EM radiation from a dipole

In summary: The radiation is in the r hat direction because the electric field is a function of the distance from the dipole.
  • #1
AriAstronomer
48
1
Hello everyone,
I'm a bit confused about electric dipole radiation. In my E&M book "Intro to Electrodynamics" by David Griffiths, it states that the electric field from an oscillating dipole is in theta hat direction. Mathematically I have the proof as to why in my book, but conceptually I'm having a hard time picturing everything.
Normally, electric field goes radially in r hat direction. Since it is a transverse wave, I picture it like a hand moving a string up and down, and the wave is perpendicular to the hand motion. I don't picture the electric field moving in circles (do they close in on themselves like magnetic field??) around a dipole. What's causing this change from an r hat direction (pointing away from a charge) to theta hat (perpendicular to a charge)?

Ari
 
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  • #2
AriAstronomer said:
Normally, electric field goes radially in r hat direction.

That's for a single point charge. Consider a "physical" dipole, which consists of two opposite charges with equal magnitude, separated by a short distance. Even for a static dipole (not oscillating) the net E field is radial only along the dipole axis.

With an oscillating dipole, you also have contributions to the E field that are related to the changing magnetic field.
 
  • #3
jtbell said:
That's for a single point charge. Consider a "physical" dipole, which consists of two opposite charges with equal magnitude, separated by a short distance. Even for a static dipole (not oscillating) the net E field is radial only along the dipole axis.

With an oscillating dipole, you also have contributions to the E field that are related to the changing magnetic field.

In other words, the static electric field of a dipole does not go around in a circle. It as a theta-hat component, but it has r-hat and phi-hat components as well.
 
  • #4
But it still radiates in the r hat direction doesn't it? I mean the fact that it reaches you from some distant point means it must have an r hat component, but I guess the point is that it's highly theta dependent?
 
  • #5
AriAstronomer said:
But it still radiates in the r hat direction doesn't it? I mean the fact that it reaches you from some distant point means it must have an r hat component, but I guess the point is that it's highly theta dependent?

Yes, exactly right.
 

1. What is a dipole and how does it produce EM radiation?

A dipole is an antenna that consists of two equal and opposite charges separated by a small distance. When an alternating current is applied to the dipole, the charges oscillate back and forth, creating a changing electric field. This changing electric field then produces an electromagnetic wave, or EM radiation, that propagates outward from the dipole.

2. What factors affect the strength and direction of EM radiation from a dipole?

The strength and direction of EM radiation from a dipole is affected by the length and orientation of the dipole, as well as the frequency of the alternating current applied to it. Longer dipoles and those oriented perpendicular to the direction of the current will produce stronger radiation, while shorter dipoles and those parallel to the current will produce weaker radiation.

3. How does the wavelength of EM radiation from a dipole relate to the length of the dipole?

The wavelength of EM radiation from a dipole is directly proportional to the length of the dipole. This means that longer dipoles will produce longer wavelengths of radiation, while shorter dipoles will produce shorter wavelengths. This relationship is important in designing and optimizing dipole antennas for specific frequencies.

4. Can the direction of EM radiation from a dipole be controlled?

Yes, the direction of EM radiation from a dipole can be controlled by adjusting the orientation and length of the dipole. By manipulating these factors, the radiation pattern of the dipole can be directed in specific directions, allowing for more targeted transmission and reception of signals.

5. What are some common applications of EM radiation from a dipole?

Dipole antennas are commonly used in radio communication, such as in cell phones, radios, and Wi-Fi networks. They are also used in radar systems for detecting and tracking objects. In addition, dipole radiation is used in medical imaging techniques like MRI and in scientific research for studying the properties of electromagnetic waves.

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