# Physics

The number of images of an object placed between two mirrors inclined at an angle of 45o are:?

A)
5

B)
6

C)
7

D)
8

Correct Answer :

7

Explanation :

If two plane mirrors are inclined at angle x, then the number of images formed in them is given by the formula:

Number of images formed = ( 360° /x ) - 1

By substituting the different values of angles between two inclined mirrors we can tabulate the number of images formed as follows:

 Angle between the two mirrors Number of images formed 45° 7 60° 5 90° 3 120° 2 180° 1 0° Infinite

A steel wire has a magnetic movement M. If the wire is bend into semicircular arc, The new magnetic movement becomes?

A)
M

B)
2M

C)
2πM

D)
(2/π)*M

Correct Answer :

(2/π)*M

Explanation : Let 2L = length of the bar magnet , then
magnetic moment,M = m * 2L

After bent into semicircle
πR = 2L (as circumference of a semi-circle = πR + 2*R )
R = 2L / π

When the magnet is bent in the form of a semicircle, the distance between the poles = 2R = 2 * (2L / π)

New magnetic moment = m * 2 * (2L / π) = 2M / π

The surface of a spherical shell is uniformly charged. Then what is the electric field inside the spherical shell?

A)
Zero

B)
constant

C)
Infinite

D)
Proportional to the distance from the center

Correct Answer :

Zero

Explanation :

According to Gauss law all charges are distributed on the surface of the spherical shell. Therefore there will be no electric flux inside the spherical shell,So that the electric field inside the shell will be zero.

A wire of resistance R is stretched to reduce its diameter to half of its original value, The new resistance is?

A)
2R

B)
4R

C)
8R

D)
16R

Correct Answer :

16R

Explanation :

The resistance R of a uniform cylinder of length L, of cross-sectional area A, and made of a material with resistivity ρ

R = ρL/A

New radius = r/2
New area = pi *(r/2)*(r/2)=1/4 of original area

As volume = length * area
new length = 4 * original length

New resistance R1 = ρ 4L/(A/4) = ρ 16*L/A = 16 R

The resistivity of the material depends on?

A)
Length of the wire

B)
Diameter of the wire

C)
Temperature of the wire

D)
Material of the wire

Correct Answer :

Material of the wire

Explanation :

The resistance R of a uniform cylinder of length L, of cross-sectional area A, and made of a material with resistivity ρ

Resistance R = ρL/A

The resistivity of a material depends on its nature and the temperature of the conductor, but not on its shape and size. A good conductor has less resistivity, whereas a bad conductor or insulator has high resistivity.
The unit for resistivity is the ohm-metre.

Calculate the energy equivalent of 1g of substance ?

A)
3*1013 J

B)
6*1013 J

C)
9*1013 J

D)
2*1013 J

Correct Answer :

9*1013 J

Explanation :

As per Einstein's equation E=mc2, where c is the speed of light (~3x108 m/s), m is the mass, and E is amount of energy.
1 g = 0.001 kg

E=0.001kg * (3 * 108 m/s)2
E=0.001 * 9 * 1016 kg*m2/s2
E=9 * 1013 kg*m2/s2

A kg*m2/s2 is a joule that measures energy

The unit of Thermal Conductivity of a material is?

A)

W / mK

B)

W / m2K

C)

Wm / K

D)

W2K / m

Correct Answer :

W / mK

Explanation :

In SI units, thermal conductivity is measured in watts per meter-kelvin (W / mK).

The most common element used in manufacturing of transistor is?

A)

Silicon

B)

Phosphorus

C)

Gallium

D)

Germanium

Correct Answer :

Silicon

Explanation :

Transistor, three-terminal, solid-state electronic device used for amplification and switching.

The first transistors were made from germanium, but now most transistors are made from silicon. Silicon (Si) and germanium (Ge) form similar crystal structures with similar physical properties, but silicon is preferred over germanium because of silicon's superior thermal characteristics.

A larger force on a rotating body results in larger?

A)

mass

B)

torque

C)

axis of rotation

D)

centre of mass

Correct Answer :

torque

What is the unit of the physical quantity, magnetic flux density?

A)

Siemens

B)

Weber

C)

Henry

D)

Tesla

Correct Answer :

Tesla

Explanation :

The tesla (symbol T) is a derived unit of the magnetic induction (also, magnetic flux density) in the International System of Units. One tesla is equal to one weberper square metre.