# Is action and reaction instantanious?

Andrew Mason
Homework Helper
nobody has addressed my very simple question. what about a hypothetical massless charged particle? it has self inductance so it would accelerate under the influence of an external field in exactly the same way that a massive particle would. the force due to self inductance exactly balancing the force due to the external field. net force is zero yet it still accelerates.
There is, of course, no such thing as a massless charged particle. A massless particle travels at the speed of light relative to all inertial frames, so it cannot accelerate.

A particle with charge q in an electric field $\vec E$ experiences a force $\vec F$ = q$\vec E$. The only thing that affects its acceleration is its mass. $\vec F$ = q$\vec E$ = ma. There is no other force.

AM

There is, of course, no such thing as a massless charged particle. A massless particle travels at the speed of light relative to all inertial frames, so it cannot accelerate.

A particle with charge q in an electric field $\vec E$ experiences a force $\vec F$ = q$\vec E$. The only thing that affects its acceleration is its mass. $\vec F$ = q$\vec E$ = ma. There is no other force.

AM

whoa. a moving charge has energy in its magnetic field. this energy must be supplied by the external force. it would not move at the speed of light.

atyy
this hypothetical massless charged particle isnt a point change. it has finite diameter. and no I dont know whits holding it together. it doesnt matter. maybe superglue.
Xiaochao Zheng's notes that I linked to above also treated the non-point charge case.

Jammer, Concepts of Mass, Chapter 11

Acceleration-dependent self-interaction effects as a possible mechanism of inertia
Vesselin Petkov
http://arxiv.org/abs/physics/9909019

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Andrew Mason
Homework Helper
whoa. a moving charge has energy in its magnetic field. this energy must be supplied by the external force. it would not move at the speed of light.
?? A moving charge has energy in its magnetic field only relative to a charge in a different intertial frame of reference. The magnetic field disappears for a charge in the same intertial frame of reference.

A charged particle cannot move at the speed of light because all particles with charge have mass. Light will always travel at the speed of light, c, relative to a particle with mass.

The only inertia, or resistance to change in speed, of a charged particle in an electric field is due to the mass of the charged particle, as explained in my previous post. (At relativistic speeds, the apparent inertia increases due to relativistic effects).

AM

atyy
Xiaochao Zheng's notes that I linked to above also treated the non-point charge case.

Jammer, Concepts of Mass, Chapter 11

Acceleration-dependent self-interaction effects as a possible mechanism of inertia
Vesselin Petkov
http://arxiv.org/abs/physics/9909019
Apparently a massless charged particle is not classically possible even if it is not a point charge. All discussions use F=dp/dt, ie. no change in momentum unless there is a net force. granpa: your superglue is referred to by Jackson (Classical Electrodynamics, Wiley 1998) as Poincare stresses.

References that seem to be regarded as standard, but not available for free are:
A.D. Yagjian, Relativistic Dynamics of a Charged Sphere, 2nd ed. Springer 2006
F. Rohrlich, Classical Charged Particles, 3rd ed. (World Scientific 2007)

On arXiv:
Radiation reaction of a classical quasi-rigid extended particle
Rodrigo Medina
http://arxiv.org/abs/physics/0508031

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it doesnt need to be possible. its hypothetical. I'm not suggesting that mass really is due to self induction. I'm saying that one can look at it in the same way.

lorentz spent a good deal of energy looking for a way to make mass be due to self induction. so apparently he didnt think it was an impossible idea.

atyy
it doesnt need to be possible. its hypothetical. I'm not suggesting that mass really is due to self induction. I'm saying that one can look at it in the same way.

lorentz spent a good deal of energy looking for a way to make mass be due to self induction. so apparently he didnt think it was an impossible idea.
All the links I gave above are referring to that idea of Lorentz and others, which they consider very seriously. I think you should study them before thinking I'm talking about something different.

Andrew Mason
Homework Helper
it doesnt need to be possible. its hypothetical. I'm not suggesting that mass really is due to self induction. I'm saying that one can look at it in the same way.

lorentz spent a good deal of energy looking for a way to make mass be due to self induction. so apparently he didnt think it was an impossible idea.
Perhaps you could give us a cite for the last statement.

Self induction is a phenomenon that occurs in a coil due to current flow. The expanding magnetic field from increasing current in one coil cuts across an adjacent coil and induces a current that opposes the increasing current.

Electrons do not experience self induction. You seem to be suggesting that there is something other than mass that causes a charged particle to resist changes in its motion.

AM