Chapter 10

Magnetism &Electromagnetism

Objectives

Explain the principles of the magnetic field Explain the principles of electromagnetism Discuss the principle of electromagnetic

induction Describe some applications of electromagnetic

induction

Roadmap

Voltage CurrentMagnetoMotiveForce

Electro-Magnetic

Field

The Magnetic Field

A permanent magnet has a magnetic field surrounding it.

A magnetic field is envisioned to consist of lines of force that radiate from the north pole to the south pole and back to the north pole through the magnetic material. Flux lines Group of flux lines is

called magnetic flux

Magnetic Flux

The force lines going from the north pole to the south pole of a magnet are called magnetic flux () Units: weber (Wb). 10e8 Lines = 1 weber

The magnetic flux density (B) is the amount of flux per unit area perpendicular to the magnetic field. Units: tesla (T).

B = .A (A is cross-section in m2)

Magnetic materialsNon-magnetic materialsFerromagnetic materials

iron, nickel, steel, cobalt, etc. They have randomly oriented magnetic

domains, which become aligned when placed in a magnetic field, thus they effectively become magnets.

Electromagnetism

Electromagnetism is the production of a magnetic field by current in a conductor.

When current passes through a conductor, an electromagnetic field is created around the conductor.

This magnetic field has no north or south pole.

Important Concepts

Permeability () is the ease with which a magnetic field can be established in a given material. The higher the permeability, the more easily a magnetic field

can be established. Relative permeability (r = / o ) of a material is the ratio of

its absolute permeability to the permeability of a vacuum. Ferromagnetic >> o

The opposition to the establishment of a magnetic field in a material is called reluctance ( = l/ A). Unit: At/m Reluctance is analogous to resistance in an electric circuit. Longer path more resistance!

Flux Direction

Magnetomotive Force

Current in a conductor produces a magnetic field.

The force that produces the magnetic field is called the magnetomotive force (Fm).

The magnetomotive force (Fm) depends upon the number of turns of wire, and the amount of current through the wire.

OppositionCauseEffect /

Example

N=5, I = 2AFm = ? If flux = 250 uWbReluctance = ?

Fm = N.I = 10 At R = Fm / Flux = 10 At / 250 uWb = 40 K At/Wb

Electromagnetic Induction

When a conductor is moved through a magnetic field, a voltage is induced across the conductor. This principle is known as electromagnetic induction.

When a wire is moved across a magnetic field, there is a relative motion between the wire and the magnetic field.

When a magnetic field is moved past a stationary wire, there is also relative motion. In either case, the relative motion results in an induced voltage in the wire. The induce voltage depends on the rate of relative motion between the wire and

the magnetic field.

Faraday’s Law

The voltage induced across a coil of wire equals the number of turns in the coil times the rate of change of the magnetic flux.

Voltage induced = N. (change of flux / change of time) = N. d /dt

Applications

Relay

Generator

SolenoidRelay

Summary