Lenz's Law and Electromagnetic Induction

In summary, Lenz's Law states that the induced current in a loop is in the opposite direction of the change in magnetic flux. When a bar magnet's north pole is directed towards a loop, the induced current will move clockwise to oppose the increasing magnetic flux. When the south pole is directed towards the loop, the induced current will move counter-clockwise. This is because the magnetic field lines of a bar magnet extend from north to south, so when the south pole is directed towards the loop, the flux increases out of the page. The direction of the induced current can also be determined using the right-hand rule for current loops. When the magnet is moved away from the loop, the direction of the current will be reversed. Additionally, the
  • #1
Soaring Crane
469
0
For Lenz’s Law, most examples involve the north pole of a bar magnet passing through first in a wire coil, or solenoid.

If the bar magnet’s north pole moves toward the loop or solenoid, the magnet’s magnetic field lines move from the south pole to north pole? The current in solenoid is then directed to the right, or the induced current moves from left to right in the coil. When the magnet moves away from the solenoid, the induced current will then be directed to the left or move from right to left.

However, if the magnet’s south pole passes through the solenoid first (in which the magnetic field lines go from the south to north pole), will the induced current’s direction be the same? When the magnet’s south pole is directed toward the solenoid, the induced current will move to the right, or from left to right? When the magnet’s south pole moves away from the solenoid, the induced current will move to the left or from right to left in the coil?



Thanks.
 
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  • #2
First things first: The magnetic field "lines" of a bar magnetic extend out from the north pole and into the south pole.

Second: Lenz's law states that the induced current in the loop is such that the induced magnetic field opposes the change in the flux.

So imagine a loop in the plane of the paper. You push the north pole into the loop (into the page), thus increasing the flux into the page. The induced magnetic field will oppose that increase and thus point out of the page. (What direction of induced current would create a magnetic field pointing out of the page?)

If you do the same thing with the south pole of the magnet, the opposite will occur--since now you are increasing the flux out of the page.
 
  • #3
I understand that the magnetic flux increases in both cases when the north or south pole is directed in the loop's direction, but I do not understand the flux's direction in the south pole first case. Why does the flux increase out of the page when the south pole is placed in the loop? (Are there any diagrams that might help me understand this point clearly?)

Thanks again.
 
  • #4
Soaring Crane said:
I understand that the magnetic flux increases in both cases when the north or south pole is directed in the loop's direction, but I do not understand the flux's direction in the south pole first case. Why does the flux increase out of the page when the south pole is placed in the loop?
Because the magnetic field itself points out of the page. The magnetic field of a bar magnet points towards the south pole, which means out of the page.
 
  • #5
In the north pole first case, the induced current (from the right hand rule) will move in the direction to the right if the magnetic field points out of the page?

For the south pole first case, the induced current will generate a magnetic field pointing into the page, so the induced current's direction will be to the left from the right hand rule??
 
  • #6
Not sure what you mean by "to the right". Current direction in a loop would be clockwise or counterclockwise viewed from above the page.
 
  • #7
In the north pole first case, I think I mean clockwise by left if the bar magnet is approaching the solenoid's right end?
 
  • #8
North pole into loop

Let's take some care to specify directions clearly. Go back to my example of a single loop in the plane of the paper. Call that plane the x-y plane--the center of the loop is at the origin: 0,0,0. The +z axis points out of the page; -z, into the page.

Given that, we can now talk about pushing the north pole of a bar magnet into the loop. We start with the magnet along the +z axis, with the north pole closer to the origin. We move the north pole of the magnet towards the loop (in the -z direction, towards the origin).
  • Which way is the bar magnet's magnetic field pointing? In the -z direction (into the page).
  • Which direction is the flux through the loop pointing? In the -z direction (into the page).
  • Which direction is the flux changing? Since the magnetic field is increasing in the -z direction, the flux is increasing in the -z direction.
  • Which way must the induced magnetic field point in order to oppose that change in flux per Lenz's law? Since the flux is increasing in the -z direction, the induced magnetic field must point in the +z direction.
  • Looking at the loop from the vantage point of the +z axis, which way does the induced current flow? Using the right-hand rule for current loops, the current must flow counter-clockwise through the loop to produce a field in the +z direction.
Make sense? Things will be exactly reversed if you turn the bar magnet around and move the south pole into the loop.
 
  • #9
So as you move the magnet away from the origin - pulling it back (with the north pole of the magnet still facing the origin), the polarity of the origin becomes south pole, and it attracts the magnet again. Does that mean that at that point the current changes in the opposite direction (into the page)?

Also when the bar magnets south pole is being directed towards the origin, does the origin become sout pole, in order to repel the magnet? is it just the same with the north pole situation, except now with south poles?
 

1. What is Lenz's Law and how does it relate to electromagnetic induction?

Lenz's Law states that the direction of an induced electromotive force (emf) in a closed circuit will always be such as to oppose the change in magnetic flux that caused it. This law is a consequence of the conservation of energy and is used to determine the direction of induced currents in electromagnetic induction processes.

2. What is electromagnetic induction and how does it work?

Electromagnetic induction is the process of creating an electric current in a conductor by placing it in a changing magnetic field. This can occur when a conductor is moved through a magnetic field or when a magnetic field itself changes. The changing magnetic field induces an emf in the conductor, which then creates a current.

3. How does Lenz's Law explain the direction of induced currents?

Lenz's Law states that the induced current will flow in such a direction to create a magnetic field that opposes the change in the original magnetic field. This means that if the original magnetic field is increasing, the induced current will flow in a direction that creates a magnetic field that opposes the increase. If the original field is decreasing, the induced current will flow in a direction that creates a magnetic field that opposes the decrease.

4. What is the relationship between Lenz's Law and Faraday's Law of Induction?

Faraday's Law of Induction states that the magnitude of the induced emf is proportional to the rate of change of the magnetic flux through the circuit. Lenz's Law determines the direction of the induced current based on this change in flux. Together, these laws explain the process of electromagnetic induction and how it results in an induced current in a closed circuit.

5. How is Lenz's Law applied in real-world applications?

Lenz's Law is applied in many real-world applications, such as generators, motors, and transformers. In generators, mechanical energy is converted into electrical energy through the process of electromagnetic induction. In motors, electrical energy is converted into mechanical energy. Transformers use the principles of Lenz's Law to step up or step down voltage levels in electrical circuits. Lenz's Law is also used in electromagnetic braking systems, where the motion of a conductor in a magnetic field is used to slow down or stop a moving object.

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