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# Multiple choice question for engineering

## Set 1

1. A source of sound frequency 600Hz is placed inside water. The speed of sound in water is 1500 m/s and in air it is 300 m/s. The frequency of sound is recorded by an observer, who is standing in air, is
a) 200 Hz
b) 3000 Hz
c) 120 Hz
d) 600 Hz

Answer: d [Reason:] The frequency of sound does not change during its refraction from water into air.

2. The ratio of the speed of sound in nitrogen gas to that in the helium gas at 300 K is
a) √(2/7)
b) √(1/7)
c) √3/5
d) √6/5

Answer: c [Reason:] v(N2)/vHe =√(γ(N2)He ×MHe/M(N2) )=√((7/5)/(5/3)×4/28)=√3/5

3. Two monatomic ideal gases 1 and 2 of molecular masses M1 and M2 respectively are enclosed in separate containers kept at the same temperature. The ratio of the speed pf spund in gas 1 to that in gas 2 is given by
a) √(M1/M2)
b) √(M2/M1)
c) M1/M2
d) M2/M1

Answer: b [Reason:] v=√(3RT/M) v1/v2 =√(M2/M1)

4. The extension in a string, obeying Hooke’s law, is x. The speed of sound in the stretched string is v. If the extension in the spring is increased to 1.5x, the speed of sound will be
a) 1.22v
b) 0.16v
c) 1.50v
d) 0.75v

Answer: c [Reason:] According to Hooke’s law, Tension (T) is proportional to Extension (x) Also, speed of sound, v∝√T v∝√1.5x v/v=√1.5=1.22 v=1.22v

5. A travelling wave in a stretched string is described by the equation y = Asin(kx-ωt). The maximum particle velocity is
a) Aω
b) ω/k
c) dω/dk
d) x/t

Answer: a [Reason:] y=Asin(kx-ωt) u=dy/dt=-ωAcos(kx-ωt) umax=ωA

6. A wave represented by the equation y=acos(kx-ωt) is superposed with another wave to form a stationary wave such that point x=0 is a node. The equation for the other wave is
a) acos(kx-ωt)
b) -acos(kx-ωt)
c) -acos(kx+ωt)
d) -asin(kx-ωt)

Answer: c [Reason:] For the formation of a stationary wave, two identical waves travelling in opposite directions must separate each other. At x=0, resultant y should be zero for getting a node. Hence -acos(kx+ωt) is the correct answer.

7. Two vibrating springs of the same material but lengths L and 2L have radii 2r and r respectively. They are stretched under the same tension. Both the strings vibrate in their fundamental modes, one of the lengths L with frequency f1 and the other with frequency f2. The ration is given by
a) 2
b) 4
c) 8
d) 1

Answer: d [Reason:] f=1/2l×√(T/m)=1/2l×√(T/(πR2 ρ))=1/2lR×√(T/πρ) As T and ρ are the same for both strings, f∝1/lR For the first string, f1∝1/L2r For the second string f2∝1/2Lr Hence f1/f2 =1

8. In the experiment to determine the speed of sound using a resonance column
a) Prongs of the tuning fork are kept in a vertical plane
b) Prongs of the tuning fork are kept in horizontal plane
c) In one of the two resonances observed, the length of the resonating air column is close to the wavelength of sound in air
d) In one of the two resonances observed, the length of the resonating air column is close to half of the wavelength of sound in air

Answer: a [Reason:] The prongs of vibrating tuning fork are kept in a vertical plane just above opening of the resonance tube

9. In a resonance tube with tuning fork of frequency 512Hz, first resonance occurs at water level equal to 30.3cm and second resonance occurs at 63.7cm. The maximum possible error in the speed of sound is
a) 51.2 cm/s
b) 102.4m/s
c) 204.8cm/s
d) 1536cm/s

Answer: a [Reason:] v= 200Hz

10. In the experiment for the determination of the speed of sound in air using the resonance column method, the length of the air column that resonates in the fundamental mode, with a tuning fork is 0.1m. When this length is changed to 0.35m, the same tuning fork resonates with the first overtime. Calculate the end correction
a) 0.012m
b) 0.025m
c) 0.05m
d) 0.024m

