# The Naked Scientists Forum

### Author Topic: Magnetic fields  (Read 9875 times)

#### Quantumcat

• The Kitty Down Under
• Hero Member
• Posts: 894
##### Magnetic fields
« on: 07/11/2003 07:49:48 »
Can someone tell me, when a charged particle is affected by a magnetic field, where does it get the energy from to move? The energy has to come from somewhere, right? The waves are going outwards and anyway there wouldn't be enough energy in a wave to like push it, and particles can be pulled too so it doesn't really work. Plus that sounds really dumb I know but it's the only thing I can think of to give the particles energy, albeit stupid. (even though it isn't really a "wave" as such ... ) All movement has to have some cause and I haven't found it with magnetic fields. I think I asked my teacher a few years ago when we were studying them I can't remember what he said, either he couldn't answer or I didn't understand what he said. Thanks if you can answer this for me.

Am I dead? Am I alive? I'm both!

#### cuso4

• Angel Delight
• Sr. Member
• Posts: 422
##### Re: Magnetic fields
« Reply #1 on: 07/11/2003 13:16:09 »
Quantum, you said a charged particle so this means the particle must carry either a positive or negative charge. They move because they are attracted to one pole of magnetic field and repelled by the other pole.

This is basically what I thought but I could be very wrong. Anyway I'll ask my teacher to see what he'll say.

Angel

"Anyone who has never made a mistake has never tried anything new." -Albert Einstein

#### Ylide

• Moderator
• Hero Member
• Posts: 905
##### Re: Magnetic fields
« Reply #2 on: 07/11/2003 20:54:29 »
Potential energy is stored in the magnetic field.  When the charged particle "sees" the field gradient, it moves in a manner corresponding to the field lines.  This is analogous to the potential energy of an elevated object.  If the object encounters a gradient that leads to lower potential energy, it will follow it.  (i.e. it will roll down the hill if its on a slope)

This is a very important physical phenomenon, as it dictates not only the behavior of the motion of matter on the macroscopic level, but also things like chemical reactions and phase changes.  A substance will always spontaneously move from higher potential to lower potential if it can.

Note that the particle must already be in motion to be affected by a magnetic field...a charged particle at rest is unaffected by a magnetic field.  I can't remember the exact equation, but the force on the particle is a function of the velocity and the magnitude of the magnetic field.  This is a cross-product function:  If the field line vector points along the x-axis and the velocity vector of the particle points along the y axis, the particle will be deflected along the z axis.

Note also that this same effect occurs in an electrical field, but is not a cross product.  The force on the particle is exclusively along the electric field vector, the direction is a function of the charge on the particle.  (positive charge moves with the field, negative charge moves against)

This is probably way more information than you wanted.  =)  Very good question though.

This message brought to you by The Council of People Who Are Sick of Seeing More People

#### Quantumcat

• The Kitty Down Under
• Hero Member
• Posts: 894
##### Re: Magnetic fields
« Reply #3 on: 08/11/2003 10:09:11 »
quote:
Originally posted by cuso4

Quantum, you said a charged particle so this means the particle must carry either a positive or negative charge. They move because they are attracted to one pole of magnetic field and repelled by the other pole.

This is basically what I thought but I could be very wrong. Anyway I'll ask my teacher to see what he'll say.

Angel

"Anyone who has never made a mistake has never tried anything new." -Albert Einstein

Oops! I meant electric fields, there's no charged particles involved in magnetic fields, but I mean where does it get the energy from to move. I don't know how to make another quote but in reply to cannabinoid's answer, you said "see" ... how does the particle "see" the electrical field??? How does the field travel, and how fast? Is it instantaneous? How does this travelling/instantaneous field push or pull the object? What you said I learned at school when we did electrical fields (I'll assume everywhere you said magnetic you mean electrical, my mistake that made you say that)

Am I dead? Am I alive? I'm both!

#### Ylide

• Moderator
• Hero Member
• Posts: 905
##### Re: Magnetic fields
« Reply #4 on: 08/11/2003 21:07:26 »
Quantum:

Charged particles ARE affected by magnetic fields.  Magnetic fields are also created by the movement of charged particles, just as electric fields are created by the existance of charged particles.  Confused?

