Ap physics c 2014 free response

[darksyesider]

Homework Statement

2a) here: http://media.collegeboard.com/digitalServices/pdf/ap/ap14_frq_Physics_C-E-M.pdf

Homework Equations

The Attempt at a Solution

Would the answer be:

stays same -- increasing -- decreasing -- stays same?

By Lentz's law, then right hand rule?

 

[Simon Bridge]

Please show your reasoning.

 

[darksyesider]

Uh first when it enters there is a CCW current so from F= ILxB, it accelerates.
Then while it is in the magnetic field there is no change so it is the same.
then when it leaves there is a CW current so it decelerates.
Then it stays the same at the end since no current.

 

[Simon Bridge]

By "accelerate" do you mean it moves faster? i.e. kinetic energy of the loop increases?

You are saying that as the loop enters the magnetic field, electrical energy is generated, so the kinetic energy of the loop also increases?

Try describing what happens in terms of conservation of energy.

 

[Redbelly98]

Uh first when it enters there is a CCW current so from F= ILxB, it accelerates.

A few questions you need to answer to figure this out:
Which end of the loop is in the field as it is entering the field?
What is the direction of I in that end, if the current is CCW?
Finally, the direction of I×B is ___?

Then while it is in the magnetic field there is no change so it is the same.

I agree.

then when it leaves there is a CW current so it decelerates.

Try to answer the questions I listed above for this case.

Then it stays the same at the end since no current.

Yes.

Try describing what happens in terms of conservation of energy.

That is an alternate way to solve the problem, but since darksyesider has already worked out the direction (CCW or CW) of the current, I suggest continuing along that line of reasoning.

 

[Simon Bridge]

Actually there is an even simpler one just from the rule
... the motion always induces the magnetic field which will oppose the motion.
Therefore...