Answer: b [Reason:] End correction=(l2-3l1)/2 =(0.35-3×0.1)/2=0.025m

## Set 2

1. A quantity of heat required to change the unit mass of solid substance, from solid state to liquid state, while the temperature remains constant, is known as
a) Latent heat
b) Sublimation
c) Hoar frost
d) Latent heat of fusion

Answer: d [Reason:] The amount of heat required to change the state of unit mass of a substance from solid at its melting point is called latent heat of fusion.

2. A constant pressure air thermometer gave a reading of 47.5 units of volume, when immersed in ice-cold water, and 67 units in a boiling liquid. The boiling point of the liquid is
a) 100°C
b) 112°C
c) 125°C
d) 135°C

Answer: b [Reason:] From Charles’s law, V1/T1 =V2/T2 47.5/(0+273)=67/T2 T2=(67×273)/47.5=385K T2=112°C

3. The bulb of one thermometer is spherical, while that of other is cylindrical. If both of them have equal amount of mercury, which one will respond quickly to the temperature?
a) Spherical
b) Cylindrical
c) Elliptical
d) Circle

Answer: b [Reason:] As the square area of cylindrical bulb is larger than a spherical bulb, heat will be transmitted quickly through a cylindrical bulb and it will respond quickly to the temperature.

4. A black body at a high temperature T K radiates energy at the rate of E Wm-2. When the temperature falls to T/2 K, the radiated energy will be
a) E/4
b) E/2
c) 2E
d) E/16

Answer: d [Reason:] E/E=((T/2)/T)4=1/16 E=E/16

5. On increasing the temperature of a substance gradually, its colour becomes
a) Red
b) Green
c) Yellow
d) White

Answer: c [Reason:] By Wien’s law ʎm is proportioanl to 1/T On increasing temperature, wavelength decreases. As yellow colour has minimum wavelength, the substance gradually becomes yellow.

6. Assertion: Good conductors of heat are also good conductors of electricity and vice-versa
Reasons: Mainly electrons are responsible for these conductors
a) Both assertion and reason are true and reason is the correct explanation of the assertion
b) Both assertion and reason are true but reason is not correct explanation of the assertion
c) Assertion is true but reason is false
d) Both assertion and reason is false

Answer: a [Reason:] Both the assertion and reason are true. Good conductors have a large number of free electrons which make them good conductors both heat and electricity.

7. A centigrade and Fahrenheit thermometers are dipped in boiling water. The water temperature is lowered until the Fahrenheit thermometer registers 140°. What is the fall in temperature registered by the centigrade thermometer?
a) 80°C
b) 60°C
c) 360°C
d) 30°C

Answer: c [Reason:] C/5=(F-32)/9=(140-32)/9=108/2=12 C=60°C On centigrade scale, the boiling point of water is 100. Therefore, fall in temperature registered by the centigrade thermometer = 100-60 = 40°C

8. If 1g of steam is mixed with 1g of ice, the resultant temperature of the mixture is
a) 270°
b) 230°
c) 100°
d) 50°

Answer: c [Reason:] Heat required to change 1g of ice at 0°C first into water at 0°C and then into water at 100°C, Q=mL+mc∆T=1×80+1×1×100=180cal Now 1g of steam carries 540cal of heat, when 1g of steam is mixed with 1g of ice, entire steam will not condense. Finally, we will have a mixture of water and steam at 100°C.

