21.

A current loop ABCD is held fixed on the plane of the paper as shown in the figure. The arcs BC (radius = b) and DA (radius = a) of the loop are joined by two straight wires AB andCD. A steady current I is flowing in the loop. Angle made by AB and CD at the origin O is 30º. Another straight thin wire with steady current I1 flowing out of the plane of the paper is kept at the origin.


Due to the presence of the current I₁ at the origin


The magnitude of the magnetic field (2) due to the loop ABCD at the origin (O) is

• The forces on AB and DC are zero

• The forces on AD and BC are zero

• The magnitude of the net force on the loop is given by 

• The magnitude of the net force on the loop is given by 

A charge Q is placed at each of the opposite corners of a square. A charge q is placed at each of the other tow corners. If the net electrical force on Q is zero, then Q/q equals

• 

• 1

• -1

• -1

An inductor of inductance L = 400 mH and resistors of resistances R1 = 2 Ω and R2 = 2 Ω is connected to a battery of emf 12 V as shown in the figure. The internal resistance of the battery is negligible. The switch S is closed at t = 0. The potential drop across L as a function of time is 

• 6^e–5t V

• 
• 
• 

The above is a plot of binding energy per nucleon Eb, against the nuclear mass M; A, B, C, D, E, F correspond to different nuclei. Consider four reactions:

(i) A + B → C + ε  (ii) C → A + B + ε
(iii) D + E → F + ε and (iv) F → D + E +ε

where ε is the energy released? In which reaction is ε positive?

• (i) and (iv)

• (i) and (iii)

• (ii) and (iv)

• (ii) and (iv)

A p–n junction (D) shown in the figure can act as a rectifier. An alternating current source (V) is connected in the circuit.

The logic circuit shown below has the input waveforms ‘A’ and ‘B’ as shown. Pick out the correct output waveform.


output is

In an optics experiment, with the position of the object fixed, a student varies the position of a convex lens and for each position, the screen is adjusted to get a clear image of the object. A graph between the object distance u and the image distance v, from the lens, is plotted using the same scale for the two axes. A straight line passing through the origin and making an angle of 45^o with the x-axis meets the experimental curve at P. The coordinates of P will be

• (2f, 2f)

• (f/2, f/2)

• (f,f)

• (f,f)

The transition from the state n = 4 to n = 3 in a hydrogen-like atom results in ultraviolet radiation. Infrared radiation will be obtained in the transition from

• 2 → 1

• 3 → 2

• 4 → 2

• 4 → 2

A mixture of light, consisting of wavelength 590 nm and an unknown wavelength, illuminates Young's double slit and gives rise to two overlapping interference patterns on the screen. The central maximum of both lights coincide. Further, it is observed that the third bright fringe of known light coincides with the 4th bright fringe of the unknown light. From this data, the wavelength of the unknown light is

• 393.4 nm

• 885.0 nm

• 442.5 nm

• 442.5 nm

The surface of a metal is illuminated with the light of 400 nm. The kinetic energy of the ejected photoelectrons was found to be 1.68 eV. The work function of the metal is (hc = 1240 eV nm)

• 3.09 eV

• 1.41 eV

• 151 eV

• 151 eV