Joy Manners - Static Fields and Potentials

Edited by Joy Manners

Course Team Chair	Robert Lambourne
Academic Editors	John Bolton, Alan Durrant, Robert Lambourne, Joy Manners, Andrew Norton
Authors	David Broadhurst, Derek Capper, Dan Dubin, Tony Evans, Ian Halliday, Carole Haswell, Keith Higgins, Keith Hodgkinson, Mark Jones, Sally Jordan, Ray Mackintosh, David Martin, John Perring, Michael de Podesta, Sean Ryan, Ian Saunders, Richard Skelding, Tony Sudbery, Stan Zochowski
Consultants	Alan Cayless, Melvyn Davies, Graham Farmelo, Stuart Freake, Gloria Medina, Kerry Parker, Alice Peasgood, Graham Read, Russell Stannard, Chris Wigglesworth
Course Managers	Gillian Knight, Michael Watkins
Course Secretaries	Tracey Moore, Tracey Woodcraft
BBC	Deborah Cohen, Tessa Coombs, Steve Evanson, Lisa Hinton, Michael Peet, Jane Roberts
Editors	Gerry Bearman, Rebecca Graham, Ian Nuttall, Peter Twomey
Graphic Designers	Javid Ahmad, Mandy Anton, Steve Best, Sue Dobson, Sarah Hofton, Pam Owen, Andy Whitehead
Centre for Educational Software staff	Geoff Austin, Andrew Bertie, Canan Blake, Jane Bromley, Philip Butcher, Chris Denham, Nicky Heath, Will Rawes, Jon Rosewell, Andy Sutton, Fiona Thomson, Rufus Wondre
Course Assessor	Roger Blin-Stoyle
Picture Researcher	Lydia K. Eaton

The Course Team wishes to thank Carole Haswell, Michael de Podesta, Keith Higgins and Steve Swithenby for their contributions to this book. The book made use of material originally prepared for the S271 Course Team by Joy Manners, Keith Meek and Shelagh Ross. The multimedia package Forces, fields and potentials was written by Joy Manners and programmed by Fiona Thomson.

First published 2000 by Institute of Physics Publishing

Published 2019 by Routledge

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First issued in paperback 2021

Copyright 2000 by The Open University

All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers.

Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

ISBN: 978-0-7503-0718-5 (hbk)

ISBN: 978-1-138-42991-8 (pbk)

DOI: 10.4324/9780429187797

DOI: 10.4324/9780429187797-1

In the earlier books of The Physical World you have learnt a great deal about motion: how to describe motion and how to predict it. You have encountered some of the forces that influence motion and what the result of that influence is. In this book you will meet an entirely new interaction, namely the electromagnetic interaction. As the name suggests, electric and magnetic effects are intertwined at a fundamental level, and you will discover the nature of that interdependence in .

To start with, however, we look at the electrostatic interaction between charged particles and draw many comparisons with the gravitational interaction between masses, with which you already have some familiarity. This is the subject of . Here you will learn how gravitational and electrostatic influences can be described in terms of fields.

further explores gravity and electrostatics, building on the field concept by introducing gravitational and electric potential, and the interrelationships between field, force, potential and potential energy.

In we are concerned with electric currents: the movement of free charges in response to electrical influences. In the modern world we are totally dependent on devices that operate using electric currents, usually in metal wires or circuits. The basic rules applying to simple circuits are explored here.

reveals the magnetic effects created and experienced by moving charges and the variety of ways in which these effects manifest themselves in the real world, for example as permanent magnets.

Open University students should be aware that the video for this book (Magnetic fields in space) is associated with . There is a prompt to view it at the end of that chapter.

DOI: 10.4324/9780429187797-2

On what do we base our scientific theories? When we look up into the sky, how have we learned what we know about the motion of the planets? The understanding of the phenomena of the physical world usually follows from observation and successive attempts at explanation, frequently spread over a period of time and with many setbacks on the way. A very interesting example of this is the explanation of the motion of the planets.

The movement of celestial bodies had, of course, been noted over thousands of years, the planets being distinguished mainly by their relatively rapid motion in the night sky. In fact, the word planet is derived from the Greek word for wanderer. This motion is illustrated in . In the ancient world, the Earth was accepted as the centre of the Universe, and the model proposed by the Greek astronomer Claudius Ptolemy in the second century AD was the accepted theory for the next 14 centuries. However, in 1543, the Polish astronomer Nicolaus Copernicus (1473-1543) proposed that the Earth executed a circular orbit centred on the Sun a heliocentric model replacing a geocentric model. This now relegated the Earth to the same status as the other planets, which were also held to be orbiting about the Sun. This hypothesis was most certainly not welcomed and was subjected both to ridicule and suppression. This is not uncommon with the breaking of totally new theoretical ground.

The night sky, showing the track of the Planet Mars over a period of about nine months. (a) The Copernican circular motion of the Earth, (b) The Keplerian elliptical motion of the Earth. (Neither diagram to scale.)

The next stage in the story arose from accurate measurement (accurate, of course, in the context of that era). Over a period of more than 30 years, using a sextant and compass, the Danish astronomer Tycho Brahe (1546-1601) made accurate measurements of all the astronomical objects he could see with the naked eye in the night sky (the telescope had not been invented). His successor, the German astronomer Johannes Kepler (1571-1630), analysed Tycho's measurements and, after laborious calculations extending over about fifteen years, discovered that the observations, initially for Mars but later for the other known planets, could be explained by a heliocentric motion that followed three laws. The first of these empirical laws required the motion to be not circular but elliptical, with the Sun at one focus of the ellipse (see ).

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