Question

The Poynting vector of electromagnetic waves has the same dimensions as that of:
1. electric field
2. electric current density
3. elctromagnetic power density
4. electric charge density

Answer 1

Correct Answer - Option 3 : elctromagnetic power density

Poynting vector:

It states that the cross product of electric field vector (E) and magnetic field vector (H) at any point is a measure of the rate of flow of electromagnetic energy per unit area at that point that is 

\( \vec S = \vec E \times \vec H\)

S = Poynting vector

E = Electric field and

H = Magnetic field

The Poynting vector describes the magnitude and direction of the flow of energy in electromagnetic waves per unit volume.

∴ The dimensions of both the Poynting vector and the electromagnetic power density are the same, i.e. M1 L-1 T-2

M = Mass

L = Length

T = Time

Notes:

<uCurrent density:

• Denoted by J, the electric current density is the rate at which charge flows (electric current) per unit area of cross-section.
• It is a vector quantity since it has both magnitude and direction.
• Its magnitude is the current per unit area and its direction is the same as that of the charges at that point.
• Current per unit area (taken normal to the current), I/A, is called current density.
  It is denoted by J.
• The SI units of the current density are A/m2 and dimension is given by M0L-2T0I¹ or [M0 L−2 T−1 Q]