Electron movement and that palm rule (1 Viewer)

[VanCarBus]

How the heck do you determine electron movement in an electric field, magnetic field. Excel books describe it quite crappily
HELP?


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http://www.hscenglishsupport.netfirms.com

 

[wogboy]

In an electric field, an electron always moves in the direction exactly OPPOSITE to the electric field (since the electron is neagtively charged). If on the other hand the particle is positively charged (e.g. a proton) then it will move IN the direction of the electric field lines.

In a magnetic field, use the right hand rule to determine the direction of motion of a moving charged particle. If you open your right hand, and you point your thumb in the direction of the particle motion, and your other fingers in the direction of the magnetic field then:

- The force on the particle is UPWARDS (i.e. AWAY from the palm of your hand), if it's POSITIVELY charged (e.g. a proton)
- The force on the particle is DOWNWARDS (i.e. INTO your palm), if it's NEGATIVELY charged (e.g. an electron)

And finally remember that in a MAGNETIC field, no force acts on an electron (or any other charged particle for that matter) unless it's moving (and in addition, not moving parallel to the magnetic field lines). By comparison, an electron or any other charged particle will always have a force acting on it when it's inside an ELECTRIC field, whether it is moving or not.

 

[Huy]

I use the right-hand palm rule, because I don't like Fleming's left hand finger rule.

stick your thumb in the direction of conventional current.
fingers point in the direction of the magnetic field.
your palm then points in the direction of the force.

x goes into the page
" . " goes out of the page

e.g.

x x x x x x
x x x x x x
----------->
x x x x x x
x x x x x x

electron flow ------>

your fingers should be pointing into the screen/page
your thumb should be pointing to the right
and your magnetic field will be generated vertically upwards

to determine the induced current,
fingers point in the direction of the magnetic field,
thumb points in the direction of the induced current,
your palm then points in the direction of the OPPOSING force.

e.g.

. . . . .
. . . . .
--------- (v pointing up/north)
. . . . .
. . . . .

then your fingers will point out of the page,
your palm should be facing towards the screen as v points 'up' on the diagram,
your thumb will then be point LEFT, however you have to reverse it to find out the direction of the force, so you will have your induced current flowing to the RIGHT

 

[stinger]

erm .. huy, you just confused me

for a conductor in a magnetic field ... don't you pretend there is a positive charge in the middle of the conductor

fingers point in direction of field
thumb in direction of VELOCITY?
palm in direction of FORCE?

so if there is a conductor moving to the left, magnetic field out of the screen, positive charges move down?

and then for an electron, you take the opposite direction (i.e. for a positive charge)

and umm, clarifiy this ... if you move a conductor in a magnetic field, are you applying a force or is that acting as the velocity?
i'm thinkin you are applyin a force ... i always get screwed on this one

 

[Huy]

LOL sorry stinger, but I'm a little unsure myself.
I know how to apply it, but I don't know what they really mean sometimes

It's funny, because in an exam, I'll get the question right, but outside of the exam conditions, I won't know anything to be honest

Conductor in a magnetic field?
I was using the electron movement (-ve charge) moving in the direction of conventional current. I don't think you pretend there's a positive charge in the middle, but I'm not sure myself.

If there's ______ (whatever it is lol!) moving to the left,
and the magnetic field points OUT of the screen, therefore it'll look like:

. . . . . . .
. . . . . . .
<---------
. . . . . . .
. . . . . . .

so if your fingers are pointing out of the screen,
and your thumb to the left,
then your palm will be facing UP/north.

BUT if you're trying to work out the induced current,
then the __________ (whatever) will be in the opposite direction, ie, DOWN (south).

That's how I figure it, it works for me, I don't really explain myself very well, but I can do it when it counts LOL?

 

[stinger]

wait ... is conventional current +ve or -ve?

+ve right?

hmmm ... i really should know that already

 

[Huy]

Originally posted by stinger
wait ... is conventional current +ve or -ve?

Conventional current flows from positive potential to negative.

+ve TO -ve

but electrons flow from -ve to +positive
(negatively charged particle being attracted to the positive end)

to sum up:
conventional current: positive to negative
electron current: negative to positive

 

[stinger]

Originally posted by Huy
Conductor in a magnetic field?
I was using the electron movement (-ve charge) moving in the direction of conventional current. I don't think you pretend there's a positive charge in the middle, but I'm not sure myself.

so do you mean when there is a conductor, with or without a current flowing through?

