Understanding Lenz Law: Impact on Current Direction in a Magnetic Field

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Discussion Overview

The discussion revolves around Lenz's Law and its implications for the direction of current and force in a square loop of wire entering a magnetic field. Participants explore the relationship between induced EMF, current direction, and the resulting forces, examining both theoretical and practical aspects of the phenomenon.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that Lenz's Law indicates the induced EMF will create a current that opposes the change in magnetic flux, potentially reducing the original current.
  • Others argue that whether the current reverses direction depends on various factors, including the strength of the driving EMF and the rate at which the loop enters the magnetic field.
  • A participant questions the direction of the force on the right side of the loop, suggesting it should be to the right based on their understanding, while a professor indicated it was to the left.
  • Some participants agree that the force on the right side of the loop should be to the right, based on the right-hand rule, but others challenge this interpretation, suggesting that the force must act to the left to oppose the increasing magnetic flux.
  • There is a discussion about the relationship between inductors and the situation described, with some noting that inductors typically consist of multiple loops and induce current via Faraday's Law.
  • A later reply introduces the idea that if there is an initial current in the loop, it would initially accelerate in the direction of motion but would slow down as counter EMF is produced, complicating the equilibrium condition.

Areas of Agreement / Disagreement

Participants express differing views on the direction of the force acting on the loop and the implications of Lenz's Law. There is no consensus on the correct interpretation of the right-hand rule in this context, and the discussion remains unresolved regarding the specific conditions under which the force direction changes.

Contextual Notes

Participants highlight the dependence of their arguments on various factors, including the strength of the driving EMF and the specifics of the magnetic field, indicating that assumptions about these variables are critical to the discussion.

jaredvert
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So say you have a square loop of wire which has a current of some positive value going clockwise and it enters a magnetic field going into the board. Now since lenz law states that the induced emf will make a current going counterclockwise (to oppose changing mag flux) then what is the new current value? Does the current just keep getting smaller and smaller and then start to actually go the other way?
 
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The induced EMF counteracts the EMF that is driving your current, so yes it will reduce the current. Whether it will force the current to go the other way depends on the specifics (how strong your current driving EMF is, how fast the loop enters the field, how strong the field is, etc.)
 
Matterwave said:
The induced EMF counteracts the EMF that is driving your current, so yes it will reduce the current. Whether it will force the current to go the other way depends on the specifics (how strong your current driving EMF is, how fast the loop enters the field, how strong the field is, etc.)
Ok well this same situation was presented in my class and it asked what direction the force was when the right side of the square just entered the magnetic field. Since it was going clockwise and the induced emf would not bring the current the other way (I predicted from common sense exactly what you said), then I said the force was to the right (right hand rule). It was a conventional current but he said the answer was to the left! Any explanation? Thanks
 
By your description, the answer should be that the force on the right side wire of the loop is going to the right.

I don't see why it would be left. Did the professor give you a reason?
 
Matterwave said:
By your description, the answer should be that the force on the right side wire of the loop is going to the right.

I don't see why it would be left. Did the professor give you a reason?
It's a high school teacher and he didn't specify but I didn't have time to think about it because class was ending
 
Perhaps the teacher just did the right hand rule wrong. By your description, you are correct in thinking the force on the right side wire is acting to the right.
 
Matterwave said:
Perhaps the teacher just did the right hand rule wrong. By your description, you are correct in thinking the force on the right side wire is acting to the right.
Yes perhaps. So is an inductor basically the same concept as this situation right here?
 
An inductor usually has many loops rather than just one (usually a solenoid). But you basically do induce a current from one source to the other via the Faraday's law.
 
jaredvert said:
Ok well this same situation was presented in my class and it asked what direction the force was when the right side of the square just entered the magnetic field. Since it was going clockwise and the induced emf would not bring the current the other way (I predicted from common sense exactly what you said), then I said the force was to the right (right hand rule). It was a conventional current but he said the answer was to the left! Any explanation? Thanks

You don't need any hand rule.
The flux of the external field through the coil is increasing because the coil enters the field.
It enters the field moving to the right. So moving more to the right will increase the flux. To oppose this a force should act to the left, opposite to the motion of the coil. This is all.
If the force were to the right, the coil will move faster and the flux will have a higher rate of change.
 
  • #10
nasu said:
You don't need any hand rule.

The flux of the external field through the coil is increasing because the coil enters the field.

It enters the field moving to the right. So moving more to the right will increase the flux. To oppose this a force should act to the left, opposite to the motion of the coil. This is all.

If the force were to the right, the coil will move faster and the flux will have a higher rate of change.
But that contradicts the right hand rule? Even if that does sound bad, are you saying the RHR only applies at certain times?
 
  • #11
Nasu's answer is true if there was no current in the loop to begin with. If you had a current to begin with, the loop would initially accelerate towards the right, but then slow in its acceleration as the counter EMF is produced. The equilibrium condition would then depend on the EMF that is providing the initial current. If, for example, the loop's initial EMF is very strong, then this EMF would effectively be driving the loop towards the right.
 

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