How shud i apply lenz's law to this?

[uzair_ha91]

Homework Statement

http://img38.imageshack.us/img38/7309/coil.png
When the switch in the circuit is closed a current is established in the coil and the metal ring jumps upward, Why? Describe what would happen to the ring if the battery polarity were reversed?

Homework Equations

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The Attempt at a Solution

Ok, for the first part: The north pole of the solenoid will point downwards and the south pole upwards. The ring's north pole (due to the induced emf creating a sort of a bar magnet just like in solenoid) will point downwards to oppse the decrease...That's why the ring jumps upward as soon we close the circuit..
But when we open the circuit, the current decreases and there is a decrease in magnetic flux,, so according to Lenz's Law, will the ring jump upwards or stay where it is?

 

[Redbelly98]

The Attempt at a Solution

Ok, for the first part: The north pole of the solenoid will point downwards and the south pole upwards.

Yes, good. That is correct.

The ring's north pole (due to the induced emf creating a sort of a bar magnet just like in solenoid) will point downwards to oppse the decrease...That's why the ring jumps upward as soon we close the circuit..

Let's take things one step at a time.

1. First, let's think about the final polarity of the ring. We are told that it jumps upward when the switch is closed. In other words, the two "magnets" (the solenoid and the ring) repel each other.

Since the two magnets repel, do they have like poles or opposite poles facing towards each other?

2. Next, use Lenz's Law. In what direction does the change in B point inside the ring?

 

[uzair_ha91]

Since the two magnets repel, do they have like poles or opposite poles facing towards each other?

Like poles for repulsion

2. Next, use Lenz's Law. In what direction does the change in B point inside the ring?

The ring's magnetic field will act upwards i.e. it's south pole will repel the south pole of the solenoid...Thus the change in magnetic field will be downwards...and to oppose this change the ring's magnetic field is hence upwards...
But here the flux is increasing through the ring, what would happen as the flux decreases (i.e. when the circuit is opened and current decreases)? I am kinda stuck here...

 

[Redbelly98]

Like poles for repulsion


The ring's magnetic field will act upwards i.e. it's south pole will repel the south pole of the solenoid...Thus the change in magnetic field will be downwards...and to oppose this change the ring's magnetic field is hence upwards...

Yes.

But here the flux is increasing through the ring, what would happen as the flux decreases (i.e. when the circuit is opened and current decreases)? I am kinda stuck here...

Who cares what happens when the circuit is opened? They are not asking about that.

 

[uzair_ha91]

oh yeah sorry ... misunderstood :-)

And if the polarity is reversed the ring would still jump upwards because now the north pole of the solenoid will repel the north pole of the ring

 

[Redbelly98]

Yes, you've got it.