PCS 125Physics: Waves and Fields

Catherine Beauchemin

Department of Physics, Ryerson University

Email: cbeau@ryerson.caURL: http://phymbie.physics.ryerson.ca/∼cbeau

Section 23.1: Properties of electric chargesPages 642–644 in your textbook

• Ancient Greek philosopher Thales of Miletus around 600 BC, reportedthat charge (or electricity) could be accumulated by rubbing fur on varioussubstances, such as amber. The Greeks noted that the charged amberbuttons could attract light objects such as hair.

• In 1600 the English scientist William Gilbert returned to the subject inDe Magnete, and coined the New Latin word electricus from ηλεκτρoν(elektron), the Greek word for ‘amber’, which soon gave rise to the En-glish words ‘electric’ and ‘electricity’.

Section 23.1: Properties of electric charges...• In 1733, Charles Francois de Cisternay DuFay, French scientist and su-

perintendent of the Jardin du Roi of Paris, made the important discoverythat there are two kinds of electricity, one produced by glass (vitreous)and the other by resin (resinous), and expressed the concept of electricityin terms of a two-fluid theory. [See original paper here]

• He also disproved certain misconceptions regarding electric charge, suchas that of Dr. Stephen Gray who believed that electric properties of a bodydepended on its colour.

Section 23.1: Properties of electric charges...• The terms vitreous and resinous were used for fifteen years until they were

replaced with positive and negative — coined independently by WilliamWatson and Benjamin Franklin.

• Benjamin Franklin, argued in favour of a one-fluid theory of electricity.Franklin imagined electricity as being a type of invisible fluid present inall matter. He posited that rubbing insulating surfaces together causedthis fluid to change location, and that a flow of this fluid constitutes anelectric current.

• He also posited that when matter contained too little of the fluid itwas ‘negatively’ charged, and when it had an excess it was ‘positively’charged.

• In reality, we now know that it is typically the transfer of electrons whichcauses a change in the charge and that a negative charge is the result ofhaving an excess of electrons.

• Arbitrarily (or for a reason that was not recorded) he identified the term‘positive’ with vitreous electricity and ‘negative’ with resinous electricity.

Section 23.1: Properties of electric charges...

Section 23.1: Properties of electric charges...• Between 1909 and 1913, Robert Millikan performed his oil-drop exper-

iments. He measured e, the magnitude of the elementary charge on anelectron and thus demonstrated that charge was quantized. [MOVIE]

q = Ne forN = 0,−1,−2,−3, ...

where e = 1.60× 10−19 C.

Millikan received the 1923 Physics Nobel Prize for this work.

Section 23.1: Properties of electric chargesYou need to remember that: [Write this in your notes]

• Charges can be either negative (electrons) or positive (protons)

• Charges of the same sign repel one another

• Charges of opposite sign attract one another

• A charged object will also attract neutral objects (with no net charge).

• In an isolated system, electric charge is always conserved: charge is sim-ply transfered from one object to another

• Electric charge exists as discrete “packets”, and we can write q = Newhere N is some integer and e = 1.60× 10−19 C is the fundamentalcharge.

• An electron has a charge of−e and a proton has a charge of +e.

• Some particles such as neutrons have no charge.

• A coulomb is the SI unit of charge. 1 C corresponds to the magnitude ofthe charge of approximately 6.24× 1018 electrons.

Section 23.2: Charging objects by inductionPages 644-645 in your textbook.

You need to remember that: [Write this in your notes]

Electrical conductors: materials in which some of the electrons are freeelectrons that are not bound to atoms and can move relatively freelythrough the material. [Charges quickly redistribute]

Electrical insulators: materials in which all electrons are bound to atomsand cannot move freely through the material. [Charges have trouble mov-ing and redistributing]

Charging an object by induction requires no contact with the object inducingthe charge. That is in contrast to charging an object by rubbing (that is,by conduction) which does require contact between the two objects.

Section 23.2: Charging objects by inductionFor a conductor

For an insulator

Quick questionsThree objects are brought close to each other, two at a time. When ob-jects A and B are brought together, they repel. When objects B and Care brought together, they also repel. Which of the following are true?(a) Objects A and C possess charges of the same sign; (b) Objects A andC possess charges of opposite signs; (c) All three objects possess chargesof the same sign; (d) One object is neutral; (e) Additional experimentsmust be performed to determine the signs of charges. Answer is?

Three objects are brought close to each other, two at a time. When ob-jects A and B are brought together, they attract. When objects B and Care brought together, they repel. Which of the following are true? (a)Objects A and C possess charges of the same sign; (b) Objects A and Cpossess charges of opposite signs; (c) All three objects possess charges ofthe same sign; (d) One object is neutral; (e) Additional experiments mustbe performed to determine the charges on the objects. Answer is?

Quick questionWhy do these people’s hair look like this?

Answer is? NOT because it’s the 80’s.

Section 23.3: Coulomb’s LawPages 645–651 in your textbookIn an experiment very similar to the Cavendish experiment, Coulomb wasable to determine the properties of the electric force

Fg =GM1M2

r2 Fe =ke|q1||q2|

r2