Quiz: Mechanical Engineering 8 June 2020

Quiz: Mechanical Engineering
Exam: UPSSSC JE
Topic: Miscellaneous

Each question carries 1 mark
Negative marking: 1/3 mark
Time: 10 Minutes
Q1. A metal block is thrown into deep lake. As it sinks deeper in water, the buoyant force acting on it
(a) increases
(b) remains the same
(c) decreases
(d) first increases and then decreases

Q2. Which of the following statements is wrong?
(a) Streamlines cannot start or end anywhere except at the interface or infinity.
(b) Streamline spacing varies directly as the flow velocity.
(c) Streamlines can meet at a stagnation point where the velocity is zero.
(d) The flow is only along the streamline and not across it.

Q3. In a Carnot engine, when working substance gives heat to the sink
(a) temperature of the sink increases
(b) temperature of the sink remains same
(c) temperature of the source decreases
(d) temperature of both the sink and source decreases

Q4. Three engines A, B and C operating on Carnot cycle respectively use air, Nitrogen and Argon as the working fluid. If all the engine operates within the same temperature limit, then which engine will have highest efficiency.
(a) Engine A
(b) Engine B
(c) Engine C
(d) All engines will have same efficiency

Q5. When a system is in equilibrium, any conceivable change in entropy would be
(a) maximum
(b) zero
(c) positive
(d) negative

Q6. Which of the following is false statements?
Some of the methods used to reduce diesel smoke are as follows
(a) using additives in the fuel
(b) engine derating i.e. reducing the maximum flow of fuel
(c) increasing the compression ratio
(d) avoidance of overloading

Q7. The instrument preferred in the measurement of highly fluctuating velocities in air flows is
(a) Pitot-static tube
(b) Propeller type anemometer
(c) Three cup anemometer
(d) Hot wire anemometer

Q8. Perpetual motion machine of second kind violates the
(a) First law of thermodynamics
(b) Kelvin-Planck statement
(c) Clausius statement
(d) Third law of thermodynamics

Q9. Second law of thermodynamics defines
(a) Heat
(b) Work
(c) Enthalpy
(d) Entropy

Q10. Consider the following statements pertaining to large heat transfer rate using fins
1. Fins should be used on the side where heat transfer coefficient is small.
2. Long and thick fins should be used.
3. Short and thin fins should be used.
4. Thermal conductivity of fin material should be large.
Which of the above statements are correct?
(a) 1, 2 and 3
(b) 1, 2 and 4
(c) 2, 3 and 4
(d) 1, 3 and 4

SOLUTION

S1. Ans (b)
Sol. As buoyant force depends only on the displaced volume of fluid by the submerged or floating body according to Archimedes principle. So, volume displaced by the metal block will remain constant from when it sinks in the fluid till it sinks deeper, hence, the buoyant force on the metal block remains same.

S2. Ans (b)
Sol. Streamline spacing is inversely proportional to the velocity, it means for accelerated flow spacing between streamlines is less and for retarded flow, this spacing will be more.

S3. Ans (b)
Sol. When working substance gives heat to the sink then its temperature remains same
because sink and source both are considered as reservoir of heat.

S4. Ans (d)
Sol. As efficiencies of Carnot engines working between same temperature limits do not depend upon working fluid of engines.
So, engine A, B and C all three will have same efficiency.

S5. Ans (b)
Sol. When a system is in equilibrium, any conceivable change in entropy would be zero.
Because when a system reaches in equilibrium state then its entropy will be maximum at that state.

S6. Ans (c)
Sol. Increasing the compression ratio is not recommended to reduce diesel smoke as it increases the weight of the engine and cost which is not worthy for reducing diesel smoke.

S7. Ans (d)
Sol. Hot wire anemometer is used for the measurement of highly fluctuating velocities in air. This can be done by measuring the heat loss of the wire which is placed in the fluid stream. The wire is heated by electrical current. The hot wire when placed in the stream of the fluid, in that case, the heat is transferred from wire to fluid, and hence the temperature of wire reduces. The resistance of wire measures the flow rate of the fluid which is the function of heat loss from the wire and change in the resistance of wire.

S8. Ans (b)
Sol. Perpetual motion machine of the second kind is a machine which produces work while exchanging heat from a single heat source. Which violates Kelvin-Planck statement of Second law of thermodynamics.

S9. Ans (d)
Sol. Second law of thermodynamics defines Entropy or energy degradation.
As second law of thermodynamics states that Heat (low grade energy) cannot be converted into work (high grade energy) completely because it violates Kelvin-Planck statement.

S10. Ans (d)
Sol. Fins are used where thermal conductivity of material is high and coefficient of heat transfer is small. We should use short and thin fins to increase heat transfer area exposed to environment.

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