Electromagnetism I

A field (scalar or vector) is a physical quantity to which a definite value can be ascribed at each point in some region of space at a particular time.

Gravitational or electric fields are real “material” agents, that act as mediators of force, conveying the force over the distance from one body to another.

The Ultimate Goal To Lay Down the Foundations Leading to an Understanding

and Use of: Maxwell’s Laws of Electromagnetism

To describe Charge and Charge ConservationTo describe the Atomic View of Charge

Lecture 1: Learning Objectives

Maxwell’s Laws of Electromagnetism (free space and in differential form):

0

div

E.E

0 div B.B

t

BEE

curl

t

E

cJBB

20

1 curl

It is not possible to fully understand Maxwell’s equations until we can appreciate the meaning of the fields E and B and the sources of these fields, the charges and the currents (hence the need for this course)

This course is a fundamental and basic course needed for an appreciation and understanding of many areas of physics. The aim of the course is to provide a sound basis for the second year core course on Electromagnetism.

Electric Charge

Experiments by Franklin, Coulomb and others showed that:

•like charge repel, unlike charges attract

(Franklin named “positive electricity” to the charge on a glass rod rubbed with silk, and “negative electricity” to that on an ebonite rod rubbed with fur)

First discovery attributed to Thales (640 - 548 BC)

Elektron amber

•there are two types of charge: positive and negative

Atomic View (unit of charge)

Atoms are made up of electrons, protons and neutrons

Particle Mass Charge

Electron 9.1 x 10-31 kg -eProton 1.7 x 10-27 kg +eNeutron 1.7 x 10-27 kg 0

Electrons and protons are stable (proton decay > 1031 years). This means that the net charge of an isolated system is constant. FURTHERMORE:

eqq ep

4310.grav

.elec

FF

The electrical neutrality of matter means that electric forces remain small or zero at large distances (greater than atomic dimensions). Mass, on the other hand, is always positive.

Comparison of gravitational and electrical forces between two electrons:

The Unit of Charge

The SI unit of charge is called one coulomb (C)

The most fundamental unit of charge is the magnitude of the charge of an electron or a proton

e = 1.60217738(48) x 10-19 C

•Everyday objects are composed of atoms, which in turn are made of protons and electrons

Summary

•Protons and electrons have a property called “electric charge”. This property comes with a sign, it is positive for protons and negative for electrons

•Atoms (and hence macroscopic objects) are electrically neutral

•Charges of different signs attract each other, charges of the same sign repel each other

•Charge is conserved and quantised