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Multiple choice question for engineering

Set 1

1. Ideally the voltage drop across a conducting diode must be
a) ∞
b) 0
c) higher than the forward biased voltage
d) equal to the forward biased voltage

View Answer

Answer: b [Reason:] Ideal conduction = No losses.

2. When reverse breakdown occurs in a diode
a) voltage increases & current is constant
b) voltage increases & current also increases
c) both are constant
d) voltage is constant & current increases

View Answer

Answer: d [Reason:] Recall the I-V curve of a diode in the 3rd quadrant.

3. Schottky diodes are also called as
a) metal diode
b) hot carrier diode
c) signaling diode
d) easy turn on diode

View Answer

Answer: b [Reason:] Due to the metal used to carry the current, it is also called as a hot carrier diode.

4. In a Schottky diode, the aluminum metal acts as a __________
a) anode
b) cathode
c) gate
d) common terminal

View Answer

Answer: a [Reason:] The Al (Metal) always acts as the anode.

5. If the doping levels of the semiconductor is increased, then the width of the depletion layer
a) increases
b) decreases
c) is unchanged
d) keeps oscillating

View Answer

Answer: b [Reason:] Higher the doping, more the number of charge carrier available to neutralize the opposite charges on the junction.

6. As compared to a p-n junction device of equal rating, the Schottky diode has
a) lower reverse voltage rating
b) lower reverse leakage current
c) higher Switching time
d) higher cut-in voltage

View Answer

Answer: a [Reason:] Low reverse voltage rating is the only drawback against a p-n junction diode.

7. In a Schottky diode, the silicon layer acts as a _____________
a) anode
b) cathode
c) gate
d) common terminal

View Answer

Answer: b [Reason:] The metal acts as the anode and the semiconductor as a cathode.

8. In a certain power electronics application, it is required that the voltage at the load terminals is to be kept within a certain range of voltages only. Among the device listed below, which would be the most ideal choice for this application?
a) P-n junction diode
b) Schottky diode
c) Zener diode
d) Fast recover diode

View Answer

Answer: c [Reason:] Zener diode is used as a voltage regulating device.

9. Shown below is the diagram of an ideal super diode. When the input voltage Vin is positive, then the output voltage Vout = ?
power-electronics-mcqs-types-power-diodes-2-q9
a) Vout/Vin
b) Vin
c) 0
d) Vin * Rl

View Answer

Answer: b [Reason:] Input is positive, hence it is amplified by the operational amplifier which switches the diode on. Current flows through the load & because of the feedback, the output voltage is equal to the input voltage.

10. Zener diodes allow a current to flow in the reverse direction, when the
a) voltage reaches above a certain value
b) temperature reaches above a certain value
c) current always flows in the reverse direction only
d) current cannot flow in the reverse direction

View Answer

Answer: a [Reason:] Zener diode has voltage regulating property. When voltage reaches above a certain value (Zener voltage), current starts to flow in the reverse direction.

Set 2

1. A gas turbine works on?
a) Carnot cycle
b) Brayton cycle
c) Dual cycle
d) Rankine cycle

View Answer

Answer: b [Reason:] A gas turbine works on 2 isentropic and 2 isobaric processes which is nothing else but Brayton cycle.

2. What is the range of Thermal efficiency in Brayton cycle?
a) 30-35%
b) 40-45%
c) 50-55%
d) 50-60%

View Answer

Answer: c [Reason:] Efficiency of Brayton cycle in actual working condition is found to be between 35 to 45%.

3. What is the maximum possible TIT(Turbine Inlet Temperature)?
a) 1000 K
b) 1500 K
c) 950 K
d) 1300 K

View Answer

Answer: d [Reason:] The maximum temperature is limited by metallurgical conditions.

4. A simple Brayton cycle uses Helium as the working fluid, has a maximum temperature of 1000 K, and a pressure ratio of 4. At the start of the compression, the Helium pressure and temperature are 50 kPa and 250 K. Based upon cold-air standard analysis assumptions, the thermal efficiency of the cycle is:
a) 0.13
b) 0.23
c) 0.43
d) 0.53

View Answer

Answer: d [Reason:] Using the pressure ratio = 1 – rp(1 – k) / k.

5. A simple Brayton cycle has a pressure ratio of 5 and a maximum temperature of 900 K. Air enters the compressor at 100 kPa, 300 K. Based upon cold-air standard analysis assumptions, the back-work ratio of this cycle is:
a) 0.23
b) 0.53
c) 0.48
d) 0.36

View Answer

Answer: b [Reason:] Using the pressure ratio and isentropic process relations, T2 = T1rp(k – 1) / k = 475 K and T4 = T3rp(1 – k) / k = 568 K. Application of the first law to the compressor and turbine processes and substituting the results into the back-work ratio definition produces rb = (T2 -T1) / (T3 -T4) = 0.53.

