Development of Magnetism About 2500 yrs ago, the Greeks discovered that certain metallic rocks from Magnesia attract iron, and attract or repel similar rocks. The first known use of magnet was in navigation. In 1600, William Gilbert discovered that the earth itself behave like a large magnet. In 1819, Hans Christian Oersted discovered the relationship between electricity and magnetism.

Cont.

In 1868, James Clerk Maxwell developed a complete theory that showed changing electric field produces a magnetic field.

Nature of Magnetism The fundamental nature of magnetism is the interaction of moving charges. The end of the magnet is always consisting of North Pole (N-pole) and South Pole (S-pole). Moving charges creates magnetic field.

Magnetic field of current-carrying conductorB

rI

B= I 2rwhere: = permeability constant = 4 x10 -7Tm/A I = current along the conductor, A r = distance from the conductor to a point, m

Right-Hand RuleMethod use to find the direction of the magnetic field by pointing the thumb of the right hand along the direction of the flow of positive charge. The fingers curl in the direction of the magnetic field.

Magnetic field of a permanent magnet

Magnetic field linesMagnetic field lines emerge from the North-pole and point toward the South-pole which continuous inside the magnet forming the closed loop.

Force on moving chargeFm = qvBwhere: Fm = magnetic force, N q = magnitude of the charge, C v = velocity of the charge, m/s B = magnetic field, T The direction of magnetic force (Fm) is perpendicular to both velocity (v) and magnetic field (B).

Force on current-carrying conductorF S B N I

Cont

Fm = IBwhere: Fm = Force on current-carrying conductor, N I = Current along the conductor, A = length of the conductor, m B= magnetic field, T

Electric motorMotor works when the magnetic field exerts force on current-carrying conductor that make it turn.

Torque on a loop of current

T = NIBAwhere: T = Torque, Nm N = no. of turns I = current along the conductor, A B = magnetic field, T A = area of the loop, m 2

LoudspeakerThe radial magnetic field created by permanent magnet exerts a force on the voice coil that is proportional to the current in the coil; the direction of the force is either to the left or to the right, depending on the direction of the current. Turning up the volume knob on the amplifier increases the current amplitude and hence the amplitude of the cones oscillation and of the sound wave produced by the moving cone.

Amperes LawAmperes law deals about two parallel currents attract each other. The magnetic field circling each wire cause forces on the current in the other wire, pulling it closer.

Faradays Law

Faradays law deals about the electromotive force generated in the wire whenever magnetic field lines cut across the wire.

Lenzs Law

Lenzs law deals about the induced current which flows in a direction to oppose the change that produced it.

Magnetic Levitation Train

Earths magnetic field

s

Magnetic domain

Magnetic poles

N

S

N

S

N

S

Reactions

N

S

FN

S

FN S S N

F

Magnetic field of current-carrying conductor

Straight wire

Circular loop

Coil of loops

Problem 1: Magnet

A long straight wire in the horizontal plane carries a current of 50 A from North to South direction. Determine the magnitude and direction of the magnetic field at a point 2.5 m exactly above the wire.

Problem 2: Magnet

A particle of mass 0.195-g carries a charge of -2.5 x10-8 C. The particle is given an initial horizontal velocity that is due north and has a 4 magnitude 4 x10 m/s. What are the magnitude and direction of the minimum magnetic field that will keep the particle moving in the earths gravitational field in the same horizontal, northward direction?

Problem 3: Magnet

A circular coil 0.05 m in radius, with 30 turns of wire, lies in a horizontal plane. It carries a current of 5.0 A in a counterclockwise viewed from above. The coil is in a uniform magnetic field toward the right, with magnitude 1.20 T. Find the magnitude of the magnetic moment and the torque on the coil.

Problem 4: Magnet

An electron travels in a circular orbit with radius 4.68 mm due to the force exerted on it by a magnetic field with magnitude 1.65 T and perpendicular to the orbit. What is the magnitude of the linear momentum of the particle?