MadSci Network: Physics Query:

### Re: What is the electromagnetic force: radiation or charge?

Date: Fri Jun 29 11:57:58 2001
Posted By: Steve Nelson, Grad student, nuclear astrophysics Ph.D. program, Nuclear Lab, Duke University
Area of science: Physics
ID: 992798858.Ph
Message:

Several physics terms are confused here. Electromagnetic fields act on
electrical charges. The electromagnetic force is the amount of force exerted on
a charge by the fields present. Electromagnetic radiation is a mixture of electric and
magnetic fields. An electric field can be conceptually viewed as an extended volume of
space in which electric field energy is present. This field is created by electric charges or
by changing magnetic fields, and it exerts a force on any charges present. At any point
the field has a particular direction. We can visualize this by drawing lines (commonly called
"field lines") along the direction that small charges would move if placed in the field.

A magnetic field is similar. Since no one has ever observed magnetic "charges"
we say that a magnetic field is created by the motion of electric charges or changing electrical
fields. It exerts a force on moving electrical charges perpendicular to their motion and
perpendicular to the magnetic field lines (similar to the electric field lines). For example,
if the motion of a particle is along the x-axis and the field is along the y-axis, then the
force is along the z-axis in a direction determined by something called the "right hand
rule." Any textbook on physics can explain these phenomena in much greater detail.

Electromagnetic radiation comes in photons. A photon is an alternating electromagnetic
field. Since changing electric fields can create magnetic fields, and vice versa, the
photon exists by constantly exchanging its energy back and forth between the two in
a wave. Because of its quantum nature, photons can only be absorbed or emitted as
whole units. For further reading on that subject, search the web for "photoelectric
effect" on any search engine.

Lastly, for the attraction or repulsion between charged particles, all charges create an
electric field. Another charge in that field will want to move along the field lines,
either towards or away from the particle that created the field. What makes the particles
in the field move is the force (the electromagnetic force) that the field exerts on the
charges themselves. The first particle will feel an equal and opposite (in direction)
force from the second charged particle, which is also creating an electric field.
Charged particles feel no force from their own electric fields. This is also explained
in greater detail in any physics textbook, with many colorful illustrations.

For a neat program that lets you place charges and draw the associated fields, as well
as links to the basic theory of electromagnetic fields and forces click here.

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