Electric field varying with distance

  • Thread starter Jahnavi
  • Start date
  • #1
848
102
[Moderator note: moved from general physics, no template.]

example.jpg


This is a solved example given in the book . Could someone help me understand how amplitude of electric field has an inverse relationship with distance ?

Only the very basics of EM waves are covered in the book so I would appreciate if someone could explain in a simple language .

Thank you .
 

Attachments

  • example.jpg
    example.jpg
    42.9 KB · Views: 751
Last edited:

Answers and Replies

  • #2
scottdave
Science Advisor
Homework Helper
Insights Author
1,813
775
For a dipole antenna, the electric field of the radiated wave will stay parallel to the antenna dipole. This means the radiation pattern will mostly be 2 dimensional as a circle, centered at the transmitting antenna. As the circumference increases the field strength is "spread out" over more circumference.
So the electric field strength is divided by the length of the circumference. The circumference of a circle is proportional to r.
 
  • #3
138
43
Recall that the irradiance (also called intensity) falls off as 1/r^2 (just think of the flux through spherical surfaces centered on the source) and is also proportional to E^2 (shown in just about every book, even if not derived). Putting both together,

[tex]I \propto 1/r^2 \propto E^2 => E \propto 1/r[/tex]
 
  • #4
848
102
Recall that the irradiance (also called intensity) falls off as 1/r^2 (just think of the flux through spherical surfaces centered on the source) and is also proportional to E^2 (shown in just about every book, even if not derived). Putting both together,

[tex]I \propto 1/r^2 \propto E^2 => E \propto 1/r[/tex]

Thanks !
 

Related Threads on Electric field varying with distance

Replies
3
Views
890
  • Last Post
Replies
16
Views
5K
Replies
1
Views
1K
  • Last Post
Replies
1
Views
923
  • Last Post
Replies
9
Views
695
Replies
3
Views
661
  • Last Post
Replies
15
Views
2K
Replies
5
Views
789
  • Last Post
Replies
9
Views
5K
Top