6. Which of the following methods can be used to increase efficiency of a Brayton cycle?
a) Regeneration
b) Increasing pressure ratio
c) Heat exchanger
d) All of the mentioned

View Answer

Answer: d [Reason:] A heat exchanger that acts as a counter-flow energy recovery device positioned within the supply and exhaust air streams of an air handling system, in order to recover the waste heat.

7. For the same maximum pressure and heat input, the most efficient cycle is?
a) Brayton cycle
b) Carnot cycle
c) Rankine cycle
d) Dual cycle

View Answer

Answer: a [Reason:] Efficiency of a Brayton cycle is directly proportional to pressure ratio.

8. The compression ratio is the ratio of?
a) Swept volume to total volume
b) Total volume to swept volume
c) Total volume to clearance volume
d) None of the mentioned

View Answer

Answer: a [Reason:] According to definition, compression ratio is the ratio of swept volume to total volume.

9. A Sterling engine uses an energy source whose temperature is 727°C and an energy sink whose temperature is 27°C. How much heat must be added to the engine to produce 1 unit of work?
a) 1.10
b) 1.19
c) 1.43
d) 2.0

View Answer

Answer: c [Reason:] Since the Sterling engine is totally reversible, its thermal efficiency is the same as that of a Carnot engine, η = 1 – Tl / Th = 0.7. Now, the definition of the thermal efficiency gives Qin / Wout = 1 / η = 1.43.

10. Which of the following is not a component of Brayton cycle?
a) Compressor
b) Combustion Chamber
c) Turbine
d) Pump

View Answer

Answer: d [Reason:] Pump is used in Rankine cycle to circulate water from Condenser to Boiler.

Set 3

1. Deaerator is an
a) closed heater
b) open heater
c) surface heater
d) none of the mentioned

View Answer

Answer: b [Reason:] One of the feedwater heater is a contact type open heater, known as deaerator.

2. What is the purpose of deaerator?
a) to remove the dissolved oxygen and carbon dioxide
b) to remove the dissolved nitrogen
c) to remove the dissolved impurities
d) to supply more oxygen to feedwater

View Answer

Answer: a [Reason:] Purpose of deaerator is to remove the dissolved oxygen and carbon dioxide in water which makes water corrosive.

3. What do you mean by vent condenser?
a) a condensing unit
b) a temperature measuring instrument
c) a heat exchanger
d) dissolves oxygen

View Answer

Answer: c [Reason:] Feedwater is passed through a heat exchanger commonly called as vent condenser.

4. How dissolved oxygen and carbon dioxides are removed?
a) by allowing it to fall from height
b) by series of chemical reactions
c) by condensing feedwater
d) by heating feedwater to saturation temperature

View Answer

Answer: d [Reason:] The solubility of these gases decreases with an increase in temperature.

5. What chemicals are added for residual dissolved oxygen and carbon dioxide?
a) sodium sulphite
b) hydrazine
c) alum
d) all of the mentioned

View Answer

Answer: d [Reason:] Na2SO3 and N2H4 are used to remove residual gasses.

6. Why deaerator is installed at a certain height from the pump?
a) to provide a net positive suction head
b) to maintain less pressure
c) to avoid leakage
d) to maintain pressure

View Answer

Answer: a [Reason:] Deaerator is installed at a certain height from the pump so as to maintain a optimum pressure before suction.

7. Why deaerator is not used in water cooled and moderated nuclear power plant?
a) radioactivity
b) not economical
c) not safe
d) not mechanically possible

View Answer

Answer: a [Reason:] deaerator is not used in water cooled and moderated nuclear power plant because of the concern regarding radioactivity release with deaeration.

8. Location of high pressure heaters
a) after the deaerator
b) before the deaerator
c) middle of deaerator
d) depends on pressure conditions

View Answer

Answer: b [Reason:] The feedwater heaters before the deaerator are often called high pressure heaters(h.p.).

9. Location of low pressure heaters
a) after the deaerator
b) before the deaerator
c) middle of deaerator
d) depends on pressure conditions

View Answer

Answer: a [Reason:] The feedwater heaters after the deaerator are often called low pressure heaters(l.p.).

10. Steam used to heat water comes from?
a) boiler
b) turbine
c) condenser
d) external power source

View Answer

Answer: b [Reason:] Steam from turbine is at high pressure and temperature.

Set 4

1. Feedwater heaters are of two types, they are
a) natural and forced
b) saturated and non saturated
c) open and close
d) water driven and steam driven

View Answer

Answer: c [Reason:] Feedwater heaters are classified by the methods of mixing steam and feedwater.

2. In open feedwater heater
a) the extracted steam is allowed to mix with feedwater
b) feedwater flows trough tubes and steam is condensed outside
c) steam flows outside tubes and feedwater inside
d) steam flows inside tubes and outside inside

View Answer

Answer: a [Reason:] In a closed heater, the extracted steam is allowed to mix with feedwater and both leave the heater at a common temperature.

