General Physics II. Spring 2013 Dr Laycock Course Elements Lectures (with Clicker 10%) Recitation (with Quiz 10%) Homework (Mastering Physics 20%) Monthly.

General Physics II. Spring 2013Dr Laycock

Course Elements

Lectures (with Clicker 10%)Recitation (with Quiz 10%)Homework (Mastering Physics 20%)Monthly Exams(3) (25% total)Final Exam (25% total) Team Research Challenge (10%)

Mastering Physics: www.MasteringPhysics.com Course Code: GPHYS2SP2012 Policies: Attend Class!!! No Clicker make-ups or exceptions

No Late HW or Make-ups without prior arrangement

Office Hrs: Wed 1-3pm and most days by Appointment (send email)Tutoring: Centers for Learning, and Physics Dept.

SyllabusWe have 26 Lecture slots, after taking 3 for exams, leaves 23 to cover the following 4 subjects, roughly 6 lectures apiece.

Section 1: Electricity and Magnetism Section 2: Light and Optics Section 3: Relativity and Quanta Section 4: Nuclei and Radiation

See the course Webpage for more informationhttp://faculty.uml.edu/slaycock

www.masteringphysics.com• General Physics II (Spring 2012)• GPHYS2SP2013 • ALL weekly homework assignments are

posted on Mastering Physics• You must register for mastering physics in

order to do the weekly homework assignments

• Most of you already have an account from last semester (they last 2 years)

• Course webpage gives DETAILED instructions on how to register and add this course to your account

Clickers “iclicker”• We will be using iclicker this

semester.• You will be issued with an

iclicker to use this semester, free of charge.

• Multiple choice questions to keep you thinking during lectures.

• Provides feedback to me, so I know if you’re “getting it” or not!

Forces of Nature

Which are FUNDAMENTAL forces of nature?

1. Nuclear, Atomic, Molecular, Magnetic

2. Gravity, Strong, Weak, Electromagnetism

3. Electricity, Magnetism, Friction, Gravity

4. Electricity, Gravity, Tension, Strong

Chapter 16

Electric Charge and Electric Field

Electricity & Magnetism - Chapters 16-21

Ch 16,17 are short, Ch 18 long, and Ch 19 long also

Key learning points for E&M Course Section

1. What is Electricity? 2. What is a Magnet? 3. And what do they have to do with each other?4. What does Light have to do with E&M ?5. Role in the fundamental workings of nature?6. How are E&M manipulated to empower humanity?7. Historical background on how we know these things.

Today’s Lecture: Electricity and Charge

• Static Electricity and the nature of Electric Charge

• Electric Charge in the Atom

• Insulators and Conductors

• Coulomb’s Law

• The Electric Field

• Gauss’s Law

Static Electricity and Electric Charge

Objects can be charged by rubbing, usually the charge is small, but can be

enormous -Why?

Do all materials respond in the same way?

Electric Charges: Positive and Negative

Charge comes in two types, like charges repel and opposite charges attract Benjamin Franklin coined the terms Positive and Negative, in line with his theory that there is single “electrical fluid”, that flows in response to some kind of pressure, from positive to negative.

Franklin’s terms were arbitrary, and in fact electricity (electrons) flow in the opposite direction to his “guess”.

Conservation of Electric Charge

Electric charge is conserved – the arithmetic sum of the total charge cannot change in any interaction.

What other Conservation Laws are there?

The Electric Force Originates with subatomic particles - an explanation for

the conservation of charge?

Atom:

Nucleus (small, massive, positive charge)

Electron cloud (large, very low density, negative charge)

An overly simplistic model for several reasons. -What might they be?

Electric Charge and its Origin

Atom is electrically neutral.

But in some elements the outer electrons are only loosely bound to the nucleus, and can be easily dislodged.

Rubbing charges objects by moving electrons from one to the other.

Electric Charge and Molecules

Polar molecule: neutral overall, but charge not evenly distributed

Insulators and Conductors

Conductor:

Charge flows freely

Metals

Insulator:

Almost no charge flows

Most other materials

Some materials are semiconductors.

Induced ChargeMetal objects can be charged by

conduction or induction:

Induced Charge and non-conductors

Nonconductors won’t become charged by conduction or induction, but will experience charge separation:

This is why dust sticks to your furniture, screen, glasses etc.

Note how the insulator becomes POLARIZED, while a conductor does not

Fully explore this in the online Homework Questions 11 and 12.

The Electroscope

The electroscope can be used for detecting charge:

An electroscope can be charged either by conduction or by induction.

The charged electroscope can then be used to determine the sign of an unknown charge.

Another Sort of Electroscope…..

