Q3. Lenz's Law states that the direction of
induced emf is such that it opposes :
(A) Very effect which produced it
(B) Very balance which causes it
(C) Very action which causes it
(D) Very cause which produces it
Answers
Q3. Lenz's Law states that the direction of
induced emf is such that it opposes :
(C) Very action which causes it
Answer:
Lenz's law, named after the physicist Emil Lenz (pronounced /ˈlɛnts/) who formulated it in 1834,[1] states that the direction of the electric current which is induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field.
Lenz's law tells the direction of a current in a conductor loop induced indirectly by the change in magnetic flux through the loop. Scenarios a, b, c, d and e are possible. Scenario f is impossible due to the law of conservation of energy. The charges (electrons) in the conductor are not pushed in motion directly by the change in flux, but by a circular electric field (not pictured) surrounding the total magnetic field of inducing and induced magnetic fields. This total magnetic field induces the electric field.
It is a qualitative law that specifies the direction of induced current, but states nothing about its magnitude. Lenz's law explains the direction of many effects in electromagnetism, such as the direction of voltage induced in an inductor or wire loop by a changing current, or the drag force of eddy currents exerted on moving objects in a magnetic field.
Lenz's law may be seen as analogous to Newton's third law in classical mechanics.[2]
Contents
DefinitionEdit
Lenz's law states that the current induced in a circuit due to a change in a magnetic field is directed to oppose the change in flux and to exert a mechanical force which opposes the motion.
Lenz's law is contained in the rigorous treatment of Faraday's law of induction, where it finds expression by the negative sign:
{\displaystyle {\mathcal {E}}=-{\frac {\partial \Phi _{\mathbf {B} }}{\partial t}},}
which indicates that the induced electromotive force {\displaystyle {\mathcal {E}}} and the rate of change in magnetic flux {\displaystyle \Phi _{\mathbf {B} }} have opposite signs.[3]
This means that the direction of the back EMF of an induced field opposes the changing current that is its cause. D.J. Griffiths summarized it as follows: Nature abhors a change in flux.[4]
If a change in the magnetic field of current i1 induces another electric current, i2, the direction of i2 is opposite that of the change in i1. If these currents are in two coaxial circular conductors ℓ1 and ℓ2 respectively, and both are initially 0, then the currents i1 and i2 must counter-rotate. The opposing currents will repel each other as a result.
ExampleEdit
Magnetic fields from strong magnets can create counter-rotating currents in a copper or aluminum pipe. This is shown by dropping the magnet through the pipe. The descent of the magnet inside the pipe is observably slower than when dropped outside the pipe.
When a voltage is generated by a change in magnetic flux according to Faraday's law, the polarity of the induced voltage is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the flux is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to oppose the change.