Attendance Quiz 21-1, 21-2

State the principle of conservation of charge in your own words, maximum of 2 sentences.

Attendance Quiz 21-1, 21-2

Principle of conservation of charge:

The algebraic sum of all the electric charges in any closed system in constant.

Electric Charge

Objective

Discuss the dichotomy, quantization and conservation of electric charge

Given the initial charge distribution, calculate the final charge distribution using conservation principles

Electric Charge

scalar; no direction

SI unit of electric charge: Coulomb (C)

Electrostatics: study of the interactions between electric charges that are at rest in an observer’s reference frame.

Three properties of electric charge:

1. Dichotomy

2. Conservation

3. Quantization

Dichotomy

Electric charge is either “positive” or “negative”

Like charges repel; opposite charges attract

3 properties of electric charge

Conservation

“The algebraic sum of all the electric charges in any closed system is constant.”

In charging, charge is neither created nor destroyed; it is only transferred from one body to another

This is a universal conservation law

3 properties of electric charge

Quantization

“The magnitude of charge of the electron or proton is a natural unit of charge.”

Basic unit of charge ≡ e = 1.602 x 10-19 C

Proton: + 1.602 x 10-19 C = +e

Electron: - 1.602 x 10-19 C = -e

Every observable amount of electric charge is always an integer multiple of this basic unit

3 properties of electric charge

Consider 2 conducting spheres of the same size, with initial charges as shown:

1.What is the final charge upon reaching equilibrium on each sphere when the two are brought into contact?

2.Are the final charges attractive or repulsive?

Q1 = +3Q Q2 = -Q

Consider 2 conducting spheres of the same size, with initial charges as shown:

1.What is the final charge upon reaching equilibrium on each sphere when the two are brought into contact?

2.Are the final charges attractive or repulsive?

Both have same sign: Repulsive

Q1 = +3Q Q2 = -Q

Net charge: +2Q so Q1 = +Q = Q2

Objective

Discuss the dichotomy, quantization and conservation of electric charge

Given the initial charge distribution, calculate the final charge distribution using conservation principles

Conductors, Insulators & Induced Charges

Objectives

Predict charge distributions, and the resulting attraction or repulsion, in a system of charged insulators and conductors

Outline the process of charging

Types of materials in terms of electric conduction

Conductors

Insulators

Semiconductors

Superconductors

Conductors

Conductors

Objects that permit easy movement of electrons through them

In most metals, mobile charges are always negative electrons: “sea of electrons”

The earth can act as an infinite source or sink of electrons: “grounding”

Conductors

Most metals are good conductors

Insulators

Objects that DO NOT permit easy movement of electrons through them

Examples include most nonmetals, ceramic, wood, plastic, rubber, air

The charges within the molecules of an insulator can shift slightly: “polarization”

Semiconductors

Objects with properties between conductors and insulators

Examples include silicon, diodes, transistors

Superconductors

Objects with zero resistance against the movement of electrons

Examples include some compounds at very low temperatures

Charging by rubbing

charge of “charger” changes; electrons transfer

Ways of charging a material

Charging by rubbing

Ways of charging a material

Charging by contact

charge of “charger” changes; electrons transfer

Ways of charging a material

Charging by induction (w/o grounding)

charge of “charger” DOES NOT change

Ways of charging a material

Charging by induction (w/ grounding)

charge of “charger” DOES NOT change

negative “charger” induces a positive charge

(positive “charger” induces a negative charge)

Ways of charging a material

Charging by induction (w/ grounding)

Ways of charging a material

Charging by polarization

charge of “charger” DOES NOT change

charged object can still attract a neutral object by polarization

Ways of charging a material

Electric Charge and the Structure of Matter

Atomic structure:

electron, proton,neutron

Proton and neutron are made up of quarks

Electric Charge and the Structure of Matter

Standard model

-current state of classification of all elementary particles

Example 1

Q1 = Q Q2 = 0

Consider 2 conducting spheres of the same size, with initial charges as shown:

What is the final charge on each sphere when the two are brought into contact?

A. Q and Q C. Q/2 and Q/2

B. Q and –Q D.Q/2 and –Q/2

Example 1

Q1 = Q Q2 = 0

Consider 2 conducting spheres of the same size, with initial charges as shown:

What is the final charge on each sphere when the two are brought into contact?

A. Q and Q C. Q/2 and Q/2

B. Q and –Q D.Q/2 and –Q/2

Index card exercise

Objectives

Predict charge distributions, and the resulting attraction or repulsion, in a system of charged insulators and conductors

Outline the process of charging

Summary

• Electric charge: fundamental quantity in electrostatics

• Electric interactions are chiefly responsible for the structure of atoms, molecules and solids.

• Conductors are materials that permit electric charge to move easily within them

• Insulators permit charge to move much less readily.

Jan 22 Discussion 1 coverage

1) Read and answer questions in “How to Succeed in Physics by Really Trying”.

2) Syllabus signature: statement of acceptance

“I hereby acknowledge that I have read, understood and accepted the contents of the Physics 72 2nd semester 2014-15 syllabus.”

Jan 23 Attendance Quiz coverage

Sec 21-3: Coulomb’s Law

Sec 21-4: Electric Field & Electric Forces