21.

In isothermal process, which of the following is not true?

• Temperature remains constant

• Internal energy does not change

• No heat enters or leaves the system

• none of the above

Two equal -ve charges -q are fixed at the point (O, a) and (O, a) on the y-axis. A positive charge Q is released from rest at the point (2a, 0) on the x-axis. The charge will :

• execute SHM about the origin

• move to the origin and remain at rest

• move to infinity

• execute oscillatory but not SHM

Who discovered X-rays?

• Roentgen

• Madam Curie

• Rutherford

•  All of these

 A charge Q is distributed uniformly in a sphere (solid). Then, the electric field at any point r,  where, r < R (r is the radius of sphere) varies as:

• ${\mathrm{r}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$

• r ^-1

• r

• r ^-2

In the given circuit, what will be the equivalent resistance between the points A and B ?

• 10 / 3 Ω

• 20 / 3 Ω

• 10 / 5 Ω

• 5 Ω

If a charge particle enters perpendicularly in the uniform magnetic field, then:

• energy and momentum both remains constant

• energy remains constant but momentum changes

• both energy and momentum changes

• energy changes but momentum remains constant 

A uniform wire of length I and having resistance R is cut into n equal parts and all parts are connected in parallel, then the equivalent resistance will be :

• R

• R/n

• R/n²

• n²R

In a L-C-R circuit, the capacitance is made one-fourth, then what should be changed in inductance, so that the circuit remains in resonance?

• 8 IMES

• 1/4 times

• 2 times

• 4 times

The light rays having photons of energy 1.8 eV are falling on a metal surface having a work function 1.2 eV. What is the stopping potential to be applied to stop the emitting electrons?

• 3 eV

• 1.2 eV

• 0.6 eV

• 1.4 eV

1f the focal length of a lens of a camera is 5f and that of another is 2.5f, what is the time of exposure for the second, if for the first one is $\frac{1}{200} \mathrm{s}$ ? (where f is focal length / unit aperture)

• $\frac{1}{200}$

• $\frac{1}{800}$

• $\frac{1}{6400}$

• $\frac{1}{3200}$