## Monday, October 12, 2009

### IIT JEE Physics Final Revision Set 35

1. If the resultant of all the external forces acting on a system of particles is zero, then from
an inertial frame, one can surely say that
(A) linear momentum of the system does not change in time
(B) kinetic energy of the system does not change in time
(C) angular momentum of the system does not change in time
(D) potential energy of the system does not change in time

2. A student performed the experiment of determination of focal length of a concave mirror
by u-v method using an optical bench of length 1.5 meter. The focal length of the mirror
used is 24 cm. The maximum error in the location of the image can be 0.2 cm. The 5 sets
of (u, v) values recorded by the student (in cm) are : (42, 56), (48, 48), (60, 40), (66, 33),
(78, 39). The data set(s) that cannot come from experiment and is(are) incorrectly
recorded, is (are)
(A) (42, 56)
(B) (48, 48)
(C) (66, 33)
(D) (78, 39)

3. Cv and Cp denote the molar specific heat capacities of a gas at constant volume and
constant pressure, respectively. Then
(A) Cp – Cv is larger for a diatomic ideal gas than for a monoatomic ideal gas
(B) Cp + Cv is larger for a diatomic ideal gas than for a monoatomic ideal gas
(C) Cp / Cv is larger for a diatomic ideal gas than for a monoatomic ideal gas
(D) Cp . Cv is larger for a diatomic ideal gas than for a monoatomic ideal gas

Paragraph for Question Nos. 4 to 6

Scientists are working hard to develop nuclear fusion reactor. Nuclei of heavy hydrogen, ²1H, known as deuteron and denoted by D, can be thought of as a candidate for fusion reactor. The D-D reaction is ²1H + ²1H → 32He + n + energy. In the core of fusion reactor, a gas of heavy hydrogen is fully ionized into deuteron nuclei and electrons. This collection of ²1H 1 nuclei and electrons is known as plasma. The nuclei move randomly in the reactor core and occasionally come close enough for nuclear fusion to take place. Usually, the temperatures in the reactor core
are too high and no material wall can be used to confine the plasma. Special techniques are used which confine the plasma for a time t0 before the particles fly away from the core. If n is the density (number/volume) of deuterons, the product nt0 is called Lawson number. In one of the
criteria, a reactor is termed successful if Lawson number is greater than 5 × 1014 s/cm3.

It may be helpful to use the following.
Boltzmann constant k = 8.6 × 10-5 eV/K;

e²/πε0 = 1.44 × 10-9 eVm.

4. In the core of nuclear fusion reactor, the gas becomes plasma because of –
(A) Strong nuclear force acting between the deuterons
(B) Coulomb force acting between the deuterons
(C) Coulomb force acting between deuteron-electron pairs
(D) the high temperature maintained inside the reactor core

5. Assume that two deuteron nuclei in the core of fusion reactor at temperature T are
moving towards each other, each with kinetic energy 1.5 kT, when the separation
between them is large enough to neglect Coulomb potential energy. Also neglect any
interaction from other particles in the core. The minimum temperature T required for
them to reach a separation of 4 × 10-15 m is in the range –

a. 1.0 × 1009 K is l.t. T is l.t. 2.0 × 109 K
b. 3.0 × 1009 K is l.t. T is l.t. 4.0 × 109 K
c. 2.0 × 1009 K is l.t. T is l.t. 3.0 × 109 K
d. 4.0 × 1009 K is l.t. T is l.t. 5.0 × 109 K

6. Results of calculations for four different designs of a fusion reactor using D-D reaction
are given below. Which of these is most promising based on Lawson criterion ?
(A) deuteron density = 2.0 × 10^12 cm–3, confinement time = 5.0 × 10^–3 s
(B) deuteron density = 8.0 × 10^14 cm–3, confinement time = 9.0 × 10^–1 s
(C) deuteron density = 4.0 × 10^23 cm–3, confinement time = 1.0 × 10^–11 s
(D) deuteron density = 1.0 × 10^24 cm–3, confinement time = 4.0 × 10^–12 s

JEE 2009 paper 1