ELECTROMAGNETISM

Magnets  A magnet from Greek word ( μαγνήτις λίθος magn ḗ tis líthos, "Magnesian stone"). μαγνήτις λίθοςmagn ḗ tislíthosMagnesian  Magnets have been known for centuries.  The Chinese and Greeks knew about the “magical” properties of magnets.  The ancient Greeks used a stone substance called “magnetite.” They discovered that the stone always pointed in the same direction. Later, stones of magnetite called “lodestones” were used in navigation.  An object that is surrounded by a magnetic field and that has the property, either natural or induced, of attracting iron or steel.

William Gilbert, an English physician, first proposed in 1600 that the earth itself is a magnet, and he predicted that the Earth would be found to have magnetic poles.

What is Magnetism  Magnetism is the properties and interactions of magnets.  The earliest magnets were found naturally in the mineral magnetite which is abundant the rock-type lodestone. These magnets were used by the ancient peoples as compasses to guide sailing vessels.  Magnets produce magnetic forces and have magnetic field lines.

Magnets have two ends or poles, called north and south poles. At the poles of a magnet, the magnetic field lines are closer together. Unlike poles of magnets attract each other and like poles of magnets repel.

The Earth is like a giant magnet!  The nickel iron core of the earth gives the earth a magnetic field much like a bar magnet.

History  In 1820, Hans Christian Oersted discovered the connection between electricity and magnetism when he observed the magnetic needle of a compass was deflected by an electric current in a nearby wire.  In 1831, M ICHAEL F ARADAY showed that a changing magnetic field can induce a current in a circuit.  In 1860, J AMES C LERK M AXWELL predicted that a changing electric field has an associated magnetic field and wrote the mathematical equations that describe the force of electromagnetism for the first time.

Cont…  In 1905, the theoretical implications of electromagnetism led to Albert Einstein's theory of special relativity.

Electromagnet  When an electric current is passed through a coil of wire wrapped around a metal core, a very strong magnetic field is produced. This is called an electromagnet.

Cont…  We can make an electromagnet stronger by doing these things: wrapping the coil around an iron core adding more turns to the coil increasing the current flowing through the coil.

Electromagnetism: Definition  The word Electromagnetism is a compound form of two Greek terms, ἢλεκτρον, ē lektron, "amber", and μαγνήτης, magn ē t ē s, "magnet".Greekambermagnet  It is a branch of physical science that deals with the physical relations between electricity and magnetism.

Introduction to Electromagnetic Fields Maxwell’s equations Fundamental laws of classical electromagnetics Special cases Electro- statics Magneto- statics Electro- magnetic waves Kirchoff’s Laws Statics: Geometric Optics Transmission Line Theory Circuit Theory Input from other disciplines

Introduction to Electromagnetic Fields  Maxwell’s Equation  Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics (classical electromagnetism), classical optics, and electric circuits.  Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents.  Classical Electromagnetism is a branch of theoretical physics that studies consequences of the electromagnetic forces between electric charges and currents.

Introduction to Electromagnetic Fields  Electromagnetics is the study of the effect of charges at rest and charges in motion.  Some special cases of electromagnetics:  Electrostatics: charges at rest (Static Electricity)  Magnetostatics: charges in steady motion  Electromagnetic waves: waves excited by charges in time-varying motion.

Introduction to Electromagnetic Fields transmitter and receiver are connected by a “field.”

Cont…  When an event in one place has an effect on something at a different location, we talk about the events as being connected by a “field”.  A field is a spatial distribution of a quantity; in general, it can be either scalar or vector in nature.

Cont…  Electric and magnetic fields:  Are vector fields with three spatial components.  Vary as a function of position in 3D space as well as time.  Are governed by partial differential equations derived from Maxwell’s equations.

Introduction to Electromagnetic Fields  Fundamental vector field quantities in electromagnetics:  Electric field intensity  Electric flux density (electric displacement)  Magnetic field intensity  Magnetic flux density units = volts per meter (V/m = kg m/A/s 3 ) units = coulombs per square meter (C/m 2 = A s /m 2 ) units = amps per meter (A/m) units = teslas = webers per square meter (T = Wb/ m 2 = kg/A/s 3 )

19 Introduction to Electromagnetic Fields  Universal constants in electromagnetics:  Velocity of an electromagnetic wave (e.g., light) in free space (perfect vacuum)  Permeability of free space  Permittivity of free space:  Intrinsic impedance of free space:

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