Lenz's Law Video Lessons

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Problem: Faraday's law of induction tells us that the magnitude of an induced EMF (voltage) is equal to the rate of change of the magnetic flux through the loop:Ε=-dΦ/dt.As the flux changes with time, an induced voltage will be produced that creates a current in the loop. In the above expression, Φis the flux through the loop. For a field that is perpendicular to the face of the loop, it is given byΦ=BA,where B is the magnetic field and A is the area of the loop. Obviously, the more rapid the change in B, and therefore Φ, the larger the induced EMF. Note that if you have more than one loop (a coil), then Faraday's law is modified by multiplying by the number of turns in the coil, .LENZ'S LAW: Lenz's law will allow you to figure out the direction of induced current flow due to a changing magnetic field. If the applied magnetic field is increasing in a loop, Lenz's law tells us that the induced current will be such that it will produce an induced magnetic field that will cancel out the increase in the applied magnetic field. Conversely, if the applied field is decreasing, the induced current will be in a direction that induces a magnetic field that will cancel out the decrease in the applied magnetic field. In other words, nature wants the field to remain constant. You can determine the required direction of the current by using the right hand rule as discussed in the previous lab.1. Thinking about Faraday's law, consider what happens as you push the north pole of a magnet into a coil. In which case do you expect the induced current in the coil to be the greatest?mark the right answerA) When the magnet is stationary within the coil.B) At the moment before the magnet is pushed into the coil.C) When the magnet is pushed into the coil rapidly.D) When the magnet is pushed into the coil slowly.2. Now let's consider the case where you have one coil inside another. In whichcase will the induced current in the outer coil be the largest?mark the right answerA) When the applied current in the inner coil is increased slowly.B) When the applied current in the inner coil is increasing rapidly.C) When the applied current in the inner coil is constant.D) When there is no applied current in the inner coil.3. The lab manual discusses the effect of placing a metal rod into the inner coil. What effect do you expect this to have on the induced current in the outer coil when the applied current in the inner coil changes?mark the right answerA) The induced current will be smaller than without the metal rod.B) The induced current will be larger than without the metal rod.C) The induced current will be the same as without the metal rod.D) The induced current will be in the opposite direction than the case without the metal r4. A loop is placed in a magnetic field as shown in the figure. If themagnitude of the field is increased, the induced current in theloop will bemark the right answerA) into the page.B) out of the page.C) counter clockwise.D) clockwise.

FREE Expert Solution

According to Lenz's law, a change in magnetic flux generates an emf.

The polarity of this emf is such that it produces a current whose magnetic field opposes the change which produces it. 

This means that the induced magnetic field inside any loop of wire will act to keep the magnetic flux in the loop constant. 

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Problem Details

Faraday's law of induction tells us that the magnitude of an induced EMF (voltage) is equal to the rate of change of the magnetic flux through the loop:Ε=-dΦ/dt.

As the flux changes with time, an induced voltage will be produced that creates a current in the loop. In the above expression, Φis the flux through the loop. For a field that is perpendicular to the face of the loop, it is given by

Φ=BA,

where B is the magnetic field and A is the area of the loop. Obviously, the more rapid the change in B, and therefore Φ, the larger the induced EMF. Note that if you have more than one loop (a coil), then Faraday's law is modified by multiplying by the number of turns in the coil, .


LENZ'S LAW: Lenz's law will allow you to figure out the direction of induced current flow due to a changing magnetic field. If the applied magnetic field is increasing in a loop, Lenz's law tells us that the induced current will be such that it will produce an induced magnetic field that will cancel out the increase in the applied magnetic field. Conversely, if the applied field is decreasing, the induced current will be in a direction that induces a magnetic field that will cancel out the decrease in the applied magnetic field. In other words, nature wants the field to remain constant. You can determine the required direction of the current by using the right hand rule as discussed in the previous lab.


1. Thinking about Faraday's law, consider what happens as you push the north pole of a magnet into a coil. In which case do you expect the induced current in the coil to be the greatest?

mark the right answer

A) When the magnet is stationary within the coil.
B) At the moment before the magnet is pushed into the coil.
C) When the magnet is pushed into the coil rapidly.
D) When the magnet is pushed into the coil slowly.


2. Now let's consider the case where you have one coil inside another. In which
case will the induced current in the outer coil be the largest?

mark the right answer

A) When the applied current in the inner coil is increased slowly.
B) When the applied current in the inner coil is increasing rapidly.
C) When the applied current in the inner coil is constant.
D) When there is no applied current in the inner coil.


3. The lab manual discusses the effect of placing a metal rod into the inner coil. What effect do you expect this to have on the induced current in the outer coil when the applied current in the inner coil changes?

mark the right answer


A) The induced current will be smaller than without the metal rod.
B) The induced current will be larger than without the metal rod.
C) The induced current will be the same as without the metal rod.
D) The induced current will be in the opposite direction than the case without the metal r


4. A loop is placed in a magnetic field as shown in the figure. If themagnitude of the field is increased, the induced current in theloop will be

mark the right answer

A) into the page.
B) out of the page.
C) counter clockwise.
D) clockwise.

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