3. Closed heaters are also called
a) contact type heater
b) shell and heat exchanger
c) drip heaters
d) shell and tube heater

View Answer

Answer: b [Reason:] Closed heaters are also called shell and heat exchanger where the feedwater flows through the tubes.

4. Expand TTD?
a) turbine temperature difference
b) total temperature difference
c) terminal temperature difference
d) turbine terminal difference

View Answer

Answer: c [Reason:] Terminal temperature difference = saturation temperature of bled steam – exit water temperature.

5. The value of TTD varies with?
a) heater pressure
b) heater temperature
c) turbine inlet temperature
d) independent of all mentioned

View Answer

Answer: a [Reason:] The value of TTD varies with heater pressure. heaters receiving wet steam, the TTD is positive.

6. Why at least one open heater is used?
a) installed vertically
b) feedwater deaeration
c) less cost
d) increase efficiency

View Answer

Answer: b [Reason:] The open heater works as a deaerator.

7. What are the advantages of open heater?
a) lower cost
b) high heat transfer capacity
c) high feedwater temperature
d) all of the mentioned

View Answer

Answer: d [Reason:] Open heaters are cheap as compared to closed heaters, in closed heaters feedwater temperature is not high.

8. What do you mean by heater drip?
a) condensate
b) heater failure
c) extract
d) maximum temperature

View Answer

Answer: a [Reason:] Saturated water at the steam extraction pressure.

9. Which of the following material can be used as a moderator?
a) Beryllium
b) Graphite
c) Heavy water
d) All of the mentioned

View Answer

Answer: d [Reason:] A neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.

10. Closed heaters are?
a) horizontal
b) vertical
c) inclined
d) depends on requirement

View Answer

Answer: d [Reason:] Closed heaters are mostly horizontal. Sometimes, they are made vertical to reduce the floor areas.

Set 5

1. For a given value of mean temperature of heat addition(Tm)
a) efficiency of Rankine is more if turbine exit temperature(Tmin) is less
b) efficiency of Rankine is less if turbine exit temperature(Tmin) is less
c) efficiency does not depend on turbine exit temperature
d) none of the mentioned

View Answer

Answer: a [Reason:] Efficiency = 1 – Tmin / Tm.

2. Mean temperature of heat addition can be increased by?
a) decreasing the amount of heat added
b) increase the amount of heat added
c) increase condensor pressure
d) decreasing Turbine Inlet Temperature(TIT)

View Answer

Answer: c [Reason:] Increasing condensor pressure decreases turbine exit temperature which in turn increases mean temperature of heat addition.

3. Internal irreversibility of Rankine cycle is caused by?
a) fluid friction
b) throttling and mixing
c) expansion and compression process are quite rapid
d) All of the mentioned

View Answer

Answer: d [Reason:] Practical analysis shows that Internal irreversibility of Rankine cycle depends upon above stated factors.

4. Which of the following statement is true?
a) efficiency is directly proportional to mean temperature of heat addition
b) efficiency is inversely proportional to mean temperature of heat addition
c) efficiency is exponential function of mean temperature of heat addition
d) efficiency does not depend on mean temperature of heat addition

View Answer

Answer: a [Reason:] Efficiency = 1 – Tmin / Tm.

5. Pulverized coal is
a) coal free from ash
b) non-smoking coal
c) coal which bums For long time
d) coal broken into fine particles

View Answer

Answer: d [Reason:] Pulverized coal also known as powdered coal or coal dust since it is as fine as face powder in cosmetic make-up.

6. What is caking in Power Plants?
a) making cakes of coal out of fine powders
b) a variety of free burning coals
c) coal produced from burning of wood in inert atmosphere
d) in boiler furnace some coals become plastic and form lumps or masses of coal

View Answer

Answer: d [Reason:] When many bituminous coals are heated, they soften and form a plastic mass that swells and re solidifies into a porous solid.

7. Water gas is a mixture of
a) CO2 and O2
b) O2and H2
c) H2, N2 and O2
d) CO, N2 and H2

View Answer

Answer: b [Reason:] Water gas is basically HHO that is H2 and 02.

8. Critical pressure of water is
a) 1 kg / cm2
b) 100 kg / cm2
c) 155 kg / cm2
d) 217.7 kg / cm2

View Answer

Answer: d [Reason:] The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature.

9. Coal used in power plant is also known as
a) steam coal
b) charcoal
c) coke
d) soft coal

View Answer

Answer: a [Reason:] Because this coal is used to generate steam.

10. The temperature to which air must be cooled in order to become saturated is the
a) minimum temperature
b) dew point temperature
c) wet-bulb temperature
d) freezing point

View Answer

Answer: b [Reason:] The dew point is the temperature at which the water vapour in air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates.