0.21 Chapter 7. magnetic materials.  (Page 2/4)

The term magnetism was thus coined to explain the phenomenon whereby lodestones attracted iron. Today, after hundreds of years of research we not only know the attractive and repulsive nature of magnets, but also understand MIR scans in the field of medicine, computers chips, television and telephones in electronics and even that certain birds, butterflies and other insects have a magnetic sense of direction.

7.1. Magnetic Circuit – an analog of Electric Circuit.

Just as in an electric circuit , voltage (V) drives current (A) through the electrical circuit resistance R where R is:

In an analogous fashion, transformer’s core is a magnetic circuit where Magneto-Motive Force in Amp-Turns(AT) drives magnetic flux in Weber overcoming the reluctance of the magnetic core where Reluctance is defined as follows:

7.2. Ampere’s circuital Law:

Andre Maria Ampere (1775-1836) gave the Ampere Circuital Law.

In the simplest form it is stated as:

James Clerk Maxwell (1831-1879) stated it as follows:

(7.4) is Differential Form. In Integral form it is as follows:

7.3. Faraday’s Law of electro-magnetic induction.

Whenever the magnetic flux linked with a closed conducting coil changes, an e.m.f. is induced which causes a eddy current to flow through the coil. The direction of eddy current is such as to create a magnetic polarity which opposes the cause of electro-magnetic induction.

If a North Pole approaches the closed coil then eddy current will make the near face of the coli North Pole so that the approaching Pole is opposed and if North Pole is receding then near face will act as South Pole so as to attract the receding Pole.

Mathematically it is:

Maxwell stated it as follows:

(7.7) in integral form is as follows:

7.4. Lorentz Force and Left Hand Rule.

A current in the magnetic field , held transverse to the magnetic field, will experience a mechanical force in the third perpendicular direction. This mechanical force is known as Lorentz Force.

Complete statement of Lorentz Force is as follows:

The first part on R.H.S. is the electro-static force and second is the magneto-static force on a moving electron. J.J. Thomson used this equation to determine q/m of an electron in Cathode Ray Experiment. He balanced the electric force and magnetic force and obtained zero deflection of the electron beam on the CRO screen.

A line of current I Amperes of length L(m) is held in Magnetic Field of B Tesla where I and B are transverse to each other then there is a force f (newtons) acting on the line of current along the third perpendicular direction. If Current is in the direction of M[i]ddle finger and Magnetic Field is in the direction of index [f]inger then the Lorentz force is in the direction of the Thu[m]b as shown in Figure 7.2.

This force F(Newtons)= B.I.L It is this force which causes an armature of a motor to rotate.

Two parallel lines carrying I amperes each will experience a magneto-static force of attraction if the current flows are in the same direction and experience a magneto-static repulsion force if they are in opposite direction.