Tag: physics

Questions Related to physics

Select the incorrect option

physics magnetic fields and electromagnetism magnetic flux density magnetic flux electromagnetic induction

  1. Luminous flux and radiant flux have same dimensions

  2. Luminous flux and luminous intensity have same dimensions

  3. Radiant flux and power have same dimension

  4. Relative luminosity is a dimensionless quantity

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Correct Option: A
Explanation:

Luminous flux ( Luminous power) is the amount of perceived power of light. It is measure energy.

Radiant flux (Radiant power) is the amount of radiated power.
While luminous intensity is ratio of luminous flux to solid angle.

A current carrying wire produces a magnetic field in its surrounding space.
The S.I. unit of magnetic flux density is 

physics magnetic fields and electromagnetism magnetic flux density magnetic flux electromagnetic induction

  1. Henry

  2. Tesla

  3. $AM^2$

  4. A-m

Show answer
Correct Option: B
Explanation:

The SI unit of Magnetic Flux density is $ Weber/m^2 $ which is also called Tesla.

The dimensional formula of magnetic flux is ___________.

physics magnetic fields and electromagnetism magnetic flux density magnetic flux electromagnetic induction

  1. $\left[ M{ L }^{ 2 }{ T }^{ -2 }{ A }^{ -1 } \right] $

  2. $\left[ M{ L }^{ 2 }{ T }^{ -3 }{ A }^{ -1 } \right] $

  3. $\left[ { M }^{ -1 }{ L }^{ -2 }{ T }^{ 2 }{ A }^{ 1 } \right] $

  4. $\left[ M{ L }^{ 3 }{ T }^{ -2 }{ A }^{ -1 } \right] $

Show answer
Correct Option: A
Explanation:

Dimensions of ${ \phi  } _{ B }=$ Unit of $B\times $ Unit of $A$
$=\dfrac { newton }{ ampere-metre } \times { metre }^{ 2 }=\dfrac { newton\times metre }{ ampere } $
$=\dfrac { kg{ ms }^{ -2 }\times m }{ A } =kg{ m }^{ 2 }{ s }^{ -2 }{ A }^{ -1 }=\left[ M{ L }^{ 2 }{ T }^{ -2 }{ A }^{ -1 } \right] $

In an experiment to measure the velocity of electrons, an electric field $(E)$ and a magnetic field $(B)$ are employed to produce zero deflection. Then :

physics magnetic fields and electromagnetism magnetic flux density magnetic flux electromagnetic induction

  1. The two fields are parallel and the velocity is given by $BE$

  2. The two fields are perpendicular and the velocity is given by $E/B$

  3. The two fields are parallel and the velocity is given by $E/B$

  4. The two fields are perpendicular and the velocity is given by $BE$

Show answer
Correct Option: B
Explanation:

The electric force on electron of charge ${e}$  is,
             $  F= {e}E $ 
                    where, E = electric field.

And Magnetic force ,
            $ F= {e}$ $(V\times B)$
               where, V= velocity of electron in magnetic field
                            B= magnetic field

From the above equations,

          $E= V\times B$
Electric field E is perpendicular to both velocity of electron and the magnetic field. So,  The two field is perpendicular to each other and the velocity of electron is $\dfrac{E}{B}$.

B. The two fields are perpendicular and the velocity is given by $\dfrac{E}{B}$.

Write the dimensions of Magnetic flux in terms of mass, time, length and charge.

physics magnetic fields and electromagnetism magnetic flux density magnetic flux electromagnetic induction

  1. $[M^1L^2T^{-1}Q^{-2}]$

  2. $[M^1L^2T^{-1}Q^{-1}]$

  3. $[M^1L^3T^{-1}Q^{-1}]$

  4. $[M^4L^2T^{-1}Q^{-1}]$

Show answer
Correct Option: B
Explanation:

B. $[M^1 L^2 T^ {-1}Q^{-1}]$


Dimension of magnetic field, 


$[B]=[M^1L^0Q^{-1}]$ 

Dimension of surface Area,

 $[A]=[L^2]$

The magnetic flux , 

$\phi=B.A$

The dimension of magnetic flux in term of mass, time, length and charge,
$[\phi]= [M^1L^2T^{-1}Q^{-1}]$

The radii of curvature of the surfaces of a double convex lens are 20 cm and 40 cm respectively, and its focal length is 20 cm. What is the refractive index of the material of the lens.? 

physics reflection of light in spherical mirrors focus and focal length spherical mirror formula and magnification reflection of light by curved surfaces

  1. $\dfrac{5}{2}$

  2. $\dfrac{4}{3}$

  3. $\dfrac{5}{3}$

  4. $\dfrac{4}{5}$

Show answer
Correct Option: C
Explanation:

Here $R _1$ = 20 cm, $R _2$ = -40 cm, f = 20 cm
Using lens maker's formula we get,
$\dfrac{1}{20} \, = \, (\mu \, - \, 1) \left ( \dfrac{1}{20} \, + \, \dfrac{1}{40} \right )$
$\dfrac{1}{20} \, = \, (\mu \, - \, 1) \dfrac{3}{40} \, \Rightarrow \, \mu \, = \, \dfrac{5}{3}$

The focal length of a plane mirror is __________ .

physics reflection of light in spherical mirrors focus and focal length spherical mirror formula and magnification reflection of light by curved surfaces

  1. positive

  2. negative

  3. zero

  4. infinite

Show answer
Correct Option: D
Explanation:

The radius of curvature of a plane mirror is infinite so focal length will also be infinite.

If we say that the focal length of a spherical mirror is $n$ times its radius of curvature, then $n$ must be

physics reflection of light in spherical mirrors focus and focal length spherical mirror formula and magnification reflection of light by curved surfaces

  1. $2.0$

  2. $1.5$

  3. $0.2$

  4. $0.5$

Show answer
Correct Option: D
Explanation:

since we know that radius of curvature is 2 times focal length i.e R=2f

hence (f=0.5R)

The focal length of a spherical mirror is half of the radius of curvature

physics reflection of light in spherical mirrors focus and focal length spherical mirror formula and magnification reflection of light by curved surfaces

  1. For all rays

  2. Only for paraxial rays near the principal axis

  3. For those rays which are far from the principal axis

  4. For those rays which subtend extremely large angles with the axis

Show answer
Correct Option: B
Explanation:
The rays that are near the principal axis (paraxial rays) and parallel to it converge to a single point on the axis after emerging from the spherical mirror. This point is called the focal point F of the lens.
And this is half of the radius of the curvature in spherical mirror.

State whether true or false.
The focal length of a spherical mirror is double its radius of curvature.

physics reflection of light in spherical mirrors focus and focal length spherical mirror formula and magnification reflection of light by curved surfaces

  1. True

  2. False

Show answer
Correct Option: B
Explanation:

Focal length of  spherical mirror is half the radius of curvature i.e f $=\cfrac { R }{ 2 } $ .