Pure magnetic field

Main Question or Discussion Point

When charged particles aren't moving they produce an electric field. As the start to move and accelerate, the electric field intensity gets lower, but a magnetic field is being made and it increases in intensity. If the particles reach the speed of light will they produce pure magnetic field without the electric field?

An atom with unpaired electrons doesn't have a pure magnetic field, there is an electric one,too, right?

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The velocity of a particle effectively has no effect on the electric field at non-relativistic speeds. A moving particle will not have a lower intensity electric field. If the particle is moving at relativistic speeds, then the effects of Lorentz contraction will actually cause a higher-intensity field perpendicular to the velocity of the particle. When a particle is traveling near the speed of light, it will have the same electric field as if it were stationary, only it will be deformed by Lorentz contraction.

Simon Bridge
Homework Helper
When charged particles aren't moving they produce an electric field. As the start to move and accelerate, the electric field intensity gets lower, but a magnetic field is being made and it increases in intensity. If the particles reach the speed of light will they produce pure magnetic field without the electric field?
No.
An atom with unpaired electrons doesn't have a pure magnetic field, there is an electric one,too, right?
A "paired" electron is usually considered to be one that is paired with another electron with the opposite spin. But "unpaired electron" could be taken to mean that the number of electrons is not the same as the number of protons. So I am going to be careful:

An atom with the same number of electrons as protons also has an electric field.

The velocity of a particle effectively has no effect on the electric field at non-relativistic speeds. A moving particle will not have a lower intensity electric field. If the particle is moving at relativistic speeds, then the effects of Lorentz contraction will actually cause a higher-intensity field perpendicular to the velocity of the particle. When a particle is traveling near the speed of light, it will have the same electric field as if it were stationary, only it will be deformed by Lorentz contraction.
What happens with the magnetic field?

For now, thanks to both of you for the answers.

Simon Bridge
Homework Helper
You can work it out yourself from the Lorenzt transformation and Maxwels equations ... the standard exercize given relativity students is to consider an infinite line of charge ... you should be able to work out the leectric field due to that. It is not moving so it has no magnetic field.

Now do the same for an observer moving parallel to the line.
Now, for that observer, the line of charges is moving - it's a current.

At relativistic speeds you'd get an appreciable length contraction - what does that do to the charge density? To the electric field? To the current? To the B field?