# Physics Most important Question For NEB Class 12

Chapterwise Most important Questions According To new Syllabus of Physics Class 12

## Most important questions From Fluid Statics

1. What is surface tension? Does it depend on surface area of fluid? Use the concept to find excess pressure in fluid drop to calculate surface tension in capillary rise method.  [1+1+3]

2. State pascals law. How fluid pressure is independent of area of container. To what height should a cylindrical vessel be filled with a liquid to make the force on the side of vessel will be equal to the force on the bottom of the vessel. [1+2+2]

3. State pascals law. Prove that fluid pressure is = ℎ where the symbol carries usual meaning. A ′′ − of horizontal portion 8 cm having uniform cross-section is filled with water from one end and glycerol (′ = 0.8  SHAPE  \* MERGEFORMAT ′) is powered from other end such that each fluid occupied 22cm of the tube. Find the position of common surface of two liquids. [1+2+2]

4. State Archimedes principle. Is it applicable for floating object? A piece of alloy of copper ( = 8.9 ) and zinc ( = 7.1 ) weighs 12.9 g in air completely immersed in water resulting weight 11.3 g in water. Find the fraction of mass metal in alloy. [1+1+3]

5. State law of floatation. Is body in equilibrium in floatation? A piece of cube of wood supporting a 200 g mass just floats in water. When the mass is removed, the cube rises by 2 cm. what is the size of cube? [1+2+2]

6. Define streamlined flow. How does streamlined flow turns into turbulent? It is desired to design an airplane with a lift 1000N per meter square with the wing area. If the velocity of air passing from lower surface of wing is 150 SHAPE  \* MERGEFORMAT , what should be the velocity at upper surface. Given that = 1.3 −3. [1+1+3]

7. What is viscosity of fluid. How does it change with temperature? An air bubble of radius 1 cm rises with constant speed of 3.5 mm/s through a liquid of density of 1.75 × 103  SHAPE  \* MERGEFORMAT 3. find the coefficient of viscosity of liquid. Neglect the density of air. [1+1+3]

## Most important questions From First law of thermodynamics

1. What is meant by thermodynamical variable? What comprises internal energy of system? Water falls from 350 high, find the temperature change assuming that the whole of energy due to fall converted into heat. = 4200−1. [1+2+2]

2. What is molar heat capacity of gas? Why is the molar heat capacity at constant pressure more that constant volume? 105 calories of heat is required to raise the temperature of 3 moles of ideal gas at constant pressure from 25 C to 30 C. calculate the amount of heat required to raise the temperature of same ga through the same range of temperature at constant volume. [1+2+2]

3. what do you mean by isothermal process. Derive relation to find work done by gas expanded from pressure ′1′ to ′2′ at constant temperature ′′.  If 5 moles of gas at 1 atm is expanded from 50 liter from 5 liter at constant temperature of 20 C. find the work done by the gas. [1+2+2]

4. State the significance of first law of thermodynamics. In constant -volume process,          = holds true but in constant-pressure process = is not true, why? Two mole of ideal gas is heated from 250 to 500  at constant pressure. Draw P-V diagram for the process and calculate the work done. [1+2+1+1]

5. State the significance P-V diagram in thermodynamics. The P-V diagram alongside shows a process involving 0.45 mol of ideal gas. What is the temperature at point ′′ and ′′. How much work was done by or on the gas in this process. How much heat is to be supplied to increase the internal energy of gas by 15000J? [2+3]

6. What do you mean by adiabatic process. The equation of state of adiabatic process can be expressed as = . Using this relation show that the work done in the process is given by = SHAPE  \* MERGEFORMAT −1 (1 − 2) where the symbol carries usual meaning. During adiabatic expansion of 0.5 mol of Argon gas the temperature falls to 70 from 520, find the work done.

