Direction of the magnetic needle

[Darshit Sharma]

Homework Statement

In which direction will the needle be deflected when the key is closed according to the given setup and is the SNOW rule correct?

Relevant Equations

Right Hand Thumb Rule.

I drew the magnetic field lines. The setup was like this: The needle below AB was in the same plane so above AB we get the magnetic field pointing inwards then looping all over Ab from behind the emanating from below AB i.e. pointing outwards. The needle is kept at that point from which the field lines appear to be emanating. I don't know how to proceed further.

So since the magnetic field is emerging out ( perpendicular to the plane AB) at the point at which the needle is kept, if we draw a tangent we get a direction telling us that the needle points towards the east.

The answer to the first part should be east. Consequently, the answer to the third part should be west.

I hope the answers are correct, aren't they?

To my surprise, my teacher taught me the SNOW rule and made us believe the following statement :

SNOW rule states that if the current is flowing in an electric circuit from the South to the North direction and a magnetic compass is placed Over the conducting wire, the needle of the compass deflects in the direction of the west.
This is a rule given to remember easily.
If the directions are reversed the needle of the compass deflects in the direction of east.

According to my teacher, the deflection should be towards west.

Isn't the SNOW rule wrong?

 

[hutchphd]

Are the objects in your diagram coplanar?? It is not clear, and I do not understand "over" and what is wrong with the right hand cross product rule?? Never before heard of SNOW.....

 

[Darshit Sharma]

Are the objects in your diagram coplanar?? It is not clear, and I do not understand "over" and what is wrong with the right hand cross product rule?? Never before heard of SNOW.....

They are not. The magnetic needle is kept in a plane perpendicularly below AB.

I also do not understand that rule but "over" here simply means above. I guess he was trying to convey that if we place a magnetic needle above a wire which carries current from the south direction to the north direction, the needle will be deflected towards the west. (I googled it and there are many webpages regarding this all conveying the wrong info)

Nothing is wrong with the right-hand rule; I used it too. However, my answer comes out to be East for the (i) part whereas his answer is west.

What do your say about the answer and the rule?

 

[kuruman]

OP posted this question on another thread and I advised to post separate questions separately. The positioning of the needle is ambiguous and I so advised OP here, post #27 and #30 for my reply
https://www.physicsforums.com/threads/a-confused-compass-needle-between-two-solenoids.1059087/

I had never heard of SNOW either. It is a mnemonic for Ampere's swimming rule which I don't understand either. From https://unacademy.com/content/question-answer/physics/what-is-the-ampere-swimming-rule/:

What is the Ampere Swimming Rule?

Answer : If a swimmer swims in the current direction while facing a magnetic needle, the north pole of the magnetic field deflects towards his left hand and the south pole towards his right hand, according to Ampere’s swimming rule.

The word ‘SNOW’ can also help remember this guideline. It indicates that if the current flows from south to north, the North Pole will be deflected to the east.

Around a permanent magnet, a continuous electric current flows in one direction. The magnetic field is stationary and referred to as a magnetostatics field. At each given time, its magnitude and trajectory remain consistent.

The amount of (dis)(mis)information therein, not to mention ambiguity, is mind-boggling.

 

[Lnewqban]

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html#c3

I risk assuming that East is coming out of the paper, while West is going into it.

 

[SammyS]

Homework Statement: In which direction will the needle be deflected when the key is closed according to the given setup and is the SNOW rule correct?
Relevant Equations: Right Hand Thumb Rule.

To my surprise, my teacher taught me the SNOW rule and made us believe the following statement :

SNOW rule states that if the current is flowing in an electric circuit from the South to the North direction and a magnetic compass is placed Over the conducting wire, the needle of the compass deflects in the direction of the West.
This is a rule given to remember easily.
If the directions are reversed the needle of the compass deflects in the direction of east.

According to my teacher, the deflection should be towards west.

Isn't the SNOW rule wrong?

It seems to me that the SNOW rule is written for the case of electron current rather than for conventional current.
As regards conventional current it gives the wrong direction for the compass needle deflection.

As for your included figure, as has already been pointed out, it's unclear as to the relative positions of the objects - particularly in a 3-dimensional sense.

 

[kuruman]

It seems to me that the SNOW rule is written for the case of electron current rather than for conventional current.

In which case Ampere's swimming rule ought to apply to swimmers facing the magnetic needle while doing the backstroke.

 

[Lnewqban]

It seems to me that the SNOW rule is written for the case of electron current rather than for conventional current.

@Darshit Sharma , please, see this animation showing the actual flow of electrons.

https://nationalmaglab.org/magnet-a...ive-tutorials/magnetic-field-around-a-wire-i/

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elecur.html#c3

 

[kuruman]

@Darshit Sharma , please, see this animation showing the actual flow of electrons.

https://nationalmaglab.org/magnet-a...ive-tutorials/magnetic-field-around-a-wire-i/

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elecur.html#c3

Nice animations. Here is a modified screenshot attempting to show the implementation of the SNOW rule and Ampere's swimming rule.

SNOW Rule
If the current flows from south to north, the North Pole will be deflected to the east.
That is indeed true here. The North Pole of the needle is red and the current is conventionally defined.

Ampere's swimming rule
If a swimmer swims in the current direction while facing a magnetic needle, the north pole of the magnetic field deflects towards his left hand and the south pole towards his right hand, according to Ampere’s swimming rule.

The swimmer is shown swimming above the needle in the direction of the current. If he swims free style and faces the needle, the rule does not work. To make work, he must flip over and do the backstroke in which case he will not be facing the needle. Unless, of course, the swim rule is meant for electron flow as @SammyS pointed out in post #6. However that would be inconsistent with the SNOW rule that uses conventional current. Instead of helping understand the swimming rule, the SNOW rule contradicts it. What a mess!!

 

[Lnewqban]

 

[Lnewqban]

 

[Lnewqban]

I risk assuming that East is coming out of the paper, while West is going into it.

If that assumption happens to be correct, the following diagram shows how I understand the OP problem.

Then, my responses would be:
(i) East
(ii) Magnetic fluxes of current-wire and compass needle get aligned.
(iii) West