9. A beaker full of hot water is kept in a room. If it cools from 80°C to 75°C in t1 minutes, from 75°C to 70°C in t2 minutes and from 70°C to 65°C in t3 minutes, then
a) t1=t2=t3
b) t1 < t2 = t3
c) t1 < t2 < t3
d) t1 > t2 > t3

Answer: c [Reason:] According to Newton’s law of cooling, Rate of cooling is proportional to temperature difference of the body and its surroundings. As the temperature of the body approaches the room temperature, rate of cooling decreases. t1 < t2 < t3

10. A black body is at 727°C. It emits energy at a rate, which is proportional to
a) (727)2
b) (1000)2
c) (727)4
d) (1000)4

Answer: d [Reason:] Eα(273+727)4 Eα(1000)4

11. Gas thermometers are more sensitive than mercury thermometer. True or false?
a) True
b) False

Answer: a [Reason:] The coefficient of expansion of a gas in very large as compared to the coefficient of expansion of mercury. For the same temperature range, a gas would undergo a much larger change in volume as compared to mercury.

12. A spherical body of area A and emissivity e = 0.6 is kept inside a perfectly black body. Energy radiated per second by the body at temperature T is
a) 0.4σAT4
b) 0.8 σAT4
c) 0.6 σAT4
d) 1.0 σAT4

Answer: c [Reason:] According to Stefan’s law, energy radiated per second by a body of emissivity e at a temperature T, E = e σAT4 E = 0.6 σAT4

## Set 3

1. In a given process of an ideal gas, dW=0 and the dQ<0. Then for the gas
a) The temperature will decreases
b) The temperature will increase
c) The pressure will remain constant
d) The temperature will increase

Answer: a [Reason:] By first law of thermodynamics, dQ = dU+dW As dW = 0 and dQ<0, so dU<0 But for an ideal gas, U is proportional to T dT<0 Hence temperature of the gas decreases.

2. The temperature of reservoir of Carnot’s engine operating with an efficiency of 70% is 1000K. The temperature of source is
a) 300K
b) 400K
c) 500K
d) 700K

Answer: a [Reason:] 0.70 = 1-T2/1000 T2=0.30×1000=300K

3. The temperature of sink of Carnot engine is 27°C. Efficiency is 25%. Then
a) 227°C
b) 327°C
c) 127°C
d) 27°C

Answer: c [Reason:] 25/100=1-300/T1 300/T1 =1-1/4=3/4 T1=(300×4)/3=400K=127°C

4. The freezer in a refrigerator is located at the top section so that
a) The entire chamber of the refrigerator is cooled quickly due to convection
b) The motor is not heated
c) The heat gained from the environment is high
d) The heat gained from the environment is low

Answer: a [Reason:] The upper cold denser air goes down while the lower warmer air goes up. The convection currents so set up quickly cool the chamber of the refrigerator.

5. Which of the following is not a thermodynamics co-ordinate?
a) P
b) T
c) V
d) R

Answer: d [Reason:] P, T and V are thermodynamic variables while the gas constant R is not thermodynamics coordinate.

6. A perfect gas is contained in a cylinder kept in vacuum. If the cylinder suddenly bursts, then the temperature of the gas
a) Becomes zero K
b) Is decreased
c) In increased
d) Remains unchanged

Answer: d [Reason:] No work is done during the free expansion of a gas. There is no change in internal energy or temperature of the gas.

7. The increase in internal energy of a system is equal to the work done in the system. Which process does the system undergo?
a) Isochoric
c) Isobaric
d) Isothermal

Answer: b [Reason:] As work done on a system is negative, so ∆U=-∆W or ∆U+∆W=0 or ∆Q=0. Hence the system undergoes an adiabatic process.

8. If the temperature of the source is increased, the efficiency of a Carnot engine
a) Increases
b) Decreases
c) Remains constant
d) First increases and then remains constant

Answer: a [Reason:] Efficiency, Ƞ=1-T2/T1 When the temperature T1 of the source increases, the efficiency increases.

9. Assertion: In adiabatic compression, the internal energy and temperature of the system gets decreased [Reason:] The adiabatic compression is a slow process
a) Both assertion and reason are true and reason is correct explanation of the assertion
b) Both assertion and reason are true but reason is not correct explanation of the assertion
c) Assertion is true but reason is false
d) Both assertion and reason are false

Answer: d [Reason:] Both the assertion and reason are false. During adiabatic compression, work is done on the system. Both the internal energy and temperature of the system increases.