To summarize:  Any charged particle (or surface containing a charge) has an electrical field.  The field lines are generally perpendicular to the surface of the charged object.  (in the case of a particle or sphere, they are radially outward or inward depending on the charge)  The field lines point "away" from positive and "toward" negative charge, so an electron would appear as a point charge with field lines pointing radially towards it.

So electric fields are created by the presence of charge...any charge, positive or negative.

Magnetic fields are created in two ways.  1) movement of current through a wire and 2) intrinsic magnetic fields in charged particles such as electrons.  This intrinsic field is a basic property, like mass and charge.

Magnetic fields affect OTHER charged particles in a manner similar but different to electric fields.  Read my post above for a description of the difference.

Anyway, back on to the point, to clarify how these fields produce a force.  It is a force very similar to gravity.  The graviational force between two objects is described by the equation F=K*M1*M2/d^2  where K is the gravitation constant, M1 and M2 are the masses of the two objects, and d is the distance between them.  Gravity is always an attractive force, for reasons physicists haven't quite figured out yet.

Now, the force between two charged particles is described mathematically as F=k*Q1*Q2/d^2  where k is a constant that contains a few symbols I don't know to type with html.  Q1 and Q2 are the respective charges.  See the similarity between the force due to gravity and the force due to charge?  Now, the electric field defined as the force on a test charge due to another charge divided by the charge of the test charge.  E=F/Q1.  The potential energy on a particle due to an electric field is a line integral of the electric field over the distance travelled.

So, with all that background now, I can answer your question.  =)

1) how does the particle "see" the electrical field???

How does a falling object "see" gravity?  It's a difference in potential energy between two points that causes an object to fall.  It can reach a lower potential energy by falling (assuming nothing is holding it in place) so it falls.  A particle in a field can reach a lower potential energy by following the field lines.  It will always follow the path to the lowest potential.  The electric field forms a gradient which the charge can follow.  Just as an intervening object can stop something that is falling, something that disrupts the electric field can stop a charged particle from being affected by it.

2) How does the field travel, and how fast? Is it instantaneous?

The field doesn't travel, per se, but changes in the field propagate at the speed of light.

3)  How does this travelling/instantaneous field push or pull the object?

Again, this is due to the gradient of the field and the potential difference it creates.  Wether it pushes or pulls depends on the vector of the field and the charge of the particle.  Use the equation I gave above, rearranged for force:  F=E*Q.  Do you know about dot products and cross products?  (from vector mathematics)  If so, this is a dot product.  If it were a magnetic field, it would the cross product between the velocity vector of the particle and the magnetic field vector.

Did I explain it better this time?  It's kind of a confusing and abstract idea.

This message brought to you by The Council of People Who Are Sick of Seeing More People

#### tweener

• Moderator
• Neilep Level Member
• Posts: 1144
##### Re: Magnetic fields
« Reply #5 on: 09/11/2003 02:58:43 »
That is a very good explanation by cannibinoid.  I can't add much except to point out that a magnetic field is generated by any sort of changing electric field, and an electric field is generated by any sort of changing magnetic field.  This is how electromagnetic waves propagate through space.  Something generates a changing electric field (like applying a voltage across the terminals of an antenna) which generates a changing magnetic field, always at the same frequency and related by the derivative of the function describing the e-field.  This magnetic field generates another e-field which generates an m-field etc. etc. and you have a wave that travels at the speed of light (it may be light) and can travel the universe until something absorbs it.

If it happens across a charged particle, then, as cannibinoid explained, it causes the particle to move.  If the particle happens to be an electron in someone's receiving antenna, then they can detect the current (which is a moving charged particle) and thus have a radio program.  However, causing the particle to move causes the wave to lose energy and thus that part of it cannot travel any more.  On the quantum level, the waves are photons whether they are light or radio, and the movement of electrons or protons behaves according to quantum mechanics.  For frequencies much lower than visible light, it is much easier to use the "classical" view of Maxwell's equations to describe and predict the fields and currents.  Maxwell's equations treat everything as waves, with no particle aspect and no quantum effects, but they are very good for real calculations on all sorts of radio and radar applications.

This doesn't help much, but I love to ramble!

----
John

#### The Naked Scientists Forum

##### Re: Magnetic fields
« Reply #5 on: 09/11/2003 02:58:43 »