Zenarthra
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http://www.physchem.co.za/OB11-ele/g...gnetic4_f1.gif

When moving a magnet in and out of a coil of wire, how do i know the direction of the flow of current?

thanks
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krisshP
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(Original post by Zenarthra)
http://www.physchem.co.za/OB11-ele/g...gnetic4_f1.gif

When moving a magnet in and out of a coil of wire, how do i know the direction of the flow of current?

thanks
Hard to explain and fully grasp as it's can be a very confusing concept, but I'll try.

In the first case, the magnet has a field around it like this

http://www.electronics-tutorials.ws/...etism/mag2.gif

As the magnet approaches the coil in your first case due to a force on it towards the right, the magnet and coil get closer, so there's an increase in flux linkage through the coil, see

Name:  image.jpg
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In your case it's a bit like initially 0 field lines through coil and then, say, 5 field lines through coil, so an increasein flux linkage through the coil

By Faraday's law there's an induced emf in the coil which has a magnitude directly proportional to the rate of change of flux linkage through the coil. The induced emf causes a current to flow in the coil and this current in the coil sets up a magnetic field around the coil, see

http://upload.wikimedia.org/wikipedi...netism.svg.png

Also using Fleming's right hand grip rule you can determine the conventional current direction if you know the field direction or you can determine the field direction if you know the conventional current direction. This explains how you use the rule

http://www.electricyouniverse.com/ey...and-rules3.jpg
http://4.bp.blogspot.com/-8e_cyTZXHw...rip-Rules2.png

By Lenz's law the induced emf acts in a direction in the coil such that it opposes the flux change that caused it. We know that originally the flux change was caused by the magnet moving to the right due to a force on it to the right, so the current in the coil causes a field around the coil which points to the left, in the opposite direction to the field from the magnet, after all field lines are like force lines, so the magnet experiences a force to the left and is pushed back to the left. Now, we know that the current in the coil forms a field pointing to the left, allowing us to use Fleming's right hand grip rule to work out the current direction in the coil.

You try the second case.


http://www.thestudentroom.co.uk/show....php?t=2552892

Try doing the question there, it can really help you grasp the concept. Then see my long post below on the thread for my explanation oaf the solution.
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Zenarthra
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(Original post by krisshP)
Hard to explain and fully grasp as it's can be a very confusing concept, but I'll try.

In the first case, the magnet has a field around it like this

http://www.electronics-tutorials.ws/...etism/mag2.gif

As the magnet approaches the coil in your first case due to a force on it towards the right, the magnet and coil get closer, so there's an increase in flux linkage through the coil, see

Name:  image.jpg
Views: 563
Size:  58.1 KB

In your case it's a bit like initially 0 field lines through coil and then, say, 5 field lines through coil, so an increasein flux linkage through the coil

By Faraday's law there's an induced emf in the coil which has a magnitude directly proportional to the rate of change of flux linkage through the coil. The induced emf causes a current to flow in the coil and this current in the coil sets up a magnetic field around the coil, see

http://upload.wikimedia.org/wikipedi...netism.svg.png

Also using Fleming's right hand grip rule you can determine the conventional current direction if you know the field direction or you can determine the field direction if you know the conventional current direction. This explains how you use the rule

http://www.electricyouniverse.com/ey...and-rules3.jpg
http://4.bp.blogspot.com/-8e_cyTZXHw...rip-Rules2.png

By Lenz's law the induced emf acts in a direction in the coil such that it opposes the flux change that caused it. We know that originally the flux change was caused by the magnet moving to the right due to a force on it to the right, so the current in the coil causes a field around the coil which points to the left, in the opposite direction to the field from the magnet, after all field lines are like force lines, so the magnet experiences a force to the left and is pushed back to the left. Now, we know that the current in the coil forms a field pointing to the left, allowing us to use Fleming's right hand grip rule to work out the current direction in the coil.

