrations in 1 sec. find the angular frequency and time period of the motion. [1+1=2]
Answer:The number of complete vibrations that a tuning fork goes through in 1 second is called its frequency, f. In this case, the frequency is 550 Hz. The angular frequency, ω, is related to the frequency by the equation:
ω = 2πf
Substituting the given value of f, we get:
ω = 2π(550)
= 1100π rad/s
The time period, T, is the time taken for one complete vibration. It is related to the frequency by the equation:
T = 1/f
Substituting the given value of f, we get:
T = 1/550 s = 0.0018 s
OR
OR
a. Define surface tension. [1]
Answer: The special property of liquid by virtue of which its surface behaves like an elastic stretched membrane and tries to occupy minimum surface area is known as surface tension.
b. State Bernoulli’s theorem. [1]
Answer: Bernoulli's theorem states that the total energy (sum of pressure energy, KE, & PE ) per unit mass of a nonviscous & incompressible fluid in streamlined flow is always constant.
c. Castor oil at 20 ̊C has a coefficient of viscosity 2.42 Ns/m²and density 940 kgm-3. Calculate the terminal velocity of a steel ball of radius 2mm falling under gravity in the oil, taking the density of steel as 7800kg/m3.(g = 10m/s2) [3]
14.Adiabatic process:
a. Define adiabatic process in thermodynamics. [1]
Answer: An adiabatic process is a thermodynamic process in which no heat is exchanged between a system and its surroundings. This means that the system is isolated from its environment and there is no transfer of heat or matter across its boundaries.
b. Derive expression for work done during adiabatic process. [3]
b. Derive expression for work done during adiabatic process. [3]
Answer: The work done during an adiabatic process can be derived using the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat transferred to the system plus the work done on the system:
ΔU = Q - W
For an adiabatic process, Q = 0, so we have:
ΔU = -W
where ΔU is the change in internal energy of the system and W is the work done on the system.
The internal energy of a gas can be expressed as:
U = (3/2) nRT
where n is the number of moles of gas, R is the gas constant, and T is the absolute temperature of the gas.
Differentiating this expression with respect to volume, we get:
dU = (3/2) nR dT
dU/dV = (3/2) nR (dT/dV)
Using the ideal gas law, PV = nRT, we can express dT/dV as:
dT/dV = -(P/nR)
Substituting this into the previous equation, we get:
dU/dV = -(3/2)PdV
The work done on the system during an adiabatic process is given by:
W = -∫PdV
Substituting the expression for dU/dV, we get:
W = -(3/2)∫PdV
Integrating this expression gives:
W = -(3/2)(P2V2 - P1V1)
where P1 and V1 are the initial pressure and volume of the system, and P2 and V2 are the final pressure and volume of the system.
c. Write the mathematical expression of entropy. [1]
c. Write the mathematical expression of entropy. [1]
Answer: The mathematical expression for entropy is given by:
ΔS = Qrev/T
where ΔS is the change in entropy of a system, Qrev is the heat transferred to the system in a reversible process, and T is the absolute temperature of the system. This equation is based on the second law of thermodynamics, which states that the entropy of an isolated system can only increase or remain constant over time. The change in entropy of a system is a measure of the degree of disorder or randomness in the system, and is related to the amount of heat that can be converted into work during a thermodynamic process.
15.a. Define an organ pipe. [1]
Answer: A hollow wooden or metallic tube used to produce musical sound is called an organ pipe.
b. Describe the various modes of vibration of the air column in a closed organ pipe. [3]
c. What is end correction? [1]
Answer: The distance between the anti-node and the open end of the pipe is called the end correction.
OR
a. State Doppler’s effect. [1]
Answer: Apparent change in frequency of a wave when there is relative motion between the source and the observer is called the Doppler effect.
b. Derive the apparent frequency of sound when an observer moves towards
a stationary source. [2]
Answer:
c. A stationary motion detector sends sound waves of 150 KHz towards a truck approaching at a speed of 120km/hr. What is the frequency of wave reflected back to detector? (Velocity of sound in air = 340m/s) [2]
Answer:
16.a. Differentiate Seebeck’s effect and Peltier’s effect. [2]
Answer: Seebeck's effect and Peltier's effect are both related to the conversion of temperature differences into electrical voltages or currents, but they are opposite in nature and occur in different situations.
b. Explain the variation of thermo-emf with temperature. [3]
Seebeck's effect, also known as the thermoelectric effect, is the phenomenon in which a temperature gradient across a material generates an electric field or voltage. This effect occurs in a circuit made of two dissimilar conductors joined at two points, which are maintained at different temperatures. The temperature difference between the two points creates a potential difference or electromotive force (EMF) that drives a current through the circuit.
