31.

Two concentric conducting thin spherical shells  A  and  B  having radii r_A and  r_B  ( r_B > r_A ) are charged to Q_A and - Q_B  (). The electrical field along a line, (passing through the centre is)

• 

• 

• 

• 

Dimension of electrical resistance is

• [ M L² T^-3 A^-1 ]

• [ M L² T^-3 A^-2 ]

• [ M L³ T^-3 A^-2 ]

• [ M L^-1 T³ A² ]

Four point + ve charges  of same magnitude (Q)  are placed at four corners of  a rigid square frame as shown in figure. The plane of the frame is perpendicular to Z-axis. If a -ve point charge is placed at a distance z away from the above frame ( z < < l ) then

• -ve charge oscillates along the Z-axis

• it moves away from the frame

• it moves slowly towards the frame and stays in the plane of the frame

• it moves slowly towards the frame and stays in the plane of the frame

Assertion: A body that is a good radiator is also a good absorber of radiation at a given wavelength. 

Reason: According to Kirchoff's law the absorptivity of a body is equal to its emissivity at a given wavelength

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

For skywave propagation of a 10 MHz signal, what should be the minimum electron density in ionosphere?

• ∼ 1.2 × 10¹² m^-3

• ∼ 10⁶ m^-3

• ∼ 10¹⁴ m^-3

• ∼ 10²² m^-3

Which of the following logic gates is a universal gate?

• OR

• NOT

• AND

• NAND

What should be the maximum acceptance angle at the air-core interface of an optical fibre if  n₁  and  n₂ are the refractive indices of the core and the cladding, respectively?

• sin^-1( n₂/n₁ )

• sin^-1$\sqrt{{\mathrm{n}}_1^{ 2} - {\mathrm{n}}_2^{ 2}}$

• $\left[{\tan }^{-1} \frac{{\mathrm{n}}_2}{{\mathrm{n}}_1}\right]$

• $\left[{\tan }^{-1} \frac{{\mathrm{n}}_1}{{\mathrm{n}}_2}\right]$

Consider an n-p-n transistor amplifier in the common emitter configuration. The current gain of the transistor is 100. If the collector current changes by 1 mA, what will be the change in emitter current?

• 1.1 mA

• 1.01 mA

• 0.01 mA

• 10 mA

A telescope has an objective lens of focal length 200 cm and an eye piece with focal length 2 cm. If this telescope is used to see a 50 meter tall building at a distance of 2 km, what is the height of the image of the building formed by the objective lens?

• 5 cm

• 10 cm

• 1 cm

• 2 cm

The ground state energy of hydrogen atom is -13.6 eV. What is the potential energy of the electron in the state?

• 0 eV

• -27.2 eV

• 1 eV

• 2 eV