THE Lenz's law is a generalization of law of faraday, which describes the phenomenon of inductionelectromagnetic. According to Lenz's law, the electromotive force induced by a magnetic flux variation it will always be formed in a sense that opposes this variation.
Before we proceed with this article, it is important that you fully understand some fundamental concepts of Faraday's law, so we suggest that you check out our text on the phenomenon of electromagnetic induction.
See too:How Transformers Work and Their Relationship to Lenz's Law
Lenz's Law
THE Lenz's law indicates what the polarity of the electromotive force induced in a loop, according to the direction in which the magnetic flux variation over that coil or coil. Created by HeinrichLenz, this law can be understood as a complement to Faraday's law, in observance of the principle of conservationgivesenergy.
Mathematically, the lawinLenz it's just a negative sign attributed to the law of inductionelectromagnetic, look at the formula:
ε – induced electromotive force (V – Volts)
ΔΦ – variation of magnetic flux (Wb – Weber or T/m²)
t –time interval(s)
The negative sign of the formula indicates that the polarity of the induced electromotive force (ε) must be contrary to the variation of the magnetic flux ΔΦ.
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Direction of the induced electrical current
To know the meaning of electric current induced, or even from the magnetic field, it is necessary that, first, we know the right hand rule. This rule states that the direction and meaning of magnetic field generated by a chainelectric they can be determined by the gesture of closing the fingers of the hand around the big toe, which must be pointed in the direction that the current passes through the conductor. In that case, the magnetic field should point to the samesense that the fingersgiveshand, as shown in the figure:
Based on this rule, when approach a turn conductive of polenorth on one magnet, this will produce a magnetic field that opposes to the increased magnetic flux in its interior, that is, it will produce a magnetic north, to repel the input of the magnet. Look at the picture:
For a north polarity to oppose the input of the north pole of the magnet, the induced current that runs through the coil must be formed counterclockwise, as shown in the figure:
In the situation we show a slightly different case, in which the polenorth of the magnet walk away of the conducting loop. When this happens, the spiral will tend to opposeto theremoval and therefore it will produce a polesouthmagnetic in front of you, so to attractthe magnet that moves away. The magnetic field, therefore, must be produced by a electric current that flows clockwise, note:
Finally, we brought a picture in which the possible situations in which the approach and departure of the north pole occur, as well as the approach and departure of the magnetic south pole are illustrated:
In the image, the magnetic field of the magnet is written in blue (B), while the magnetic field induced in the coils is written in red. ΔB, in turn, indicates the direction of variation of the magnetic field in each situation.
Lookalso: A summary of what's most important about electrostatics
Solved exercises on Lenz's law
Question 1) About the phenomenon of electromagnetic induction, tick only the false alternatives.
I – When a magnet is approached to a conducting loop, an induced magnetic field in the loop arises to oppose the passage of the magnet.
II – When we try to pass a magnet through the interior of a coil, a magnetic field is induced in the coil to favor the passage of the magnet.
III - In the case where a magnet has zero relative speed in relation to a coil, the electromotive force induced in the coil will be nonzero.
Are false:
a) I and II
b) II and II
c) I, II and III
d) I and III
e) II and III
Template: Letter e
Resolution:
Let's look at the alternatives:
I - TRUE. The induced magnetic field always opposes the flux variation of the magnetic field.
II - FALSE. In this case, the induced magnetic field will try to make the magnet stop moving.
III - FALSE. In the case where there is no relative velocity between the magnet and the loop, there will not be any induced electromotive force.
Analyzing the answers, we see that the false alternatives are letters II and III, so the answer is letter E.
Question 2) A magnet has its magnetic south pole facing towards a conducting loop, when that loop moves away. In relation to the situation described, tick what is correct:
I – The magnet will be attracted by the coil.
II – The magnet will be repelled by the spiral.
III – The electrical current formed in the loop will have a clockwise direction.
IV – It is not possible to know the direction of the electric current, since we do not know if the loop is to the right or left of the magnet.
They are correct:
a) I and II
b) I, II and III
c) I and IV
d) II and III
e) III and IV
Template: Letter C
Resolution:
In the case described, the conducting loop will produce a magnetic north pole in the vicinity of the magnet, in order to attract it, thus opposing its furthermore, as we do not know the arrangement of the magnet and the coil, it is not possible to state the direction of the electric current induced in the coil. Therefore, the correct alternative is the letter C.
By Me. Rafael Helerbrock