## Most important questions From Second law of thermodynamics

1. State Kelvin’s statement of second law of thermodynamics. If a Carnot’s engine is operating between temperature 1 and 2, that is its efficiency? A heat engine absorbs 500 calories heat from source at 200° and rejects 200 calories to the sink whose efficiency is 75%. Find the temperature of sink and by what magnitude of sink be lowered so that efficiency reaches 85%. [1+1+3]

2. What is petrol engine? Why is spark plug is needed in petrol engine? Sketch the P-V diagram for otto cycle. Write the relation of efficiency of petrol engine in term of compression ratio. [1+1+2+1]

3. A P-V diagram of an engine is present alongside, which engine does it represent? Explain why? What does a straight line ′′ represent? Does this type of engine require spark plug, why/why not? Explain. [2+2+1]

4. What do you mean by coefficient of performance of refrigerator? Why is external agency needed to operate the refrigerator? A refrigerator absorbs 200 J of heat and power supply delivers 100 J of work to the machine. What is the co-efficient of performance of the machine? [1+2+2]

5. what is meant by entropy? Introduce the concept of entropy from first law of thermodynamics. Find the entropy of change in melting 1 kg of ice at 0° to the same temperature. [1+2+2]

6. Why is expansion stroke of engine called power stroke? Why should compression ratio be more in diesel engine? A petrol engine consumes 5 kg petrol per hour. If the power of engine 20 KW and calorific value of petrol is 11 × 103 k cal per kg. Calculate efficiency of engine. [1+1+3]

## Most important questions From Magnetic effect of current

1. The magnetic force experienced by a charge particle is ⃗ = (⃗ × ⃗⃗), justify that a proton entering in the magnetic field perpendicular to its velocity traces a circle. Can that proton experiences a zero force while moving inside the magnetic field, how? find the force experienced by a wire of length 5 m carrying a current of 2A inside magnetic field of 2T such that the area vector of cross-section wire is parallel to the current. [2+2+1]

2. What is Right-Thumb rule in magnetism? Explain.  Show that the magnetic force experienced by a charge particle in side magnetic field is ⃗ = (⃗ × ⃗⃗), where the symbol carries usual meaning. Use this expression to define 1T magnetic field at a point. [1+3+1]

3. State right hand palm rule of magnetic force. A student claim that the magnetic force on particle does no work however produces torque, explain how is it possible. An electron moves in the magnetic field with velocity 2 × 106 SHAPE  \* MERGEFORMAT   in the field region of 7.6 × 10−2   in unspecified direction. Find the maximum and minimum possible force on it. [1+2+2]

4.  what is Lorentz force? A student claim that the lightning having current in the order of 105 stokes the metal flag and the bole is bend, is student correct, how? A flat square surface with side length 2.5 cm is in − at = 0. Calculate the magnitude of flux through the surface by the field of ⃗⃗⃗ = 0.25 ̂ + 0.30 ̂ − 0.45 ̂. [1+2+2]

5.  A rectangular coil of length ′′ and breadth ′′ carries a current ′′ inside uniform magnetic field ′⃗⃗′.

a.   What is the force experienced by each arm of coil. 

b.  Is this force is capable of produce torque? Justify with calculation. 

c.   How is it useful in construction of moving coil galvanometer? Explain. 

6. What happen if a current carrying specimen is placed in magnetic field such that the magnetic field and current are perpendicular to each other? A copper strip of thickness 2mm and breadth of 2 cm carries a current 0.1 A and it is placed in magnetic field of 2T such that current is perpendicular magnetic field. The voltage difference across the upper and lower surface is 2 then find current density, electric field and hence the electron concentration. [2+3]

7. State Biot-Savarts law. For what kind of current, it is useful? Use to find the magnetic field at the center due to a current carrying circular coil. If the magnetic field produced at the center of a coil of 500 turns carrying 0.2 A current is 2.5 . Find the diameter of coil. [1+2+2]

8. State Ampere’s circuital law. Use it to find the magnetic field at the center of solenoid of length ′′ having ′′ turns of coil per unit length carrying current ′′ be each turn. A MRI machine possess 5 million turns of insulated copper wire of radius 0.5 m. To diagnosis the issue of a person in spinal cord, radiographer passes the current of 7.25 A in the coil. What is the magnetic field experienced by the body of patient lying in the center of the MRI machine?

