## Monday, May 11, 2009

### Kerala Engineering Entrance (KEAM) 2008 Questions on Nuclear Physics

Science without religion is lame, religion without science is blind.

– Albert Einstein

KEAM Engineering 2009 Exam will begin on 25th of this month. Here are the questions on nuclear physics which appeared in last year’s physics eam:

(1) The nuclear radius of a certain nucleus is 7.2 fm and it has a charge of 1.28×10–17C.The number of neutrons inside the nucleus is

(a) 136

(b) 142

(c) 140

(d) 132

(e) 126

The nuclear radius R is related to the mass number A as

R = 1.2 A1/3

On substituting for R in this equation we obtain A1/3 = 6 so that A = 216.

Since the total charge of the nucleus (carried by all the protons) is given as 1.28×10–17 coulomb and the charge on a proton is 1.6×10–19 coulomb, the number of protons (Z) in the nucleus is (1.28×10–17) /(1.6×10–19) = 80.

Therefore, the number of neutrons in the nucleus, N = A Z = 216 – 80 = 136.

(2) The energy released in the fission of 1 kg of 92U235 is (Energy per fission = 200 MeV)

(a) 5.1×1026 eV

(b) 5.1×1026 J

(c) 8.2×1013 J

(e) 5.1×1023 MeV

1 kg of 92U235 contains 1/0.235 moles and each mole contains 6.02×1023 atoms. Therefore1 kg of 92U235 contains (6.02×1023)/ 0.235 atoms.

Therefore, the energy E released in the fission of 1 kg of 92U235 is given by

E = (6.02×1023) ×200 / 0.235 MeV = 5.1×1026 MeV = 5.1×1032 eV

Since 1 eV = 1.6×10–19 joule, the above energy is (5.1×10 32)×(1.6×10–19) joule. This works out to approximately 8.2×1013 J.

(3) Which one of the following statements is true if half life of a radioactive substance is 1 month?

(a) 7/8th part of the substance will disintegrate in 3 months

(b) 1/8th part of the substance will remain undecayed at the end of 4 months

(c) The substance will disintegrate completely in 4 months

(d) 1/16th part of the substance will remain undecayed at the end of 3 months

(e) The substance will disintegrate completely in 2 months

The number of nuclei N remaining undecayed at the end of n half lives is given by

N = N0/2n where N0 is the initial number.

Therefore, at the end of 3 months (3 half lives in the present case) the number remaining undecayed is N0/23 = N0/8.

This means that 7/8th part of the substance will disintegrate in 3 months [Option (a)].

You will find a few more multiple choice questions (with solution) on nuclear physics here.