# Lenz's Law: Direction of Flux & Current

• defetey
In summary, the question is about the direction of current in a solenoid when a vertical magnetic field is switched off. According to Lenz's Law, the current will be counterclockwise in order to counter the change in flux from the switched off field. The direction of the flux and the side of the solenoid that creates the "counter-"magnetic field are not specified in the given information.
defetey

## Homework Statement

I don't have the question with me, it was on a test I did. It was something like this though:

A coil is placed vertically on a table. The magnetic field there is also vertical. If the magnetic field is suddenly switched off, in which direction would the current in the solenoid move?

A) Clockwise
B) Counter-clockwise
C) There would be no current
-and I forgot the rest of the choices.

## The Attempt at a Solution

Well it would look something like this:

http://img43.imageshack.us/img43/2058/emfb.jpg

So, current is produced when there is a change in the magnetic field, so there is obviously would be current if the field is turned off. And that current's field would try to counter the flux.

But some things aren't clear to me;

1) Since the field is even on all points of the solenoid, which side of the solenoid (inner or outer) would be the one that creates the "counter-"magnetic field against the flux? Would the field be countered from within or from the outside?

2) Which direction would the flux be (that creates the current) if the "field is suddenly switched off"?

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Your current would be counterclockwise. Lenz's Law states that this coil will attempt to fight the change in flux that is brought about when the B field is switched off. Since that original B field was pointed up, the current must be CCW to produce a B field in that same direction in an attempt "to fight" the change in flux from some flux to zero.

Lenz's Law states that the direction of an induced current is always such that it opposes the change that caused it. In this case, the change that caused the current is the sudden switching off of the magnetic field. Therefore, the current in the solenoid would move in a direction that creates a magnetic field that opposes the original magnetic field.

1) The direction of the induced current would depend on the direction of the original magnetic field. If the original field was directed downwards, then the induced current would flow clockwise, creating a magnetic field that points upwards to oppose the original field. If the original field was directed upwards, then the induced current would flow counterclockwise, creating a magnetic field that points downwards to oppose the original field.

2) The direction of the induced current would also depend on the rate at which the magnetic field is switched off. If the magnetic field is switched off slowly, the induced current would be weaker and the flux would be in the same direction as the original field. If the magnetic field is switched off quickly, the induced current would be stronger and the flux would be in the opposite direction to the original field.

## 1. What is Lenz's Law?

Lenz's Law is a fundamental law of electromagnetism that describes the direction of induced current in relation to the changing magnetic field that causes it. It states that the direction of the induced current will always be such that it opposes the change in magnetic flux that caused it.

## 2. How does Lenz's Law relate to Faraday's Law?

Lenz's Law is a consequence of Faraday's Law, which states that a changing magnetic field induces an electric field. Lenz's Law specifically describes the direction of the induced current resulting from the changing magnetic field.

## 3. What is the direction of the induced current according to Lenz's Law?

The direction of the induced current is always in such a way that it opposes the change in the magnetic flux that caused it. This means that if the magnetic flux is increasing, the induced current will flow in a direction that creates a magnetic field that opposes the increase in flux. Conversely, if the magnetic flux is decreasing, the induced current will flow in a direction that creates a magnetic field that aids the decrease in flux.

## 4. How is Lenz's Law applied in practical situations?

Lenz's Law is applied in many practical situations, such as generators, motors, and transformers. In generators, Lenz's Law is used to ensure that the induced current flows in the same direction as the external current, producing a continuous flow of electricity. In motors, Lenz's Law is used to determine the direction of rotation based on the direction of the external current. In transformers, Lenz's Law helps to regulate the voltage and current in the secondary coil.

## 5. What happens if Lenz's Law is violated?

If Lenz's Law is violated, it would result in a violation of the law of conservation of energy. This is because Lenz's Law ensures that the induced current always opposes the change in magnetic flux, which in turn ensures that the energy in the system remains constant. If Lenz's Law is violated, the induced current could potentially add energy to the system, leading to a violation of the law of conservation of energy.

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