Can Magnetic Fields decelerate a charged particle?

[IronHamster]

The equation for the force of a magnetic field on a moving charged particle would say no:

F = q * v x B,

which means the force is always perpendicular to v, and never has a component along v. Thus a magnetic field can't accelerate a particle, only deflect.

This is correct, right? If so, why do permanent magnets attract or repel each other?

 

[John37309]

A magnetic field CAN decelerate a charged particle, but only if the particle has magnetic properties. Think of it like this; If the particle is made of iron dust, iron is a ferris metal, so it will be effected by magnetic fields. But if your particle is made of very fine silica glass that does not stick to permanent magnets, then the magnetic field will not have any effect on the particle.

John.

 

[zebidieB]

When accelerating or decelerating, any charged particle (such as an electron) radiates away energy in the form of electromagnetic waves.

 

[IronHamster]

Cool! How about an ordinary conducting wire or a moving electron? Those don't have magnetic properties right?

 

[John37309]

Cool! How about an ordinary conducting wire or a moving electron? Those don't have magnetic properties right?

Electrons can be bent and manipulated in dipole magnetic fields. Like in the old CRT TV tubes.

A dipole magnetic field can induce a current to flow in a wire, if the wire forms an electric circuit.

John.

 

[JaredJames]

A magnetic field CAN decelerate a charged particle, but only if the particle has magnetic properties.

A charged particle moving generates a magnetic field, therefore surely it must have "magnetic properties".

 

[John37309]

A charged particle moving generates a magnetic field, therefore surely it must have "magnetic properties".

I suppose you need to clarify the specific situation to explain it properly. IronHamster seemed to need clarification between dipole magnetic fields and static electric fields, which are two different sides of a similar coin. But they are different.

An electron being shot at the screen of a TV is a charged particle, and it can have magnetic properties. But that is not what we are using that electron for in the cathode ray tube. We just want to electron to excite the phosphor on the screen.

But in a different situation, in a simple battery light bulb circuit, electrons are still charged particles, but that's not what we are using them for in this case. In the light bulb circuit, we are channelling the electrons through a copper wire to excite the light bulb filament. In this case, there is a magnetic field created around the wires by the flow of the electrons.

So really, it very much depends on what context you are discussing.

John.

 

[JaredJames]

An electron being shot at the screen of a TV is a charged particle, and it can have magnetic properties. But that is not what we are using that electron for in the cathode ray tube. We just want to electron to excite the phosphor on the screen.

But in a different situation, in a simple battery light bulb circuit, electrons are still charged particles, but that's not what we are using them for in this case. In the light bulb circuit, we are channelling the electrons through a copper wire to excite the light bulb filament. In this case, there is a magnetic field created around the wires by the flow of the electrons.

What does the eventual use of a charged particle have to do with whether or not we can accelerate it with a magnetic field?

A moving charge produces a magnetic field. In your first example, the magnetic property allows you to control where the electron goes. In the latter, the magnetic property isn't utilised, however you could easily replace bulb with motor (but that is irrelevant).

This magnetic field means you can interact with it using other magnetic fields to accelerate / decelerate / control etc.

To the OP: how do you think they accelerate particles at CERN? It's not magic, it's via magnets.

 

[cragar]

Electrons can be bent and manipulated in dipole magnetic fields. Like in the old CRT TV tubes.

A dipole magnetic field can induce a current to flow in a wire, if the wire forms an electric circuit.
John.

Are you saying a static B field can cause a current in a wire, Cause it cant. We need a changing B field to induce an E field. And we can model a magnetic dipole with a current loop.
And at cern the particles are accelerated with an Electric field and deflected with magnets to keep them in a circle. B fields can't do work. I guess you could argue that a B field cause centripetal acceleration but it doesn't change their speed.

 

[singhvi]

As I understand, you want to know that when the magnetic force is vXB then why should there be any increase in the speed of the electron (exchange of energy).

The thing that is different in the context of two magnets attracting or repelling each other is that static magnetic fields do not result in any transfer of energy, but a changing magnetic field can.

When two magnets attract or repel, the magnetic fields are obviously changing with time, which is the reason why vXB is not the only force term there, there is an induced electric field which is the term that represents the energy transfer from the magnetic field to the motion of the magnets.

I hope that clears it up, and also why just stating vXB as the force is not a valid assumption, as even the slightest of perturbation in the position/motion of a bound electron, will result in a force on the other electrons and the perturbation of these electrons will act as a positive feedback as the configuration is not stable when two magnets ineteract.

 

[singhvi]

I also think that to actually get a better picture of why the magnets attract, you should look at the relativistic effects that are responsible for what we call the magnetic force, which is actually nothing different than the electric force with considerations of relativistic effects like length contraction and retarted potentials.