 

[Huy]

Originally posted by stinger
so do you mean when there is a conductor, with or without a current flowing through?

I'm completely confused now as well, stinger!

I always use conventional current, UNLESS it is an electron.
If it's an electron, I will point my thumb in the direction of electron flow, not conventional current (positive to negative).

once that's done, the palm does the rest. (fingers pointing towards the magnetic field, checking X and "."s).

if it's induced current, then i'll use the same method, but change the direction of the thumb after i've figured everything out, as this is the direction which tells me the opposing force.

other than that, i can't really explain it too well here

 

[wogboy]

and umm, clarifiy this ... if you move a conductor in a magnetic field, are you applying a force or is that acting as the velocity?

If you move a conductor in a magnetic field, this is acting as the VELOCITY. You are not *necessarily* applying a force to move the conductor. So the direction you move the conductor is the "thumb" direction.

The only time you need to apply a force on a conductor to move it through a magnetic field is when you draw induced current from that conductor (i.e. when you connect a connecting wire with a resistive load to both ends of the conductor which you're moving through the magnetic field. A part of this connecting wire MUST be outside the magentic field for the current to flow). Whenever such a current flows, a new magnetic field is formed and this new magentic field ALWAYS opposes the original one, in accordance with Lenz's Law.

This new opposing magentic field, in conjunction with the current flowing through the conductor, causes a new force and this new force ALWAYS opposes the direction which you move the conductor. Hence you need to apply a force in order to overcome this opposing "Lenz's Law" force. Just as a side note, because you're applying a force over a certain distance (the distance in which you move the conductor), you are doing work (supplying energy) in doing so (W=Fs). This energy, not surprisingly, is equal to the total electrical energy dissipated in the conductor as induced current flows through it (conservation law of energy).

This is the only case where you would need to apply a force to move a conductor (when drawing current from it), otherwise no force is needed to move it.

 

[Huy]

Thanks for clearing the air wogboy

I didn't know what stinger was saying there.

 

[stinger]

hmmm ... i just read that induced current is in the oppostie direction to the right hand palm rule

wtf???? i did not know this

is this right? where does this apply to?
a conductor ... or a conductor with a resistive load??

i think this is what huy said that made me confused

 

[wogboy]

hmmm ... i just read that induced current is in the oppostie direction to the right hand palm rule

The thing that causes confusion here is the magnetic field. Which magnetic field are we talking about? Which way is it going?

As you move a conductor (with a resistive load around it which is outside the magetic field) through a big "natural" magnetic field (lets call this large magnetic field b1) and the current is induced, the little magnetic field (call this b2) which arises from this induced current is always OPPOSING the original magnetic field, b1.

So for example the magnetic field b1 is going NORTH (remember this is the "natural" magnetic field created by the fixed magnets inside the generator, and this is almost always the magnetic field you'll see drawn in a question diagram), then b2 is going SOUTH.

Now for the most important part, when determining the direction of the induced current, your thumb (velocity/motion direction) points in the direction that you're pushing the conductor. Your other fingers (not your thumb) point in the direction of b2, NOT b1. Then your palm points in the direction of the induced current (coming outwards of your palm).

Alternately you can point your thumb in the direction which you push the conductor (as before), and point your other fingers to b1 (not b2 this time), and now the induced current will be going INTO your palm rather than coming OUT of it as before.

Have a look here to see some diagrams in case you're confused.

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

 

[stinger]

ahhhh .. i see

yea, i think we were just confused with whether there was a 'resistive load'

so if there is not a resistive load (just a rod for example), you take the magnetic field as b1 as you called it

 

[wogboy]

so if there is not a resistive load (just a rod for example), you take the magnetic field as b1 as you called it

If there's no resistive load, then there can't possibly be any induced current! However there will be an induced emf in the rod (due to separation of charges). To determine the positive end of this conducting rod, just use the right hand rule with the thumb in the direction of motion, and the other fingers in the direction of b1 in this case. Then your palm will point up towards the positive end of the rod.

 

[stinger]

yea ... thats what i meant