Point Charges & Magnetism: Relativity's Role

[lovetruth]

Magnetism is said to arise due to relativity. When an extended charged object move, length contraction occurs which increases the objects charge density thus increasing electric field which is then perceived to be caused by magnetism (if we do not account for relativity). Thus, magnetism is a relativistic effect.
for More info: http://galileo.phys.virginia.edu/classes/252/rel_el_mag.html

My Question is : Will point charges show magnetism as they do not have any length which can contract?

 

[Mentz114]

Magnetism is said to arise due to relativity. When an extended charged object move, length contraction occurs which increases the objects charge density thus increasing electric field which is then perceived to be caused by magnetism (if we do not account for relativity). Thus, magnetism is a relativistic effect.
for More info: http://galileo.phys.virginia.edu/classes/252/rel_el_mag.html

My Question is : Will point charges show magnetism as they do not have any length which can contract?

Your explanation is not quite right. The magnetic field appears in a moving frame to counter-act the change in the electric field, so all observers will agree on the path of a charge in a field.

From the article

So observers in the two frames will agree on the rate at which the particle accelerates away from the wire, but one will call the accelerating force magnetic, the other electric. We are forced to the conclusion that whether a particular force on an actual particle is magnetic or electric, or some mixture of both, depends on the frame of reference—so the distinction is rather artificial.

Yes, a moving point charge has a magnetic field.

 

[lovetruth]

Your explanation is not quite right. The magnetic field appears in a moving frame to counter-act the change in the electric field, so all observers will agree on the path of a charge in a field.

From the article



Yes, a moving point charge has a magnetic field.

Please elaborate wat u mean by " counter-act the change in the electric field, so all observers will agree on the path of a charge in a field." I didnt quite get it.

 

[Mentz114]

Please elaborate wat u mean by " counter-act the change in the electric field, so all observers will agree on the path of a charge in a field." I didnt quite get it.

Suppose there's an experiment where an electron is slung through an electric field, which deflects it so it hits a certain spot on a screen.

An observer, moving inertially relative to the lab, would see the electric field as changed, and therefore would not expect the electron to hit the same spot, if they consider only the electric field. But they must see the electron hit the same spot as the observers in tha lab, and indeed they do. The reason is that a magnetic field would also be present for the moving observer, and this (in their calculations) would do just enough to steer the electron through the hole. It is almost as if the magnetic field is there to ensure that no causal paradox happens.

 

[lovetruth]

Suppose there's an experiment where an electron is slung through an electric field, which deflects it so it hits a certain spot on a screen.

An observer, moving inertially relative to the lab, would see the electric field as changed, and therefore would not expect the electron to hit the same spot, if they consider only the electric field. But they must see the electron hit the same spot as the observers in tha lab, and indeed they do. The reason is that a magnetic field would also be present for the moving observer, and this (in their calculations) would do just enough to steer the electron through the hole. It is almost as if the magnetic field is there to ensure that no causal paradox happens.

The link i have provided proves that : Magnetism is just a Relativistic effect and not a fundamental force like Electric force; just as centrifugal is a pseudoforce. Now, magnetism arises due to length contraction of extended charged bodies which increases charge density thus leading to magnetism. Electron is an extended body i.e. it has finite volume but what about a point charge? It is just a point and cannot be squeezed further.

 

[Vanadium 50]

The link i have provided proves that : Magnetism is just a Relativistic effect and not a fundamental force like Electric force; just as centrifugal is a pseudoforce.

I'm afraid it doesn't. One statement is "a moving charge produces a magnetic field". Another statement is "Given an arbitrary E and B field, there always exists a frame where the field is purely electric." Statement 2 does not follow from Statement 1.

 

[kmarinas86]

Your explanation is not quite right. The magnetic field appears in a moving frame to counter-act the change in the electric field, so all observers will agree on the path of a charge in a field.

Every inertial observer will agree with the first and higher derivatives of the path of the charge, but not every observer will agree on how much distance the charge traveled over time. Nor will they agree on the electric and magnetic components of the field. Nor will they agree on the angle between the acceleration $\mathbf{x}''$ of the charge and the velocity $\mathbf{x}'$ of the charge. The acceleration $\mathbf{x}''$ of the charge is the same for all inertial observers, but the velocity $\mathbf{x}'$ of the charge is the same only for co-moving inertial observers.

 

[kmarinas86]

Now, magnetism arises due to length contraction of extended charged bodies which increases charge density thus leading to magnetism.

Whether a particle experiences a gain of magnetism, or a loss, is relative to the observer.
Whether a particle experiences a gain of charge density, or a loss, is relative to the observer.
Whether a particle experiences a gain of length contraction, or a loss, is relative to the observer.

...if Lorentz ether theory is not correct.

 

[lovetruth]

I'm afraid it doesn't. One statement is "a moving charge produces a magnetic field". Another statement is "Given an arbitrary E and B field, there always exists a frame where the field is purely electric." Statement 2 does not follow from Statement 1.

The concept of magnetism is only invoked when relativistic effect like length contraction are ignored. When Maxell wrote electromagnetic equation, he didn't kno about relativity. Thus, magnetism was used as a scapegoat to explain the relativistic phenomena.

 

[lovetruth]

Whether a particle experiences a gain of magnetism, or a loss, is relative to the observer.
Whether a particle experiences a gain of charge density, or a loss, is relative to the observer.
Whether a particle experiences a gain of length contraction, or a loss, is relative to the observer.

...if Lorentz ether theory is not correct.

Lorentz ether theory is correct excluding the concept of ether. Lorentz transformation is correct and is incorporated in SR.
So there is no doubt in the 3 statements u stated. Remove the word 'whether' as the 3 statements r absolutely correct.

 

[member 11137]

Every inertial observer will agree with the first and higher derivatives of the path of the charge, but not every observer will agree on how much distance the charge traveled over time. Nor will they agree on the electric and magnetic components of the field. Nor will they agree on the angle between the acceleration $\mathbf{x}''$ of the charge and the velocity $\mathbf{x}'$ of the charge. The acceleration $\mathbf{x}''$ of the charge is the same for all inertial observers, but the velocity $\mathbf{x}'$ of the charge is the same only for co-moving inertial observers.

Could you please elaborate more on the last affirmations (preservation of the acceleration in a change of inertial frame) or suggest references to learn more about these items?

In advance: thanks.