Electric Forces: Coulomb’s Law

Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them.

Compare this with the law of Gravitation F = GM1M2

r2

Like Charges Repel, Opposites Attract: They also Obey Newton’s 3rd Law….

The forces act along the line connecting the charges, and are equal and opposite

Coulomb’s Law and the Strength of the Electric Force

Unit of charge: coulomb, C

The proportionality constant in Coulomb’s law is:

Charges produced by rubbing are typically around a microcoulomb:

One Coulomb may seem huge, but is only the amount of charge passing through a household lightbulb in about one second. Can any of you prove this for next time?

Charge on the electron

Electric charge is quantized in units of the electron’s charge.Robert Millikan's oil-drop experiment (1908-13) demonstrated this fact directly, and measured the elementary charge. His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. Knowing the electric field, the charge on the droplet could be determined. Repeating the experiment for many droplets, Millikan showed that the results could be explained as integer multiples of a common value (1.6x 10−19 coulomb), the charge on a single electron.

This was a key piece of evidence for the existence of electrons, atoms and molecules

Calculate the number of electrons in a bucket of water, and thus the resulting force between two buckets of water

Lets do this on the Document Camera, where I can write….

Coulomb’s Law

The proportionality constant k can also be written in terms of , the permittivity of free space:

(16-2)

Using Coulomb’s Law for Multiple ChargesCoulomb’s law strictly describes point charges.

Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.

Where would this sort of thing be important?

The net force on a charge is the vector sum of all the forces acting on it.

Wait a minute……Where do Electric Forces Come From?

Before we get bogged down with math, what’s all this about charges attracting and repelling each other?

How do they do it?

Notice that the charges do not need to touch.

The Electric Field

Definition: The electric field E is the force exerted per unit charge, assuming the “test charge” is extremely small, so it doesn’t add its own field to the mix!

Early scientists and philosophers struggled with the idea of “action at a distance”.

How was the electric force propagated?

Michael Faraday proposed that a “field” extended outwards from all charged objects, and that these fields interacted with one another.

Fields are also a great mathematical convenience

Visualizing Electric Fields: A Single Point-Charge

The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge.

The electric field is stronger where the field lines are closer together.

Visualizing Electric Fields: Two Charges

The lines emanating from two equal charges, opposite in sign will connect to form a Dipole (two poles).

While if the charges are the same, the lines will avoid each other, and the charges repel

Calculating the Electric Field

For a point charge Q, we calculate its Electric Field using an imaginary (minute) test charge “q”:

Since the force between 2 charges is given by Coulomb's law, the force felt by our tiny test charge q would be F= k Qq/r2

Thus the force per unit charge (Electric field) would be E= F/Q

We can work that out: E = k Qq/r2

q

the q’s cancel, leaving ->

If we know the Field, its easy to find the force exerted on charges anywhere in it!

Force on a point charge q in an electric field:

For a complex distribution of Charges, we just add up the contribution of each’s field at the point in question:

This is called the Superposition principle for electric fields

Vector Electric Field Calculation - on Doc-Cam

Similar to Example 16.9 in the book

Find the Direction and Magnitude of the Electric Field due to a a pair of unequal Charges.

(takes 10 min, so do next time if not today)

More Complex Field Lines and Symmetry

The electric field between two closely spaced, oppositely charged parallel plates is constant. -where might this configuration occur?

Electric Fields and Conductors

The static electric field inside a conductor is zero. The free charges “instantly” align themselves to totally cancel the external field.

The net charge on a conductor is all on its surface. -Charges want to be as far apart as possible.

Faraday Cage, Car in Thunderstorm etc…

Gauss’s Law

The net number of field lines through the surface is proportional to the charge enclosed, and also to the flux, giving Gauss’s law:

This can be used to find the electric field in situations with a high degree of symmetry.

For example the total charge on a thundercloud. NEXT TIME!

Photocopy Machines and Computer Printers Use Electrostatics

Photocopy Machines and Computer Printers Use Electrostatics

Laser printer is similar, except a computer controls the laser intensity to form the image on the drum

• Two kinds of electric charge – positive and negative

• Charge is conserved

• Charge on electron:

• Conductors: electrons free to move

• Insulators: nonconductors

Summary of Chapter 16

Summary of Chapter 16

• Charge is quantized in units of e

• Objects can be charged by conduction or induction

• Coulomb’s law:

• Electric field is force per unit charge:

Summary of Chapter 16

• Electric field of a point charge:

• Electric field can be represented by electric field lines

• Static electric field inside conductor is zero; surface field is perpendicular to surface

• Electric flux:

• Gauss’s law: