Lenz’s Law

Lenzs law is used to determine the direction of induced emf in the conductor or coil.

According to Lenz’s law, the electromagnetically induced emf and hence current flows in the conductor or coil in such a direction that the magnetic field set up by it always oppose the cause, which produces it.

Consider a coil having a large number of turns connected to a galvanometer as shown in the figure. A bar magnet is placed near the coil so that its flux links with the coil. When North pole of a bar magnet is taken nearer to the coil, an emf is induced in the coil and hence current flows through it in such a direction that side Y of the coil attains North polarity which restricts the movement of the bar magnet.

Whereas, when North pole of the bar magnet is taken away from the coil, the direction of induced emf in the coil is reversed and side Y of the coil attains South polarity which again restricts the movement of the coil.

Right-Hand Thumb Rule

When electric current flows through a conductor, a magnetic field is set up around the conductor. In this magnetic field, the direction of magnetic lines of force can be determined by right-hand thumb rule which is stated below.

To find out the direction of magnetic lines of force, hold the current carrying conductor in the right hand so that thumb points in the direction of flow of current. The fingers curling around the conductor will point in the direction of magnetic lines of force.

Fleming’s Left-Hand Rule

When a current carrying conductor is placed in a magnetic field, a force is experienced by it. The Fleming’s left-hand rule is applied to determine the direction of this force, which is stated below.

Stretch thumb, first finger and second finger of your left hand mutually perpendicular to each other. If the first finger represents the direction of magnetic field, the second finger represents the direction of flow of current then thumb will indicate the direction of the force acting on the conductor.

Fleming’s Right-Hand Rule

Fleming’s right-hand rule is used to determine the direction of induced emf and hence current in a coil or conductor, which is stated below.

Stretch, thumb, first finger and second finger of your right hand mutually perpendicular to each other. If thumb indicates the direction of motion of conductor, the first finger indicates the direction of magnetic field then the second finger will indicate the direction of induced emf in the conductor.

The two Faraday’s laws of electromagnetic induction are as flows:

First law: Whenever a coil or conductor cuts magnetic flux, an emf is induced in that conductor.

Second law: It states that the magnitude of induced emf is equal to the rate of change of flux.

Coulomb’s Law of Magnetic Force

First Law: Like poles repel each other and unlike poles attract each other.

Second Law: Force, F exerted by one pole on the other pole is directly proportional to the product of the pole strengths of the two poles and inversely proportional to the square of distances, d between them.

Mathematically, F α (m1m2)/d2
or, F = (km1m2)/d2

where, m1, m2 = pole strengths,
d = distances between the poles,
k = constant which depends upon the nature of the surrounding of material.

Magnetism and Electromagnetism — 1 | Objective Type Question Answers

#1 The materials having low retentivity are suitable for making

temporary magnets

paramagnetic

ferromagnetic

#4 Which of the following is a vector quantity?

Magnetic field intensity

#5 A material which is slightly repelled by a magnetic field is known as

diamagnetic material

susceptibility

zero

#8 Materials subjected to rapid reversal of magnetism should have

high permeability and low hysteresis loss

Soft iron

nickel

#11 Paramagnetic materials have relative permeability

slightly more than unity

#12 Degaussing is the process of

demagnetizing metallic parts

#13 Ferrites are a sub-group of

feri-magnetic materials

#14 Gilbert is a unit of

magnetomotive force

#15 The working of a meter is based on the use a permanent magnet. In order to protect the meter functioning from stray magnetic fields

a soft iron shielding is used

reluctivity

Ebonite

#18 Out of the following statements, concerning an electric field, which statement is not true ?

The electric field intensity at a point is numerically equal to the force exerted upon a charge placed at that point.

#19 Super-magnetic materials are composed of

ferromagnetic particles in a non-ferromagnetic matrix

#20 Susceptibility is positive for

ferromagnetic substances

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