You try the second case.


http://www.thestudentroom.co.uk/show....php?t=2552892

Try doing the question there, it can really help you grasp the concept. Then see my long post below on the thread for my explanation oaf the solution.
Ahh ok, for example a magnet dropped through a conducting rod, is there no induced emf when the magnet is travelling inside the rod because there is no change in magnetic flux?
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uberteknik
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(Original post by Zenarthra)
Ahh ok, for example a magnet dropped through a conducting rod, is there no induced emf when the magnet is travelling inside the rod because there is no change in magnetic flux?
What do you mean by 'conducting rod'?

Do you mean a hollow cylinder with a magnet travelling through it?

Take a look at this:

http://www.4physics.com/phy_demo/New...Nightmare.html
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Zenarthra
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(Original post by uberteknik)
What do you mean by 'conducting rod'?

Do you mean a hollow cylinder with a magnet travelling through it?

Take a look at this:

http://www.4physics.com/phy_demo/New...Nightmare.html
Name:  magnet.png
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Yes thats what i meant.:P
Could you please take a look at this diagram?
Thanks!
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uberteknik
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(Original post by Zenarthra)
Name:  magnet.png
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Yes thats what i meant.:P
Could you please take a look at this diagram?
Thanks!
The yellow object is the bar magnet not the cylinder.

In the static condition (free standing with nothing in it) the cylinder does not have an associated magnetic field there is no induced current because there is no changing external magnetic field to produce any.

When the magnet is dropped into the tube, the tube cuts the lines of flux produced by the magnet and by Faraday's and Lenz's rules, that motion constitutes a changing flux which therefore produces a circulating current around the circumference of the tube.

NB. Both the north and south poles of the magnet will produce a current in the cylinder and each will be in the opposite direction to the other.

By inspection, the currents induced in the cylinder are changing because the magnet is moving through the cylinder. i.e. the currents also have a vector component travelling at the same velocity with the magnet because the magnetic lines of flux are being cut by the cylinder as it passes through.

By Faraday's and Lenz's laws, that travelling current will produce an associated magnetc field in the walls of the cylinder. This magnetic field is set up so as to oppose the magnets field which produced it.

THIS IS THE IMPORTANT BIT:

As the magnet passes through the cylinder under the external force of gravity, the magnets lines of flux try to spin-align the immediately adjacent electrons and magnetic domains of the cylinder. Since the magnet is continuously moving past new static domains and electrons, new lines of flux are continuously cut throughout the length of the cylinder. This is the reason why the travelling magnet is slowed throughout the length of the cylinder and not just at the ends where the magnet enters and exits.
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Zenarthra
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(Original post by uberteknik)
The yellow object is the bar magnet not the cylinder.

In the static condition (free standing with nothing in it) the cylinder does not have an associated magnetic field there is no induced current because there is no changing external magnetic field to produce any.

When the magnet is dropped into the tube, the tube cuts the lines of flux produced by the magnet and by Faraday's and Lenz's rules, that motion constitutes a changing flux which therefore produces a circulating current around the circumference of the tube.

NB. Both the north and south poles of the magnet will produce a current in the cylinder and each will be in the opposite direction to the other.

By inspection, the currents induced in the cylinder are changing because the magnet is moving through the cylinder. i.e. the currents also have a vector component travelling at the same velocity with the magnet because the magnetic lines of flux are being cut by the cylinder as it passes through.

By Faraday's and Lenz's laws, that travelling current will produce an associated magnetc field in the walls of the cylinder. This magnetic field is set up so as to oppose the magnets field which produced it.

THIS IS THE IMPORTANT BIT:

As the magnet passes through the cylinder under the external force of gravity, the magnets lines of flux try to spin-align the immediately adjacent electrons and magnetic domains of the cylinder. Since the magnet is continuously moving past new static domains and electrons, new lines of flux are continuously cut throughout the length of the cylinder. This is the reason why the travelling magnet is slowed throughout the length of the cylinder and not just at the ends where the magnet enters and exits.
So when a magnetic field passes through the conductor, do the electrons in the conductor move perpendicular to the magnetic field lines and this causes the induced current?
Could you share a link that explains the spin aligning of electrons please?

thanks.
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uberteknik
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(Original post by Zenarthra)
So when a magnetic field passes through the conductor, do the electrons in the conductor move perpendicular to the magnetic field lines and this causes the induced current?
Yes.