9. Define 1A current in terms of magnetic force. Why parallel conductors carrying current in same direction attracts each other? Two parallel wires separated at distance 2.5 m exerts  5 × 10−5  SHAPE  \* MERGEFORMAT and one of them carries 0.5 A current, what’s the current in other? Is the current in these wire is same? [1+2+2]

## Most important questions From Electromagnetic induction

1. State Faradays law of electromagnetic induction? A bicycle spoke is spinning inside magnetic field such that the length of spoke is normal to field, how much induced emf will be produced. A plane of wing 40 m is travelling due to north through magnetic field of 5 × 10−4 of (Vertical component of Earth) at velocity of 500 SHAPE  \* MERGEFORMAT , what is the voltage developed across the wing. [1+2+2] ℎ

2. What do you mean by self-induction? What will be the self-induction of solenoid of ′′ turns and radius ′′ carrying current ′′. A 60V constant self-induced emf is to be maintained in a coil of 2 H self-inductance, how is it possible? [1+2+2]

3. What is mutual induction? How is it useful in transformer? A secondary coil maintains 400V when there is change of current in primary coil from 0 to 5A in . Find the co-efficient of mutual induction of coil. [1+2+2]

4. What do you mean by electromagnetic induction? What will be the induced current in a closed loop having number of turns ′′that suffers the flux change 1 to 2 in the interval of ′′. Develop the relation to find the induced emf in a straight metal of length ‘l’ moving in uniform magnetic field ‘B’ with velocity ‘v’ making angle ′′ with magnetic field. [1+1+3=5]

5. State Lenz law? How is it useful in understanding of electromagnetic induction? Does it hold good for conservation of energy? [1+2+2=5]

6. What do you mean by self-inductance? Develop the relation to find self-inductance of ideal solenoid. A air cored solenoid of length 0.5m and radius 2 cm has 100 turns per cm, what is its self-inductance? [0 = 4 × 10−7−1]  [1+2+2=5]

7. State faradays law of electromagnetic induction. How a student can use faradays law of electromagnetic induction to find the induced emf in a rotating coil placed in uniform magnetic field, develop the relation and state the nature of induced emf and current. Class 12 students were asked to find the number of turns of coil of dimension 75 cm x 1.5 m used in generator that produces 220 V (at maximum) required for college during power cut. Student are given that the uniform magnetic field inside generator to be 0.75 tesla and frequency of rotation to be 1500 revolution per minute. [1+3+1+3=8]

8. What is transformer? How is it useful in long-distance power transmission? A primary coil maintain 200 V has current 5A and it transmit power at its 85% efficiency at same voltage. What is the current in secondary coil?  [1+2+2]

9. Explain how solenoid stores energy which time varying current is passed through it. The energy stored in solenoid is = 1 SHAPE  \* MERGEFORMAT  2, use it to prove the energy density is =  SHAPE  \* MERGEFORMAT 1 2. A solenoid of                                        2                                                20self-inductance 500  and radius 15 mm carries a steady current 2A, what is the energy stored? Find the energy density too. [1+2+2]

## Most important questions From (AC) Alternating current

1. What is phasor? How can one represent alternating current with help of phasor? Why is rms value of AC is more useful (meaningful) in daily life than mean value? Explain.

2. What is inductive reactance? How is it useful in household appliances to reduce power consumption? An AC (220 , 50 ) is applied across the heater of 5 Ω resistance and the current should be maintained at 0.2 , what should be the inductance of coil connected in series with resistor?

3. The diagram alongside is the phasor of L-R circuit, is current leading the voltage? Why? The current amplitude in pure inductor of a radio receiver is to be 250 when the voltage amplitude is 3.60 V at frequency of 1.60 , what is the inductive reactance of coil and selfinductance of coil. [1+2+2]

4. What is electrical resonance? How do you achieve it in L-C-R series circuit? Obtain the expression for resonant frequency of such circuit. A radio possesses a resistor of 5Ω, capacitor of 850 and inductor of 0.25 H, at what frequency the radio will be tuned? Find the power of the circuit, current during tuning, phase angle between current and voltage too?  [1+2+2+3]

5. An AC Emf is represented by = 220 sin(314 ), state the nature of current. Justify that this current can passes through capacitor. Find the peak voltage and inductive reactance developed by the given AC supply in capacitor of 2. [1+2+2]

6. Define wattless current. Justify that an ideal solenoid consumes no power when current passes through it. How are quality factor and power factor differing, explain? [1+2+2]   SHAPE  \* MERGEFORMAT

7. The impedance of L-C-R series is given by = √( − )2 + 2  then sketch the variation of impedance with frequency. Can impedance of L-C-R circuit be zero, explain. If 220 be the rms voltage of supply is connected to L-C-R series circuit having resistance 50Ω, what is the current in circuit during resonance. [1+2+2]

