Light with an energy flux of $ 25 \times 10^4 W m^{-2} $ falls on a perfectly reflecting surface at normal incidence. If the surface area is $ 15 c m^2 $, the average force exerted on the surface is
- $1.25 \times 10^{-6} N $
- $2.50\times10^{-6} N$
- $1.20\times10^{-6}N$
- $3.0\times10^{-6}N$
The Correct Option is B
Approach Solution - 1
Energy flux, $I= 25 \times {10}^4 W m^{-2}$
Area, $ A = 15 c m^2 = 15 \times {10} ^{-4} m^2 $
Speed of light, $ c= 3 \times {10}^8 m s^{-1}$
For a perfectly reflecting surface, the average force exerted on the surface is
$ F= \frac{ 2IA}{ c} = \frac{ 2 \times 25 \times 10^{4} W m^{-2} \times 15 \times 10^{-4} m^{2} }{ 3 \times 10^{8} \,m\, s^{-1}}$
$ = 250 \times {10}^{-8} N = 2.50 \times {10}^{-6} \,N $
Approach Solution -2
Ans. Electric Flux is the rate of flow of an electric field through an area. Electric flux is proportional to the number of electric field lines passing through a virtual surface.
- Flux refers to the presence of a force field in a physical medium.
- In electronics, flux refers to an electrostatic field and any magnetic field.
- Flux is shown as "lines" in a plane that contains or intersects electric charge poles or magnetic poles.
- The total number of electric field lines passing a given area in a unit of time is defined as the electric flux.
Similar to the example above, if the plane is normal to the flow of the electric field, the total flux is given as ɸP = EA
The S.I. unit of electric flux is Volt Metres (V m). The dimensions of the electric flux physics are NC−1 m2 or Kg m3 s−3 A−1.
In Electrostatics, Electric Flux is described as several electric field lines, passing per unit area. It is an additional physical quantity that is used to measure the strength of an electric field and build the fundamentals of electrostatics in the world of Physics. Electric Flux can also be described as the product of the electric field and surface area projected in a direction perpendicular to the electric field.
On tilting this plane at an angle of θ, the calculated area is given by Acosθ and the total flux over this surface is given as:
ɸP = EACosΘ
Where,
- E = magnitude of the electric field
- A = area of the surface through which the electric flux is to be determined
- Θ = angle made by the plane and the axis parallel to the direction of flow of the electric field
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Concepts Used:
Electromagnetic waves
The waves that are produced when an electric field comes into contact with a magnetic field are known as Electromagnetic Waves or EM waves. The constitution of an oscillating magnetic field and electric fields gives rise to electromagnetic waves.
Types of Electromagnetic Waves:
Electromagnetic waves can be grouped according to the direction of disturbance in them and according to the range of their frequency. Recall that a wave transfers energy from one point to another point in space. That means there are two things going on: the disturbance that defines a wave, and the propagation of wave. In this context the waves are grouped into the following two categories:
- Longitudinal waves: A wave is called a longitudinal wave when the disturbances in the wave are parallel to the direction of propagation of the wave. For example, sound waves are longitudinal waves because the change of pressure occurs parallel to the direction of wave propagation.
- Transverse waves: A wave is called a transverse wave when the disturbances in the wave are perpendicular (at right angles) to the direction of propagation of the wave.