Physics - Electromagnetic Induction

bonkysleuth SG

A soft iron bar wound with a coil is connected to a centre-zero galvanometer. Magnets P and Q are placed on a conveyor belt that moves in a uniform speed below the soft iron bar. As magnet P moves below soft in iron bar, the needle of centre-zero galvanometer first deflects left, then deflects right.

Explain why the needle first deflects left, then deflects right.

Is it due to the fact that the magnet P is constantly moving while on the conveyor belt and thus the switch in current direction? But even so, how do i fully explain this?

SBS2601D SG

The direction of the flow of current depends on the position of the magnets on the conveyer belt.

If I'm not mistaken, as the magnets approach, the current will flow such that the magnetic field induced will OPPOSE the magnetic field of the conveyer belt magnets.

And as the magnet on the conveyer belt passes and hence gets further from the soft iron core, the current flow will be such that the magnetic field induced will be the oppostie of what's cited above.

Hence you'll see the current move in 2 directions as shown by the galvanometer.

bonkysleuth SG

2 coils of copper wire are wound on a soft-iron rod. Coil P is connected to a centre-zero galvanometer. Coil Q is connected in series to a battery and a switch. The coils can slide freely on the soft iron rod As switch is closed, a deflection is seen on galvanometer for a short time, and the coils slide apart a little.

Describe the reading of the galvanometer

(a) after the switch if left on for some time

(b)as the switch is opened

What change would you expect in the deflection of the galvanometer as the switch is closed if the soft iron rod is replaced by an identified glass rod?

I wrote it doesn't deflect but I think the answer is there is a short-lived deflection. Don't understand why. How can the glass rod be magnetised?

And yes, I drew out the current direction myself. Please help verify if they are correct.

I also have one question regarding transformer. A compass is placed at the secondary side. It says,"When the switch was closed, needle of the compass that was placed above this wire had deflected. The needle again deflected when switch was opened.(why does needle deflect when switch was opened?) No deflection occurred when steady current was supplied to coil at primary side.

Why is this so? (paragraph was taken from textbk)

Next, When switch is opened, the current in primary coil decreases to 0. This creates a decreasing magnetic field which induces a current in the opposite direction in secondary coil and lamp flashes again.

I found the statement in BOLD funny. Would someone please explain to me the explanation behind these observations?

thanks

SBS2601D SG

Only a change in the magnetic field will induce a current.

As long as the magnetic field induced by the current remains the same, there will be no current induced on the other circuit. Which implies that only when the current changes will there be a deflection of the needle.

anpanman SG

Hi. Just a query for all.

Does the diagram drawn by bonkysleuth operate behind the same principle to that of the transformer? If yes, then is the answer for part (a) of the question being that the galvanometer will not deflect since after some time, the current in circuit becomes steady and there is no changes. Hence no change in magnetic field. This as a result will cause galvanometer to not deflect.

For (b), should it be that the galvanometer deflects because as switch is opened, current decreases to 0. This will create a decreasing magnetic field for which current is induced in the other circuit. However, galvanometer now deflects in opposite direction that to in (a).

Not sure if my concept is sure because I apply this using the workings behind the transformer yea.

anpanman SG

By the way, can someone tell me the exact function of the slip ring, commutator, and carbon brushes? these cannot be found in the textbook and neither do the teachers tell us about it .

SBS2601D SG

Hi. Just a query for all.

Not sure if my concept is sure because I apply this using the workings behind the transformer yea.

If you're interested, you can google "magnetic flux density". You'll understand things better that way.

Edit: Save you the trouble. Look up "Faraday's Law of Induction".

eagle SG

By the way, can someone tell me the exact function of the slip ring, commutator, and carbon brushes? these cannot be found in the textbook and neither do the teachers tell us about it .

I shall just answer this short question first... Rushing quite a few things at once :(

Slip ring is to allow contact between the coil and the circuit while the coil is rotating. Used for AC circuits.

Commutator, or split ring, is to allow contact between the coil and the circuit, and because it is splitted, it also changes the direction of current in the coil every 180 degrees. Used for DC circuits.

Carbon brushes are usually made of graphite. U know graphite can be used as a lubricant right? And it can also conduct electricity right? So it is used for maintaining contact between the circuit and slip ring/commutator, while at the same time act as a lubricant because of the rotating/moving parts.

anpanman SG

Describe the reading of the galvanometer

(a) after the switch if left on for some time

(b)as the switch is opened

What change would you expect in the deflection of the galvanometer as the switch is closed if the soft iron rod is replaced by an identified glass rod?

Hi SBS2601D, after looking up from google, I still think that the answers for questions(a) and (b) which I have answered seemed logical to me... Anyway does anyone know the answer to this question posed by bonkysleuth?

What change would you expect in the deflection of the galvanometer as the switch is closed if the soft iron rod is replaced by an identified glass rod?

thanks a lot.

SBS2601D SG

What change would you expect in the deflection of the galvanometer as the switch is closed if the soft iron rod is replaced by an identified glass rod?

thanks a lot.

Hey sorry.

I think I didn't read carefully.

I was thinking the galvanometer should not show any movement.

Um.

Maybe the galvanometer showed a slight deflection for a short time because the glass rod hit the circuit when it was passing through?

Haha. Joking.

I go and trawl for the answer if I can.

SBS2601D SG

Ok.

Maybe this is the reason.

The soft iron core is not compulsory to cause a magnetic field to form around the coil carrying the current.

The soft iron core is basically there to amplify the magnetic field only.

In other words, you don't have to magnetise whatever rod that's inside the coil to induce a current at the other coil.

So if you apply the same theory of "Faraday's Law of Induction", you'll also be able to deduce as well the direction of flow of current induced.

As for why the deflection is just a short moment, I think I don't have to explain that already, is that ok?

anpanman SG

Just wondering, in the above set-up, if you close the switch, galvanometer deflects. (eg. to left)So does it deflect another way when switch is opened(eg. to right)?

eagle SG

Just wondering, in the above set-up, if you close the switch, galvanometer deflects. (eg. to left)So does it deflect another way when switch is opened(eg. to right)?

yes

this is a common TYS question as well

anpanman SG

yes

this is a common TYS question as well

Thanks!

By the way, in the experiment described by bonkysleuth, does the coils slide apart due to the repulsion of like poles? Cuz the current induces in the other circuit which is nearer to the intial circuit a pole of opposite polarity (Lenz's law)?