Poynting Vector

Tuesday, 25 September 2018

12:45 PM

• EM waves carry energy
• The rate of flow of energy can be described by the Poynting Vector

Its direction is the direction of propagation

Time dependent
- Magnitude varies
- Magnitude reaches a maximum when E and B are maximum

• The magnitude  represents the rate at which energy flows through a unit surface area perpendicular to the driection of the wave propagation

 

Power can therefore be translated from

 to

 

Intensity

Wave intensity  is the time average over the cycles -

 

Density

Electric -

Magnetic -

 

 

 

Instantaneous Energy - Sum of the Individual Densities

 

Average Energy

 

 

Momentum - EM waves transport momentum

 

Pressure

 

  is the Poynting Vector!