10. The efficiency of Carnot’s engine operating between reservoirs, maintained at temperature 27°C and -123°C is
a) 50%
b) 24%
c) 0.75%
d) 0.4%

Answer: a [Reason:] Ƞ=1-T2/T1 Ƞ=1-(273-123)/(273+27)=1-150/300=1/2=50%

11. The temperature of sink of Carnot’s engine is 27°C. If the efficiency of the engine is 25%, then the temperature of source must be
a) 27°C
b) 127°C
c) 227°C
d) 327°C

Answer: b [Reason:] Ƞ=1- T2/T1 0.25=1- (273+27)/T1 T1=300/0.75=400K=127℃

## Set 4

1. In which of the following the magnetic moments align themselves parallel to each other?
a) Paramagnetic material
b) Ferromagnetic material
c) Ferrimagnetic material
d) Diamagnetic material

Answer: b [Reason:] In a ferromagnetic material, the number of unpaired electrons is more. Most of these spin magnetic moments point in one direction. Hence even in the absence of external field, the magnetic moments align themselves parallel to each other and give rise to magnetic field.

2. Which of the following is a strong magnet?
a) Diamagnetic material
b) Paramagnetic material
c) Antiferromagnetic material
d) Ferromagnetic material

Answer: d [Reason:] The ferromagnetic materials have permanent dipole moment. So they act as strong magnets.

3. Which of the following exhibits spontaneous magnetisation?
a) Paramagnetic material
b) Ferrimagnetic material
c) Diamagnetic material
d) Ferromagnetic material

Answer: d [Reason:] The ferromagnetic materials exhibit magnetisation even in the absence of external field. This property is called spontaneous magnetisation. Hence ferromagnets exhibit spontaneous magnetisation.

4. When does a ferromagnetic material become paramagnetic material?
a) At Curie temperature
b) Below Curie temperature
c) Above Curies temperature
d) Never

Answer: c [Reason:] Curie temperature is the temperature at which the magnetic properties of a material changes. When the temperature is greater than curie temperature, ferromagnetic material becomes paramagnetic material.

5. Which of the following materials have permanent magnetic moment?
a) Ferromagnetic material
b) Ferrimagnetic material
c) Diamagnetic material
d) Paramagnetic material

Answer: a [Reason:] In ferromagnetic material there will be large number of unequal electron spins and hence there exists enormous amount of permanent magnetic moment.

6. In which of the following the increases and then decreases?
a) Ferromagnetic material
b) Antiferromagnetic material
c) Paramagnetic material
d) Diamagnetic material

Answer: b [Reason:] The susceptibility is very small and is positive. It is given by, susceptibility = C/(T+θ) for T>TN. Where, TN is the Neel temperature. Initially the susceptibility increases slightly as the temperature increases and beyond a particular temperature, known as Neel temperature, the susceptibility decreases with temperature.

7. What is the material used in two port device?
a) Ferromagnets
b) Ferrites
c) Antiferromagnets
d) Paramagnets

Answer: b [Reason:] The ferrites have low hysteresis loss and eddy current loss. Hence they are used in two port devices such as gyrator, circulator and isolator.

8. A paramagnetic material had a magnetic field intensity of 104 A/m. If the susceptibility of the material at room temperature is 3.7×10-3, calculate the magnetisation.
a) 37A/m
b) 3.7A/m
c) 370A/m
d) 0

Answer: a [Reason:] Susceptibility = I/H Intensity of magnetisation I = Susceptibility × H Intensity of magnetisation = 37A/m

9. A paramagnetic material had a magnetic field intensity of 104 A/m. If the susceptibility of the material at room temperature is 3.7×10-3, calculate the flux density in the material.
a) 37Wb/m2
b) 3.725Wb/m2
c) 0.012612Wb/m2
d) 0

Answer: c [Reason:] Flux density B = μ0[H+I] B = 0.012612Wb/m2

10. A piece of ferric oxide with magnetic field intensity 106 A/m and susceptibility is 1.5 ×10-3. Find the magnetisation of the material.
a) 15A/m
b) 150A/m
c) 1.5A/m
d) 1500A/m

Answer: d [Reason:] I = Susceptibility × H I = 1500A/m

## Set 5

1. Clinical thermometer can be sterilized by boiling. True or false?
a) True
b) False

Answer: b [Reason:] The range of clinical thermometer is usually from 95°F to 110°F and the boiling point of water is 212°F. So on sterilization by boiling, the capillary of the thermometer will burst due to thermal expansion of mercury in the capillary.