(Original post by Zenarthra)
Could you share a link that explains the spin aligning of electrons please?
thanks.
This gets well beyond the A-level syllabus very quickly. For a full understanding you will need to understand the Lorentz force, magnetic moments, Maxwells equations, Quantum Field Theory to name a few. These and the maths describing them is well beyond FP4 level and normally are part of a Physics degree course.

For now, accept the laws as read together with the basic equations in the 2nd link provided.

http://bio.groups.et.byu.net/mri_tra...c_Fields.phtml

http://www.school-for-champions.com/...sm_lorentz.htm

http://en.wikipedia.org/wiki/Lorentz...ion_of_E_and_B

http://www.maxwells-equations.com/

http://en.wikipedia.org/wiki/Electro..._dipole_moment

Good luck!
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Zenarthra
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(Original post by uberteknik)
As the magnet passes through the cylinder under the external force of gravity, the magnets lines of flux try to spin-align the immediately adjacent electrons and magnetic domains of the cylinder. Since the magnet is continuously moving past new static domains and electrons.
Thanks for the links, i will read them soon.
But is it this that causes the magnet to kind of wobble side to side in which flux will constantly be cutting the rod.

And if this did not happen the flux cutting would not change and hence a emf will not be induced?
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uberteknik
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(Original post by Zenarthra)
Thanks for the links, i will read them soon.
But is it this that causes the magnet to kind of wobble side to side in which flux will constantly be cutting the rod.


Wobbling probably due to imbalance in the mass of the magnet and/or by not placing the magnet completely centrally in the cylinder, or an inherent non symmetrical magnetic field of the magnet, turbulent air flow etc. All of which can induce SHM as the magnet moves through the cylinder..


(Original post by Zenarthra)
And if this did not happen the flux cutting would not change and hence a emf will not be induced?
At what points on the surface of the cylinder are you trying to measure the emf? There will be an emf generated between points on the same plane around the circumfernece of the cylinder. The emf will change as the magnet passes through as two spikes. First in one direction and then in the other as the NS poles traverse the measuring positions.

Read through the notes provided carefully. I have already explained the mechanism why the current is induced.
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Zenarthra
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(Original post by uberteknik)


Wobbling probably due to imbalance in the mass of the magnet and/or by not placing the magnet completely centrally in the cylinder, or an inherent non symmetrical magnetic field of the magnet, turbulent air flow etc. All of which can induce SHM as the magnet moves through the cylinder..


At what points on the surface of the cylinder are you trying to measure the emf? There will be an emf generated between points on the same plane around the circumfernece of the cylinder. The emf will change as the magnet passes through as two spikes. First in one direction and then in the other as the NS poles traverse the measuring positions.

Read through the notes provided carefully. I have already explained the mechanism why the current is induced.
I meant when its in the middle of the rod, like from B-C in the diagram.
What equation could be used to measure the induced emf from B-C?
Because the flux doesn't change but is just cut.

Thanks, im just getting alil confused, I apologize for the stupid questions xD
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uberteknik
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(Original post by Zenarthra)
I meant when its in the middle of the rod, like from B-C in the diagram.
What equation could be used to measure the induced emf from B-C?
Because the flux doesn't change but is just cut.

Thanks, im just getting alil confused, I apologize for the stupid questions xD
Are you doing this as an experiment for a controlled assessment?
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Zenarthra
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(Original post by uberteknik)
Are you doing this as an experiment for a controlled assessment?
Im preparing for 1 yes
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uberteknik
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(Original post by Zenarthra)
Im preparing for 1 yes
OH.
I've probably inadvertently given you way more information than I should already.
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Zenarthra
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(Original post by uberteknik)
OH.
I've probably inadvertently given you way more information than I should already.
Its ok, i like learning.
Back to question :P

What equation could be used to measure the induced emf from B-C?
Because the flux doesn't change but is just cut.
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DannySmith420
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I need help understanding Lenzs law. I understand that a current carrying conductor has its own magnetic field, hence why when a coil has an induced current, its magnetic field produced by the induced current opposes the motion of the magnet. However if it not a complete circuit no current is induced only e.m.f and if no current is induced where does the force come from to oppose the motion because without current it has no magnetic field?
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