On the other hand, Peltier's effect is the opposite of Seebeck's effect, in which an electric current flowing through a junction of two different materials causes a temperature difference across the junction. This effect is often used for cooling or heating applications, such as in thermoelectric refrigerators or heat pumps.
b. Explain the variation of thermo-emf with temperature. [3]
Answer: Thermo-emf, or the voltage generated by a thermocouple due to a temperature difference between its two junctions, depends on the materials used and the temperature difference between the two junctions. The variation of thermo-emf with temperature can be explained by the Seebeck coefficient, which is a measure of the sensitivity of a material to temperature differences.
The Seebeck coefficient is usually temperature-dependent, and may be positive or negative depending on the materials used. When two dissimilar metals are used to form a thermocouple, the Seebeck coefficient for each metal determines the magnitude and sign of the thermoelectric voltage. For a given metal, the Seebeck coefficient usually increases with temperature, up to a certain point where it may start to decrease. This temperature is known as the Curie temperature, and it varies depending on the material.
In general, the thermo-emf generated by a thermocouple increases with the temperature difference between its junctions, up to a certain limit where it may saturate due to the limitations of the materials used. The relationship between thermo-emf and temperature difference can be expressed by the Seebeck equation, which relates the thermo-emf to the Seebeck coefficient and the temperature difference.
17.When a charge particle moves in a uniform magnetic field, it experiences a force, called the Lorentz force.
a. What is the vector representation of Lorentz force? [1]
Answer: The Lorentz force is a vector quantity that describes the force experienced by a charged particle moving in a magnetic field. It is given by the formula F = q(v x B), where F is the force vector, q is the charge of the particle, v is its velocity vector, and B is the magnetic field vector.
b. State Fleming left hand rule. [1]
b. State Fleming left hand rule. [1]
Answer: Fleming's left-hand rule is a mnemonic device used to determine the direction of the force experienced by a current-carrying conductor in a magnetic field. It states that if the forefinger, middle finger, and thumb of the left hand are held mutually perpendicular to each other and the middle finger is made to point in the direction of the magnetic field, the forefinger will point in the direction of the conventional current in the conductor and the thumb will point in the direction of the force experienced by the conductor.
c. A horizontal straight wire 5 cm long weighing 1.2gm−1 is placed perpendicular to a uniform horizontal magnetic field of flux density 0.6T. if the resistance of the wire is 3.8Ωm−1 , calculate the p.d. that has to be applied between the ends of the wire to make it just self-supporting. [3]
c. A horizontal straight wire 5 cm long weighing 1.2gm−1 is placed perpendicular to a uniform horizontal magnetic field of flux density 0.6T. if the resistance of the wire is 3.8Ωm−1 , calculate the p.d. that has to be applied between the ends of the wire to make it just self-supporting. [3]
Answer:The weight of the wire in the magnetic field creates a downward force, which is balanced by the upward magnetic force on the wire. The current flowing through the wire produces a magnetic field that interacts with the external magnetic field, creating a force on the wire in the opposite direction to the weight. The wire will be self-supporting when the two forces are equal in magnitude.
Using the formula F = BIL, where F is the magnetic force, B is the magnetic field, I is the current, and L is the length of the wire in the magnetic field, we can find the current required to balance the weight of the wire:
F = BIL = mg
where m is the mass per unit length of the wire and g is the acceleration due to gravity.
Solving for I, we get:
I = mg / BL
The resistance of the wire is given as 3.8 Ωm^(-1), and its length is 5 cm or 0.05 m. The potential difference required to produce a current I through the wire is given by Ohm's law as V = IR, where R is the resistance of the wire:
V = IR = (mg / BL)R
Substituting the given values, we get:
V = (1.2 x 0.05 x 9.81) / (0.6 x 3.8) = 0.31 V
Therefore, the potential difference that has to be applied between the ends of the wire to make it just self-supporting is 0.31 V.
