The Hall Effect - Clarifications

[Chriss]

I was reading about the Hall Effect and i thought i understood it, then as I was reading further I don't.

 

What i understood is that if you place a magnet near a copper wire the electrons go in the opposite direction.

 

 

Theory says that a curent is flowing trough the wire from top to bottom, and the driftting electrons are flowing from bottom to the top. And this is confusing. What do i miss in my knowledge ?

 

 

And second i was reading about Magnetic force exerted on a current carryin wire.

I attach a picture . Can someone explain teach me the intuition of thing in the picture ?

Edited November 4, 2013 by Chriss

[studiot]

You are asking about the Lorenz force

 

http://en.wikipedia.org/wiki/Lorentz_force

[Chriss]

What do i understand by this picture ?

http://en.wikipedia.org/wiki/File:Regla_mano_derecha_Laplace.svg

 

I don't get it. Why is the force on 90 degrees and not in the same dirrection as the magnetic field?

Edited November 4, 2013 by Chriss

[imatfaal]

What do i understand by this picture ?

http://en.wikipedia.org/wiki/File:Regla_mano_derecha_Laplace.svg

 

I don't get it. Why is the force on 90 degrees and not oposite ?

 

The Magnetic field B is completely arbitrary in its "direction" - ie there is no physical reason for the arrow to be pointing in that direction or 180 reversed. In order to be able to communicate with each other we decide on conventions. This diagram is the Lorentz force as studiot mentioned - and the force is perpendicular to both the direction of travel of the charge and the magnetic field. There are two solutions to this unless we apply the usual convention (note this rule is for positive charge and conventional current).

 

That diagram shows how to determine which way around things are. The current runs along a wire in the direction shown by the thumb - in magnetic field that we say is aligned in the direction of the fingers - then the force on the wire will be upwards in the direction of the palm.

In reality we can measure the force and observe the direction of the movement of the charge - so the arbitrary bit is the direction of the magnetic field. In tests you will most likely be give two of three and be asked to determine the direction of the the third

[Chriss]

So when the wire is atracted and when is repeled by the magnetic field ?

[studiot]

Are you asking about the force on the wire or the Hall effect, they are different.

[swansont]

So when the wire is atracted and when is repeled by the magnetic field ?

The force is given by the right hand rule, since F = qv X B. If you point your fingers in the direction of the current, and bend them in the direction of B, your thumb points in the direction of the force.

[Chriss]

Which is the direction of the current ? Is the same as as electrons flowing in one direction ?

[swansont]

Which is the direction of the current ? Is the same as as electrons flowing in one direction ?

We use the positive current flow as the convention, so it's opposite of the direction the electrons are going.

[Enthalpy]

In the Hall effect, as well as the Lorentz force, the effect is at 90° from both the speed and the induction - not opposite.

 

The Lorentz force is a Hall effect, just discovered by different people bearing different names. Charge carriers deflected to the side of the wire are a little bit more concentrated there, and the resulting electrostatic force appears as a Lorentz force.

 

About the direction of current, induction, speed, induced voltage... It's good to know the rules to answer exams. In real life, you have 50% risk to be wrong using them, so everyone tries and, if not the desired polarity, swaps the terminals.

[Chriss]

I think i start to get it. I'm gonna read further on the subject to understand better.

Edited November 5, 2013 by Chriss

[Chriss]

Here is a video showing... How do i apply the right hand rule here ?

 

Edited November 6, 2013 by Chriss

[swansont]

Initial motion is to the right, so hand points to the right. Magnetic field is up (out of the screen), so your palm as to be up to point your fingers up. Thumb points down. The force is always perpendicular to the motion, so you get a circular path while the charge in in the field with the force directed to the center of the circle.

[Chriss]

Ok. I see now.

Edited November 7, 2013 by Chriss

[Chriss]

I started getting confused again as i reading and watching this figure.

How do i apple the right hand rule here ? In the direction of the current ? And also how do i know the direction of the current ?

So when i apply the right hand rule, it's only in the direction of the current.

Edited November 12, 2013 by Chriss

[imatfaal]

Current runs in the x direction which is marked (read up about conventional current if you do not get the direction the charges are shown moving in) and the field in the y. The right hand rule applies to positive charges/conventional current moving in a magnetic field. Fingers in direction of field, thumb in direction of current - the force will be up through the palm. The right hand rule does not apply to a negative charge moving in a magnetic field - you can either reverse it in your head or more easily think of a negative charge moving in one direction as a positive charge moving in the exact opposite direction.

[derek w]

conventional direction of current was before the discovery of electrons and which direction they flow.

[Chriss]

I don't understand the difference between current and voltage. Aren't they proportional ?

Edited November 29, 2013 by Chriss

[imatfaal]

I don't understand the difference between current and voltage. Aren't they proportional ?

 

That's a bit beyond the scope of a forum answer - it is a basic physics class and will take a while to explain. Why not take it upon yourself to do a bit of reading

 

I love this resource

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

You would want the Electricity and Magnetism section.

 

Alternatively why not listen to Walter Lewins introduction to Electromagnetism

https://www.edx.org/course/mitx/mitx-8-02x-electricity-magnetism-608

it starts in February and will run for an entire semester - it has weekly homework and 3 intermediate exams and a final. But you will know a lot more when you finish.

[Chriss]

Yes. I think i asked a stupid question. Is it possible to delete it?

[imatfaal]

Yes. I think i asked a stupid question. Is it possible to delete it?

 

No need - it is far better to ask a 'stupid question' than to sit and wait for enlightenment. And I really wasn't joking about the utility of on-line courses; they used to be very hit and miss but now there are some serious courses available for free from world leading educators in their subject.

 

Of course there is also Khan Academy which I forgot to mention earlier

[MigL]

Not a stupid question at all.

Voltage is a potential and the analogy you may be more familiar with is gravitational potential.

Think of the potential or voltage as the height of a waterfall. The current would then be the mass of water going over the falls.

The potential provides the motive "force" for the current.