Electricity & Magnetism

Lecture 1: Basic Phenomena

Methods of Charging

Today’s Topics

• Why study electromagnetism

• Some electrostatic phenomena– Chapter 21: [21.1, 21.2]

Lightning

Properties of Charge

Elementary Electrostatics

• Rubbing a balloon on a wool jumper makes the balloon attract your hair

• The balloon is said to be “charged” or to have an “electric charge”

• Similarly glass rubbed with silk/fur will become “charged”

• Charged glass will attract a charged balloon• Two charged balloons will repel each other

Experiments Show….

• Benjamin Franklin (1706-1790)– Two types of charge

• Charles Coulomb (1736-1806)– Coulomb’s Law

• Robert Milikan (1868-1953)– Quantisation (1909)

Properties of Charge: Two types of charge

• Arbitrarily named –Positive (+ve) e.g. glass

–Negative (-ve) e.g. wax & rubber

Like Charges Repel …

+ve+ve +ve+ve

-ve -ve -ve-ve

& Opposites Attract…

+ve -ve+ve -ve

Properties of Charge: Charge is Quantised

• Whenever we measure the amount of charge we get a value that is an integer multiple of a unique number e

• q = N e

Charge Integer

Fixed number

Properties of Charge: Charge is always conserved

– Charge is never created or destroyed

– The process of “charging” is really “moving” charge from one place to another

QuizWhy does your hair stand on end in a lightning storm?

Quiz

• A: more than before rubbing?

• B: the same as before?

• C: less than before?

Rub a balloon on your hair, the balloon attracts you hair. Is the total amount of charge in the balloon and in your hair ...

Equipment and methods of Charging

The ElectroscopeUsed to detect and crudely measure charge

Earthing

The Earth is a practically limitless supply (or sink) of charge

Van de Graf Generator

Different Methods for Charging

• Friction– e.g. rubbing a balloon with wool

• Conduction– e.g. touching an electroscope

• Induction– e.g. balloon sticking to a wall

Charging by Friction

When two different insulators are rubbed together, electrons can be transferred from one insulator to the other. The body which has gained electrons has a negative charge and the one which lost electrons has a positive charge of equal magnitude.This process is called charging by friction.

Charging by Conduction

InductionBalloon on wall

Wall is neutral and an insulator

+ve Charges move slightly towards balloon

-ve Charges move slightly away from balloon

Wall is still neutral but surface has small residual charge, sufficient to hold baloon

Charging an Electroscope by Induction

Induction to Charge Object

Quiz

• A: the water moves away from the rod

• B: the water doesn’t move

• C: the water moves towards the rod

If a positively charged rod is brought near a trickle of water the water moves towards it. What happens if we use a negatively charged rod?

Quiz

• a: A is positive• b: A is negative• c: A is neutral• d: not enough information

Object A attracts object B. If we know that B is positively charged what can we say about A?

Experiments and applications of Static Electricity

• Milikan’s Oil Drop experiment

• The Van de Graaff Generator

• The electrostatic Painting

• Photocopies and Laser Printers

Summary: Lecture 1

• Equipment and Techniques– Electroscope “Measures

charge”

– Earthing Earth as a limitless supply or sink of charge

– Van de Graaff Generator “Generates charge”

• Introduced the concept of charge

• Properties of charge– Two types +ve & -ve

• Like repel

• Opposites attract

– Charge always conserved

– Charge quantised

• Methods of charging– Friction

– Conduction

– Induction