MAGNETIC EFFECTS OF CURRENT AND MAGNETISM

CH 3(A) MAGNETIC FIELD DUE TO CURRENT

#) Magnetic effect of current. It is phenomenon by virtue of which a current produces a magnetic field. A straight current produces a circular magnetic field and a circular current produces a straight magnetic field at the centre of the circular coil.

#) S.I unit of magnetic field strength is teals. If a charge of 1 coulomb, while moving with a velocity of 1ms-1 at a point, perpendicular to the direction of magnetic field, experiences a force of 1 Newton, the strength of magnetic field at that point is 1 teals.

1 teals = 1N A-1 m-1

#) Dimensions of B = [MA-1 T-2 ].

#) Magnetic field induction at the centre of a circular coil of radius r carrying current 1 is given by. The direction of this magnetic field induction is given by Right Hand palm rule..

#) Magnetic dipole moment of the current loop is given by. M=nLA = nl a2.

Where n is the number of turns in the loop, carrying current I and a is the radius of the loop. The unit of M is ampere (meter)2 [ A-m2 ]

#) A solenoid consists of an insulated long wire closely wound in the form of a helix. Its length is very large as compared to its diameter.

#) Magnetic field induction at a point well inside the solenoid of length l, having N turns carrying current I is given by. B=µ0 Ni / l.

Magnetic field induction at a point on one end of the solenoid carrying current is B = µ0 Ni/ 2l.

CH 3(B) FORCES ON CHARGED PARTICLES IN ELECTRIC AND MAGNETIC FIELDS

#) Force on a charged particle in an electric field is F =qE. This force acts in the direction of electric field. The charged particle describes a parabolic path if electric field is acting perpendicular or at some angle with the initial direction of motion of the particle.

#) Lorentz force. The force experienced by a charged particle moving in space where both electric and magnetic fields exists is called Lorentz force.

#) Two long parallel conductors carrying currents in the same direction attract each other and carrying currents in the opposite direction, repek each other. The force acting per unit length of each conductor will be

F=µ0/ 4

Where I1 ,I2 = currents in the two linear parallel conductors, placed distance r apart.

#)One ampere of current is that much current which when flowing through each of the two parallel uniform linear conductors each of length one meter placed in free space at a distance of one meter from each other will attract or repel each other with a force of 2x 10-7 N per meter of their length.

#) Radial magnetic field. It is that magnetic field, in which the plane of the coil in all positions remains parallel to the direction of the magnetic field.

#) Current sensitivity of a galvanometer. It is defined as the deflection produced in the galvanometer, when unit current flows through it.

#) Voltage sensitivity of a galvanometer. It is defined as the deflection produced in the galvanometer when a unit voltage is applied across the two terminals of the galvanometer.

#) Shunt. It is a low resistance wire connected in parallel with the galvanometeror ammeter. It protects the galvanometer or ammeter from the strong currents.

#) Conversion of galvanometer into ammeter

A galvanometer can be converted into an ammeter of given range by connecting a suitable low resistance S in parallel to the given galvanometer.

An ideal ammeter has zero resistance.

#) Conversion of galvanometer into voltmeter

A galvanometer can be converted into voltmeter of given range by connecting asuitable resistance R in series with the galvanometer, whose value is given by

R= V/1g - G

Where V is the voltage to be measured, 1g is the current for full scale deflectionof galvanometer and G is the resistance of galvanometer.

The resistance of an ideal voltmeter is infinity.

#) Ch 3 (c) MAGNETS AND EARTH'S MAGNETISM

#) According to molecular theory, every molecule of a magnetic substance, is a complete magnet in itself.

In an unmagnified substance, the molecular magnets are randomly oriented such that they form closed chains. That is why resultant magnetism of the unmagnified specimen is zero. On magnetizing the substance, the molecular magnets are realigned in the direction of the field.

#) Magnetic lines of force are hypothetical lines which enable us to understand certain phenomena in magnetism.

Tangent to field line at a point gives us the direction of magnetic field B at that point. No two magnetic lines of force can intersect each other.

Magnetic lines of force are continuous curves from north to south, outside the body of the magnet and form south to north inside the body of the magnet.

A current carrying solenoid from outside resembles a bar magnet. Inside the solenoid, there is a strong magnetic field, which can magnetize a specimen. Thesolenoid is hollow from inside, whereas the bar magnet is solid

#)A loop of current acts as a dipole of magnetic moment M = 1 An where I is strength of current through the loop and A is area enclosed by the loop.

#) Terrestrial magnetism deals with the magnetic field of earth. It is described in terms of three quantities, which are called magnetic elements of earth. These are:

(I) Magnetic declination

(II) Magnetic inclination or Dip

(III) Horizontal component.

Magnetic declination at a place is the angle between magnetic axis and the geographic axis. It is also equal to the angle between magnetic meridian and geographic meridian at the place.

Magnetic inclination or dip at a place is defined as the angle, which the direction of total intensity of earth's magnetic field makes with a horizontal line in magnetic meridian.

Horizontal component is the component of total intensity of earth's magnetic field (R) in the horizontal direction in magnetic meridian.

#) Neutral points are the points where net field intensity due to the field of the bar magnet and field of earth is zero. When a magnet is placed with its N pole towards geographic north, neutral points lie on equatorial line of the magnet.

Where H is horizontal component of earth's magnetic field.

#) Tangent Law. When a magnet is suspended freely under the combined actionof two uniform magnetic fields of intensities F and H acting at 900 to each other,the magnet comes to rest making an angle 0 with the direction of H, such that. F = H tan0

This is tangent law. A tangent galvanometer is based on tangent law.

Ch 3(d) CLASSIFICATION OF MAGNETIC MATERIALS

#) Hysteresis is the phenomena of lagging of magnetic induction (B) and intensity of magnetization (I) behind the magnetizing field (H), when a specimen is taken through a cycle of magnetization.

From the hysteresis loop of a material, we can study about retentively, coercively etc of the material. The loss of energy per unit volume of the specimen per cycle of magnetization is equal to area of the I - H loop. The studyof these characteristics enables us to select suitable materials for different purposes.

D) Classification of magnetic materials

#) On the basis of magnetic properties, different materials have been classified into three categories: Diamagnetic, Paramagnetic and Ferromagnetic.

Diamagnetic substances are those in which the individual atoms/molecules/ions do not possess any net magnetic moment on their own. These are feebly magnetized in a direction opposite to that of the magnetizing field in which they are placed. For example,bismuth,antimony,copper,gold,quartz,mercury,water,alcohol,air,hydrogen,etc. The permeability of diamagnetic substance is less than one and their susceptibility is positive. The susceptibility of such material decreases with rise in temperature.

Ferromagnetic substances are those in which each individual atom/molecule/ion has a none zero magnetic moment on its own. They are magnetized strongly in the direction of the magnetizing field in which they are placed. For example, iron, cobalt, nickel and a number of their alloys. Ferromagnetic materials show all the properties of paramagnetic substance to amuch greater degree.

At a certain temperature, ferromagnetic passes over to paramagnetic. This transition temperature is called Curie temperature. For soft iron, curie temp. Is 1000 K.

The three kinds of behavior (die, para, Ferro) have been explained on the basis of electron theory of magnetism.