Basic Electromagnetic Radiation Concepts

Learning Objectives

Electric Fields

All matter contains electrons and protons. Electrons have a negative charge, while protons have a positive charge. A charged particle produces an electric field in all directions. This field produces a force that is either directed away or toward the original particle (Figures 1 and 2). This force attracts oppositely charged particles and repels particles with the same charge. If the particle moves, such as electrons in an antenna, the associated electric field moves with it. A difference in electric field strength between two locations is called a voltage. So when we apply a voltage across two ends of a wire the electrons in the wire move toward the positive voltage and away from the negative voltage.

Figure 1. Diagram of a positively-charged particle showing the lines of force created by the electric field.

Figure 2. Diagram of a negatively-charged particle showing the lines of force created by the electric field.

Magnetic Fields

If a charged particles moves, it creates a magnetic field. Unlike the electric field, the force lines are directed at right angles to the direction of motion (Figure 3). So if electrons are moving in a wire, there has to be a magnetic field encircling the length of the wire, the direction of the magnetic force depending on the direction of the electron movement. Magnetic and electric fields interact such that a changing magnetic field creates an electric field and a changing electric field creates a magnetic field. The magnetic field shown in Figure 3 is created by the moving electric field associated with the charged particle. Conversely, if a magnetic field moves, an electric field is generated. Moving a magnet past a wire will create a voltage that moves the electrons in the wire.

Figure 3. Relation of magnetic field force lines with respect to an electron moving forward and backwards. Click on the "Play" button to animate.

Electromagnetic Radiation

We can visualize the creation of EM waves as follows. If a charged particle accelerates (moves faster, slower or changes direction), it produces both an electric field (because the particle is charged) and a magnetic field (because the particle is moving). Furthermore because the motion of the particle is changing, the electric field is changing and the magnetic field is changing. The changing electric field creates a new magnetic field and the changing magnetic field produces a new electric field. The collapsing and regeneration of the electric and magnetic fields is what allows EM radiation to propagate. Play the following animation to visualize this process.

Figure 4. Diagram of an electromagnetic (EM) wave generated by an accelerating electron. The z and x directions are on the plane of your screen and the y direction is coming out of the screen. As the electron moves up and down the associated electric field (represented by the black curve) also moves up and down. The electron motion also generates a magnetic field (magenta curve) coming out and going into the screen at right angles to the electron motion and the electric field. The magnetic field has the same sine wave shape as the electric field curve, but is distorted in the animation due to perspective. Because the particle's motion is changing (i.e. it is accelerating) the electric and magnetic fields are continually forming, collapsing and forming again in the opposite direction. The changing electric field generates another magnetic field to the right and the changing magnetic field generates another electric field to the right, thus the EM wave and associated force fields and energy to move in the direction of the black arrow at the speed of light. The small moving black and magenta arrows in the center of the animation represents the varying electric and magnetic force fields as the EM wave propagates past the green point.

 

Photons

Earlier we discussed how EM radiation also has the characteristics of a particle. We can think of it this way. When a charged particle, such as an electron accelerates, it releases a small packet of energy called a photon. Photons have no mass, they are pure energy. These photons moves along with the EM wave at the speed of light. The existence of photons has some important consequences, for example it means EM energy comes in tiny but finite packets that can't be divided. However for the purposes of this class, we will only need to consider the wavelike properties of EM radiation and the term "photon" will be used as just another term for an EM wave.

Polarization, Phase and Coherence

The EM wave represented in Figure 4 represents the wave field associated with one photon. In the real world, EM radiation consists of an enormous number of photons. If all the photon waves are oriented in the same way, for example, if all the electric fields are oriented along the vertical plane such as in Figure 4, the EM radiation is said to be polarized. EM radiation can be vertically polarized as above, or have some other polarization. If all the photon waves are oriented in different ways from each other, the radiation is unpolarized. However for each photon the electric and magnetic force fields will always be at right angles to each other, as shown above.

If all the peaks and valleys representing the electric and magnetic fields of different photons all line up with each other, the EM radiation is said to be in phase or coherent radiation. Most radio or radar transmissions are coherent. Visible and IR radiation is usually incoherent, an exception being lasers, which are coherent.

Summary

Charged particles create an electric force field. Moving charged particles create a magnetic force field. Accelerating charged particles produce changing electric and magnetic force fields which propagate as EM waves. EM radiation that has all the electric and magnetic field variations along the same plane is polarized. Polarized radiation that is also in phase is coherent.

Study Questions

  1. Say there are two charged particles close to each other. The electric fields from each particle point opposite to each other in the region between the two particles. Do the particles have the same or opposite charges?
  2. Do the particles in 1. repel or attract each other?
  3. Now suppose the electric fields in the region between the two particles go in the same direction. One particle is positively charged. What is the charge of the other particle?
  4. Do the particles in 3. repel or attract each other?
  5. If an electron is moving to the right, what is the direction of the magnetic field force lines above the electron?
  6. In Figure 3, what does the electric field look like? What is the angle between the electric force lines and the magnetic force lines?
  7. Why does a charged particle need to accelerate (change speed or direction) in order to produce an EM wave?

Last Update: 1/12/2004