Use Lenz’s law to determine the direction of induced emf whenever a magnetic flux changesUse Faraday’s law with Lenz’s law to determine the induced emf in a coil and in a solenoid


You are watching: The magnet moves as shown. the field of the magnet through the coil is

The direction in which the induced emf drives current around a wire loop can be found through the negative sign. However, it is usually easier to determine this direction with Lenz’s law, named in honor of its discoverer, Heinrich Lenz (1804–1865). (Faraday also discovered this law, independently of Lenz.) We state Lenz’s law as follows:


The direction of the induced emf drives current around a wire loop to always oppose the change in magnetic flux that causes the emf.


Lenz’s law can also be considered in terms of conservation of energy. If pushing a magnet into a coil causes current, the energy in that current must have come from somewhere. If the induced current causes a magnetic field opposing the increase in field of the magnet we pushed in, then the situation is clear. We pushed a magnet against a field and did work on the system, and that showed up as current. If it were not the case that the induced field opposes the change in the flux, the magnet would be pulled in produce a current without anything having done work. Electric potential energy would have been created, violating the conservation of energy.

To determine an induced emf

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Check Your Understanding Find the direction of the induced current in the wire loop shown below as the magnet enters, passes through, and leaves the loop.


To the observer shown, the current flows clockwise as the magnet approaches, decreases to zero when the magnet is centered in the plane of the coil, and then flows counterclockwise as the magnet leaves the coil.

is directed outward perpendicular to the plane of a circular coil of radius and normal to the area unit vector

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The accompanying figure shows a conducting ring at various positions as it moves through a magnetic field. What is the sense of the induced emf for each of those positions?

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a. CCW viewed from the magnet; b. CW viewed from the magnet; c. CW viewed from the magnet; d. CCW viewed from the magnet; e. CW viewed from the magnet; f. no current


A single-turn circular loop of wire of radius 50 mm lies in a plane perpendicular to a spatially uniform magnetic field. During a 0.10-s time interval, the magnitude of the field increases uniformly from 200 to 300 mT. (a) Determine the emf induced in the loop. (b) If the magnetic field is directed out of the page, what is the direction of the current induced in the loop?


When a magnetic field is first turned on, the flux through a 20-turn loop varies with time according to

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where is in milliwebers, t is in seconds, and the loop is in the plane of the page with the unit normal pointing outward. (a) What is the emf induced in the loop as a function of time? What is the direction of the induced current at (b) t = 0, (c) 0.10, (d) 1.0, and (e) 2.0 s?


The magnetic flux through the loop shown in the accompanying figure varies with time according to

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where is in milliwebers. What are the direction and magnitude of the current through the
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resistor at (a)
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; (b)
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and (c)
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a. 150 A downward through the resistor; b. 46 A upward through the resistor; c. 0.019 A downward through the resistor




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Glossary

Lenz’s lawdirection of an induced emf opposes the change in magnetic flux that produced it; this is the negative sign in Faraday’s law