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Name: Areial
Status: student
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Does a charge moving with a constant velocity have both electric and magnetic field?


Wow, that was an easy question!

Ask for further clarification if that leaves you with questions. Einstein's wrestling with this same concept led to his development of the special theory of relativity.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


There will be a magnetic effect. The electric field at any point will change. According to Maxwell's equations, electric field changing over time will generate a magnetic field. Ampere's Law as adjusted to include for induction tells us that the integral of magnetic field around a circle is proportional to the rate of change over time of the electric flux through that same circle. It also tells us that the faster the charge at a given location, the greater the magnetic field in the space round the charge.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Hello Areial,

The answer to your question is, "it depends upon how you look at it!" And understanding that question led to some of the greatest achievements of classical physics.

In the absence of any external factors :

Situation 1, you are sitting still and observing a charged particle move past you in a straight line. In such a case you could observe a magnetic field and an electric field from the charged particle. Both the magnetic and electric fields at your location would vary with time.

Situation 2, you are moving at the same speed and in the same direction as the charged particle. Now you will not observe any magnetic field, but you will observe a constant electric field from the charged particle.

Understanding the importance of reference frames in these two situations is part of what led first to Maxwell unifying our understanding of electricity, magnetism and light into a single framework, and eventually leading Einstein to the theory of special relativity. It is curious to note that his seminal paper on special relativity contains neither of those words in the title. Rather it is titled (excuse my rough German translation), "Concerning the electrodynamics of moving bodies."

Michael Pierce

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