Renewable Energy Sources Questions and Answers

31.

In the following fusion energy reaction \(D + T = {}_{0}n^{1} + X\) identify X

A.Li
B.H\(_{2}\)
C.He\(^{3}\)
D.He\(^{4}\)

Exam Year(s)

UGC NET - 2012,

Answer: D

In the D-T fusion reaction, deuterium and tritium fuse to give a neutron and helium-4: D + T = n + He-4, with release of about 17.6 MeV. Mass numbers balance, because deuterium (2) plus tritium (3) is 5, equal to the neutron (1) plus helium (4). Charges balance too, since 1 + 1 = 2 for the helium nucleus. He-3 would leave the mass number at 4, not 5, and lithium or H2 do not satisfy the conservation of mass number and charge.

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32.

Assertion (A) : Natural gas is a very attractive ecofriendly fuel.

Reason (R) : It produces few pollutants and less carbon dioxide per unit energy than any other fossil fuel on combustion. Identify the correct answer :
Codes :

A.Both (A) and (R) are true and (R) is the correct explanation of (A).
B.Both (A) and (R) are true and (R) is not the correct explanation of (A).
C.(A) is true but (R) is false.
D.(A) is false but (R) is true.

Exam Year(s)

UGC NET - 2012,

Answer: A

Both (A) and (R) are true and (R) is the correct explanation of (A). Natural gas is mostly methane and is regarded as an eco-friendly fossil fuel because it burns cleanly. It produces few pollutants, with little sulphur dioxide or particulate matter, and gives less carbon dioxide per unit of energy than coal or oil, because methane has a high hydrogen-to-carbon ratio. That is exactly why it is attractive, so (R) explains (A) and the choice that the reason is not the explanation is wrong.

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33.

The efficiency of a MHD power generator for applied electric field \(\vec{E}\), magnetic field \(\vec{B}\) and velocity of hot ionized gas \(\vec{U}\) is

A.\(\dfrac{|\vec{E}|}{|\vec{U}|\,|\vec{B}|}\)
B.\(\dfrac{|\vec{U}|\,|\vec{B}|}{|\vec{E}|}\)
C.\(\dfrac{|\vec{E}|^{2}}{|\vec{U}|\,|\vec{B}|}\)
D.\(\dfrac{|\vec{E}|^{2}}{|\vec{U}|\,|\bar{B}|^{2}}\)

Exam Year(s)

UGC NET - 2012,

Answer: A

In a magnetohydrodynamic generator the ionised gas moving at velocity U through magnetic field B induces an electric field of magnitude U times B. The efficiency is the ratio of the field actually applied across the load to this induced field.\[ \eta = \frac{|\vec{E}|}{|\vec{U}|\,|\vec{B}|} \]The reciprocal in the second option is greater than one in normal operation, and the options with E squared have the wrong dimensions for a dimensionless ratio, so the first is correct.

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34.

The maximum specific power output (p) from a MHD power generator varies with the velocity (u) of hot ionized gas as

A.\(p \propto u\)
B.\(p \propto u^{2}\)
C.\(p \propto u^{3/2}\)
D.\(p \propto u^{3}\)

Exam Year(s)

UGC NET - 2012,

Answer: B

The power output of an MHD generator per unit volume is proportional to the electrical conductivity of the gas and to the square of the velocity and of the magnetic field, \(P \propto \sigma u^{2} B^{2}\). So the maximum specific power varies as \(u^{2}\). The options with \(u\), \(u^{3/2}\) and \(u^{3}\) do not agree with this relation.

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35.

The slow neutrons initiating nuclear fission with \(\mathrm{U^{235}}\) have energies of the order of

A.0.25 MeV
B.0.15 MeV
C.0.25 eV
D.0.025 eV

Exam Year(s)

UGC NET - 2012,

Answer: D

Slow (thermal) neutrons, which are the ones that initiate fission of U-235, have an energy of about 0.025 eV, the average thermal energy at room temperature (\(kT\) at about 293 K). The values 0.25 MeV and 0.15 MeV are the energies of fast neutrons, and 0.25 eV is ten times the thermal value. The answer is 0.025 eV.

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36.

For energy to be produced from nuclear fusion of Deuterium (D) and Tritium (T), the mixture of D + T has to be heated up to energies of at least

A.1 KeV
B.10 KeV
C.500 eV
D.1 MeV

Exam Year(s)

UGC NET - 2012,

Answer: B

For fusion of deuterium and tritium the fuel has to be heated to temperatures of the order of 100 million kelvin, which corresponds to energies of about 10 keV. A thousand eV is too low for the reaction rate to be useful, and 1 MeV is much higher than is needed. The key gives 10 keV.

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37.

In an ideal Magneto Hydro Dynamic Power Generator, the separation between the plates = 0.5 m, magnetic flux density = \(2\ Wb/m^{2}\) and average fuel velocity = 500 m/sec. The voltage developed across the plates will be

A.500 V
B.1000 V
C.2000 V
D.5000 V

Exam Year(s)

UGC NET - 2014,

Answer: A

In an ideal MHD generator the voltage developed across the plates is the product of the magnetic flux density, the separation of the plates and the velocity of the conducting gas:

\[ V = B\,l\,v \]

\[ V = 2 \times 0.5 \times 500 = 500 \ \text{V} \]

So the answer is (A), 500 V. The larger values would result from a wrong multiplication, such as taking the plate separation as 2 m or the velocity as 2000 m/s.

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38.

In an ideal MHD power plant, the electrical efficiency corresponding to maximum power production is

A.50%
B.75%
C.25%
D.100%

Exam Year(s)

UGC NET - 2014,

Answer: A

In an ideal MHD generator the power output is greatest when the load is matched to the internal resistance, so the electrical efficiency at maximum power is 50%. This is the usual result for a generator delivering power to a matched load.

75% and 100% are higher than the matched load efficiency, and 25% is lower, so they do not correspond to maximum power.

In a Faraday type MHD generator the voltage is the induced field times the channel width and the load factor is the ratio of the load voltage to the induced voltage, which is 0.5 at the point of maximum power.

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