18.Electron is deviated in electric and magnetic field.
a. What path does the electron follow in electric field when the electron is projected normally in the field? [1]
Answer: When an electron is projected normally in an electric field, it follows a straight-line path in the direction of the field. This is because the electric field exerts a force on the electron in the direction of the field, causing it to accelerate in that direction.
b. An electron passes through a space without deviation. Does it mean there is no field? [2]
b. An electron passes through a space without deviation. Does it mean there is no field? [2]
Answer: No, it does not necessarily mean that there is no field. If an electron passes through a space without deviation, it could mean that it is moving parallel to the direction of the field, or that it is moving in a direction such that the forces due to the field are balanced. For example, if an electron is moving in a direction perpendicular to the direction of the field, and the field has a uniform magnetic and electric field component, then the forces due to the field will balance, and the electron will not experience any deviation.
c. Is there any condition that an electron does not experience any force inside the magnetic field? [2]
c. Is there any condition that an electron does not experience any force inside the magnetic field? [2]
Answer:Yes, there is a condition under which an electron does not experience any force inside a magnetic field. This occurs when the electron is moving parallel or antiparallel to the direction of the magnetic field, since the force due to the magnetic field is proportional to the cross product of the velocity of the electron and the magnetic field. If the velocity is parallel or antiparallel to the magnetic field, the cross product is zero, and the force due to the magnetic field is zero.
19.a. What is rectification? [1]
Answer: Rectification is the process of converting an alternating current (AC) signal into a direct current (DC) signal by allowing current to flow in only one direction.
b. Explain rectifier circuit operation with two diodes. [3]
Answer: A rectifier circuit with two diodes is known as a "full-wave rectifier." The circuit typically includes a transformer that steps down the voltage of the AC signal, two diodes, and a load resistor. The diodes are placed in a "bridge" configuration, with one diode in series with the load resistor and the other diode in series with the transformer. When the AC signal is positive, the diode in series with the load resistor conducts, allowing current to flow through the resistor and into the load. At the same time, the other diode is reverse-biased and does not conduct. When the AC signal is negative, the diode in series with the transformer conducts, allowing current to flow through the transformer and into the load. At the same time, the other diode is reverse-biased and does not conduct. By alternating which diode conducts, the circuit effectively converts the AC signal into a pulsing DC signal, with the pulsations occurring at the frequency of the AC signal.
c. What happens when one of the diodes becomes functionless?[1]
Answer:When one of the diodes in a rectifier circuit becomes functionless, the circuit will still work but at a reduced efficiency. The diode that is still functioning will continue to rectify the AC signal, but the resulting DC signal will be less smooth and may contain more ripple than it would with both diodes functioning. This is because the remaining diode will only conduct during half of the AC signal's cycle, and during the other half of the cycle, the load resistor will be disconnected from the circuit. Additionally, the load resistor may experience a higher voltage drop, which could cause it to heat up or even fail.
GROUP C [8×3=24]
20.a. Write sustainable conditions for interference. [2]
Answer: In order for interference to be sustainable, the following conditions need to be met:
The waves interfering with each other should have the same frequency and wavelength.
The waves should be coherent, which means that they maintain a constant phase relationship with each other.
The waves should be able to overlap in space and time, which means that they should have similar amplitudes and arrive at the same location at approximately the same time.
The waves should be able to propagate in the same medium or media.
c. In a Young’s double slit experiment, the separation of first to fifth fringes is 2.5 mm when the wavelength used is 620nm. The distance from the slits to the screen is 80 cm. Calculate the separation of two slits. [3]
21.Kirchhoff’s laws in electricity are very useful in solving in solving the
complicated circuit connections.
a. What is the significance of first law? [1]
b. State and explain second law with circuit diagram. [2]
c. Apply these laws to calculate unknown value of resistance. [3]
d. What is meter bridge? Write name of material used to construct meter bridge. [1+1=2]
OR
a. Derive the expression for impedance in L-C-R circuit. [3]
b. Find the condition of resonance L-C-R circuit. [2]
c. A circuit consists of a capacitor of 2μF and a resistor of 1000Ω. An alternating emf of 12V (rms) and frequency 50Hz is applied. Find the current flowing, the voltage across capacitor and the phase angle between applied emf and current. [3]
22.a. State Bohr’s postulate of atomic model. [2]
b. Derive an expression for radius of nth orbit in H-atom. [3]
c. Calculate de Broglie wavelength of electron when it is accelerated by 500 Volt. (mass of electron = 9.1 × 10−31 kg, Plank’s constant = 6.62 × 10−34 Js, charge of electron = 1.6 × 10−19 C) [3]
OR
Radioactivity is the spontaneously occurring phenomenon in nature.
a. What is radioactivity? [1]
b. Obtain N = Noe−λt in radioactive decay law. [3]
c. Describe the significance of decay curve showing the longest life time of radio-isotopes. [1]
d. The half-life of radium is 1620 years. After how many years 25% of a radium block remains undecayed? [3]
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