2. A pressure cooker reduces cooking time for food, because
a) Heat is more evenly distributed in the cooking space
b) The higher pressure inside the cooker crushes the food material
c) Cooking involves chemical changes helped by a rise in temperature
d) Boiling point of water involved in cooking is increased

Answer: d [Reason:] As the pressure inside the pressure cooker increases, the boiling point of water increases.

3. Which of the following is more close to a black body?
a) Blackboard paint
b) Green leaves
c) Black holes
d) Red roses

Answer: a [Reason:] Blackboard paint is close to a black body. A good absorber is good emitter but a black hole does not emit all radiations.

4. Heat current is maximum in which of the following?
a) Copper
b) Steel and copper
c) Copper and steel
d) Steel

Answer: a [Reason:] Heat current, dQ/dt is proportional to K As the thermal conductivity of copper is maximum, heat current will be maximum in copper.

5. The sprinkling of water reduces slightly the temperature of a closed room because
a) Temperature of water is less than that of the room
b) Specific heat of water is high
c) Water has large latent heat of vaporisation
d) Water is a bad conductor of heat

Answer: c [Reason:] Water has a large latent heat of vaporisation. When it is sprinkled over a large area, its vaporisation occurs which, in turn, causes cooling.

6. At a common temperature, a block of wood and a block of metal feel equally cold or hot. The temperatures of block and wood are
a) Equal to the temperature of the body
b) Less than the temperature of the body
c) Greater than the temperature of the body
d) Has no change

Answer: a [Reason:] The temperature of the block and wood are equal to the temperature of the body as both feel equally hot or cold.

7. On a cold morning, a metal surface will feel colder to touch than a wooden surface, because
a) Metal has high specific heat
b) Metal has high thermal conductivity
c) Metal has low specific heat
d) Metal has low thermal conductivity

Answer: b [Reason:] Due to higher thermal conductivity of metal than wood, heat begins to flow readily from our body to the metal surface and so we feel colder.

8. Woollen clothes keep the body warm, because wool
a) Is a bad conductor
b) Increases the temperature of body
c) Decreases the temperature
d) Has no effect

Answer: a [Reason:] Woollen fibres enclose a large amount of air in them. Both wool and air are bad conductors of heat and do not allow loss of heat from our body due to conduction.

9. Ratio of the amount of heat radiation, transmitted through the body to the amount of heat radiation incident on it, is known as
a) Conductance
b) Inductance
c) Transmittance
d) Absorbance

Answer: c [Reason:] The ratio of amount of heat transmitted through an object to the amount of heat incident on it is called transmittance.

10. If the amount of heat energy received per unit area from sun is measured on earth, mars and Jupiter, it will be
a) The same for all
b) In decreasing order Jupiter, Mars, Earth
c) In increasing order Jupiter, Mars, Earth
d) In decreasing order Mars, Earth, Jupiter

Answer: c [Reason:] Heat energy received on a planet varies inversely as the square of the distance from the sun. So Jupiter is lesser than Mars is lesser than Earth

11. Assertion: For higher temperatures, the peak emission wavelength of a black body shifts to lower wavelengths [Reason:] Peak emission wavelength of black body is proportional to the fourth power of temperatures
a) Both assertion and reason are true and reason is the correct explanation of the assertion
b) Both assertion and reason are true but reason is not correct explanation of the assertion
c) Assertion is true but reason is false
d) Both assertion and reason is false