## Most important questions From Wave optics

1.What do you mean by wavefront? How can we produce plane wavefront? Use wave concept of light to prove the refraction phenomenon. [1+1+3]

2. What is wave-particle duality of light mean? State the Huygens concept of light propagation and use to prove reflection of light. [1+2+2]

3.What do you mean by coherent light? How can we produce coherent light? The phase difference of interfering wave in Young’s Double slit experiment is = 2 (), whare the symbol has their usual meaning. Use it to show the fringe width of bright and dark band are same. [1+1+3]

4. What is diffraction of light. How is primary maximum produced in single slit diffraction? How wide is the central maximum peak on a screen 3m behind a 0.01 mm slit is illuminated by 500 nm light source? [1+2+2]

5. Why is diffraction of light is not evident in our daily life however we observe sound diffraction. In single slit diffraction, the angular location of nth secondary minimum is sin = ,where the symbol carries their usual meaning. Use it to find the angular width of central maximum and prove that the central maximum has width double the width of first secondary maximum. [2+3]

6. What is diffraction grating? How is grating considered as multiple slit diffraction? A light of wavelength 5792 ̇ is used to determine the grating element of unknown diffraction grating and third order spectrum is measured at an angle of 60.300 through spectrometer when the light fall normally on grating. Find grating element. [1+2+2]

7. What is meant by resolving power of instrument? Describe the Rayleigh criteria for resolution of objects. James Webb space satellite has disc of 6.5 diameter and operates at smallest wavelength of 0.6 . Find its resolving power. [1+2+2]

8. Light is transvers in nature, how is polarizer useful to understand it? Explain. State and explain the principle described by the diagram alongside. A parallel beam of unpolarized light is incident at angle of 550 on the surface of glass and undergone complete polarization. What is the refractive index of glass and angle of refraction in glass? [3+3+2]

1. At polarizing angle, the reflected and refracted ray are at

. 180                                  . 0                      . 90                                  .

2.Polarization phenomenon occurs in

.    b.   . ℎ.   .

c. Which of the following is correct in case of polarization phenomenon

. = 1      . = 1                    c. =         d. = 1

d. If whole the apparatus of Youngs experiment is placed in water, the fringe width of patterns

a. Increases             b. Decreases   c. doesn’t change        d. can’t be said

e. The limit of resolution of a meter radio telescope for radio wave of wavelength 10 cm is about

a. 17⁰      . 70        c. more than one arc-second     d. 1 arc-minute

f. As spherical wave propagates, its radius of curvature

a. decreases             b. increases                c. remains same    d. depends on medium

## Most important questions From Semiconductor 1. What is shown in diagram alongside? Draw the nature of output. Explain its working mechanism too. [1+2].  What happens if another diode is connected between ‘B’ and ‘L’ in same fashion to that of ‘crystal diode’, explain.   2. What do you mean by biasing of diode. How is the diode alongside is biased? Why? Discuss its mechanism to regulate voltage when the magnitude is variable in your supply. 3. What do you mean by digital logic gate? Which gate is shown in diagram alongside? Discuss its operation with required truth table. [1+1+3]

## Most important questions From Radioactivity

1. A ′′ particle is equivalent to electron however it’s not an electron, how? In an experiment of decay of radio-active material the following thing was observed. Name the particle ′′  and ′′ with justification. Explain why the particle ′′ remained undeviated? [1+1+2+1]

2. i. State the law of radio-activity. 

ii. Use this law and develop the relation = 0−, where the symbol has their usual meaning. 

iii. Develop the relation for half life from above equation. 

iv. A radio-active substance undergone decay at the rate of 1200 / to 120 / in 10 days. Find the decay constant and half life of the substance. 

3. i. Define one curie. 

ii. What does the graph alongside represent. Write the equation that satisfies the curve. [1+1]

iii. From graph, calculate half life and mean life of substance. 

iv. If 5 million particle of this substance is allowed to decay then how many particles be decayed after 300 seconds? 

## Most important questions From Recent trend in physics

1. What is seismic wave? How does P and S wave carry energy from hypocenter to epicenter? Why is love wave is more hazardous, explain. [1+2+2]

2.What is nano-technology? Discuss the advantages of nano-technology over other current technology of material and explain the application of it in future. [1+2+2]