(radiation) monpoles vs. dipoles

In summary, Griffiths explains in Chapter 11 that an electric monopole does not emit radiation. However, a point charge with an electric dipole moment \mathbf{p} (t) = q \mathbf{d} (t) (where \mathbf{d} (t) is the instantaneous coordinate of the charge) does emit radiation with power P = \mu_0 q^2 a^2/(6 \pi c), where \mathbf{a}(t) = \ddot{\mathbf{p}} (t). The term "monopole" refers to a point charge that does not move, although any distribution of charge that produces a spherically symmetric field outside a certain
  • #1
lion8172
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In the chapter on radiation (Chapter 11), Griffiths notes that an electric monopole does not radiate, but also that a point charge of electric dipole moment [tex]\mathbf{p} (t) = q \mathbf{d} (t) [/tex] (where [tex] \mathbf{d} (t) [/tex] is the instantaneous coordinate of the charge with respect to a fixed origin ) radiates with power
[tex] P = \mu_0 q^2 a^2/(6 \pi c) [/tex], where [tex] \mathbf{a}(t) = \ddot{\mathbf{p}} (t) [/tex]. By "monopole," does he simply mean a point charge that doesn't move?
 
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  • #2
That would be one case. Any distribution of charge which gives a spherically symmetric field outside of a certain region is an electric monopole.
 
  • #3


Yes, in this context, Griffiths is referring to a stationary point charge as a monopole. This is because a monopole by definition has no dipole moment, and therefore does not have any acceleration or change in position over time. This lack of movement means that there is no changing electric field, which is necessary for radiation to occur. On the other hand, a dipole has a non-zero dipole moment and therefore experiences acceleration, resulting in the emission of electromagnetic radiation. This is why a stationary monopole does not radiate, while a moving dipole does.
 

1. What is the difference between a monopole and a dipole?

A monopole is a type of antenna that only has one pole, while a dipole has two poles. Monopoles are typically used for receiving and transmitting radio waves, while dipoles are used for both radio and TV signals.

2. Which type of antenna is more efficient, a monopole or a dipole?

In general, dipoles are more efficient at transmitting and receiving signals compared to monopoles. This is because dipoles have two poles, which allows for a more balanced and symmetrical distribution of energy.

3. Can a monopole be used as a substitute for a dipole?

No, a monopole cannot be used as a substitute for a dipole in most cases. While a monopole can function as an antenna, it is not as efficient as a dipole and may not be able to transmit or receive certain types of signals.

4. What are the advantages of using a dipole over a monopole?

Dipoles have several advantages over monopoles. They have a wider bandwidth, meaning they can receive and transmit a greater range of frequencies. They also have a higher gain, which allows for a stronger and more reliable signal. Additionally, dipoles have a more predictable radiation pattern, making them more suitable for specific applications.

5. Are monopoles or dipoles better for use in urban areas?

In urban areas, dipoles are generally better for use as they have a more predictable radiation pattern and can better avoid interference from nearby buildings and structures. Monopoles may also be affected by reflections and multi-path signals in urban environments, making them less ideal for use.

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