WELCOME TO PHYSICS 1302

Discussion sectionsTalk it over; ask questions

Lab sectionsReal World!

HomeworkDo it!

office hours

Lectures+in class

questions

TextbookRead it!

Self studyexams

Lectures• Lectures M,T,W 08.00-08.50. The Friday 08.00 spot is for

quizes or if I need to move a lecture.– Watch for announcements, in lectures and on the web

• Read the text BEFORE the lecture.– I will tell you what you need to read for the next lecture.

• Listen and think during the lecture.

• The slides will be on the web after the lecture.– You don’t need to take notes unless it helps you to concentrate.

• Read the slides and text again after the lecture. – Now is the time to make sure you understand

• Don’t get behind. If you don’t understand ASK– In the lecture.

– In the discussion sessions.

– In my or the TA’s office hours.

In class Questions

• There will be in-class questions from time to time.– You need a transmitter (buy it from the book store).

• They will make up 5% of your final grade.– 2 points for a correct answer.

– 1 point for any other answer .

• You may need a calculator and/or pencil and paper.

• They are an opportunity for you to ask if you don’t understand and for me to see whether or not you do understand.

Labs and Discussion groups• The labs are compulsory, they make up 15% of your final

grade.

• YOU MUST GET 60% ON YOUR LAB GRADE TO PASS THE WHOLE COURSE.

• This is a writing intensive course, your lab report is the writing part. It must be in reasonably grammatical and well spelt English.

• The discussion groups are compulsory. They are your opportunity to consolidate what you have learnt from the course and/or ask if you don’t understand.

– A group quiz question will be done in your discussion group.

– If you have missed one discussion session in the previous three weeks without permission you will lose 50% of the group question points.

– If you have missed two sessions you will get zero points.

Homework

• Usually you will be given two homework questions in you discussion session, to be handed in to your TA before the next session.

– One question will be marked for 4 points.

– Handing in the other question will get 1 point.

• The homework will count for 10% of your final grade.

• To become fluent with the problem solving you need to do more problems than the homework.

– There is a list of suggested problems from the text book, do them and more if you find them difficult

– The answers to the problems will be on the web

– Try them first, then look at the answers.

Office Hours

• If you need help contact;1. The TA office

2. Make an individual appointment with your TA

3. Come and see me My office hours are on the web but I am fairly

flexible as long as I am not overwhelmed

Quizes and Final

• There will be four quizes on Thursdays (group) and Fridays (individual)

– 1st-2nd February

– 22nd-23rd February

– 22nd-23rd March

– 12th-13th April.

• The final will be on Wednesday 10th May at 18.30

• You can drop one quiz

• Points– 15% for each of 3 quizes + 25% for the final or

– 15% for each of 4 quizes + 10% for the final

You will need

• Fishbane, Gasiorowicz and Thornton– Chapters 12, 21-34

• Lab Manual II, Electricity and Magnetism

• Competent Problem Solver, Calculus version

• An interwrite PRSRF transmitter for in-class questions

• Read the syllabus on the class web page at www.physics.umn.edu/classes

The Challenges of Physics 1302

1. Nearly zero direct experience with E&M in everyday life- for scientists and engineers, though, E&M is everywhere

2. The course is full of NEW concepts: a few per lecture!!- each concept has many different consequences- these concepts are interrelated

3. The course “builds upon itself” sequentially - if you miss a lecture or discussion session you

MUST make it up or you will be LOST

4. Maths is the language of physics and here you need to learn to work with it- calculus: line integrals, surface integrals, gradients- vectors: addition, dot-product, cross-product, decomposition

All of Physics 1302 in 5 lines

( ) F q E v B

0

closed surface

QE dA

B 0

closed surface

dA

surface

dE ds B dA

dt

0 0 0

surface

dB ds I E dA

dt

Outline of physics 1302

• Coulomb’s Law gives force acting on charge Q1 due to another charge, Q2.

– superposition of forces from many charges

– Electric field is a function defined throughout space

– here, Electric field is a shortcut to Force

– later, Electric field takes on a life of it’s own!

1 212 122

12

1ˆ

4 o

QQF r

r

totaltotal EQF

1

1 121

1ˆ

4n

total nn o n

QF Q r

r

12r̂

Q1

Q2

Q313r̂

• Gauss’ Law– an integral formulation of Coulomb’s law

– very useful in problems with geometrical symmetry

– vector calculus:

– symmetry -> integral easy

– Like the gravitational force

Outline of physics 1302 (cont.)

• Potential Energy– charges subject to electric forces

– work is done to “raise a charge to a location with higher potential energy”

– electric potential energy versus electric potential -> U = QV

– voltage on a battery is an example of electric potential value

– charge on a capacitor is chargestored at an electric potential

+++++++++++Q

-Qvolts

‘Rus

- - - - - - - - - -

+

-

Outline of physics 1302

• Magnetism – permanent magnets and magnetic fields• Moving charges (currents) give rise to magnetic

fields Ampere’s Law and Law of Biot and Savart

– unchanging currents givestatic magnetic fields that“ring” around the current

• Magnetic fields give a force that acts on moving charges

– not a central force

• Magnetic fields and electric fields store energy– in capacitors E-field stores energy

– in “inductors” magnetic field (B) stores energy

– along with resistors, capacitors and inductors form elements of simple circuits (passive elements)

Outline of physics 1302

• Simple circuits– DC circuits

– AC circuits

• Faraday’s Law– time varying magnetic fields generate electric fields

– transformers, inductors in AC circuits

– ignition coils in cars

– electromagnetic cannon

• Time varying electric fields generate magnetic fields– repaired Ampere’s Law

• Maxwell’s equations describe all of electromagnetism– Electromagnetic waves, light, x-rays, microwaves etc

The World According toPhysics 1301

• Motion and interactions

• Specified by geometry, mass and forces

• Forces

• Gravity:

• Others: Tension, Normal, Friction

12212

2112 r̂

r

mmGF

m1

m212r̂

amdt

pdF

• Forces

• Gravity:

• Electric:

• Magnetic:

The World According toPhysics 1302

• Fields and Forces ( )

• Specified by geometry and mass and charge

12212

2112 r̂

r

mmGF

12212

2112 ˆ

4

1r

r

QQF

o

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11Q1

v

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Q1

Q2

BE

,

Electric Charge• Charge is an intrinsic property of matter

– Protons have positive (+ve) charge

– Neutrons have no charge

– Electrons have negative (-ve) charge

– The charge on an electron EXACTLY equals the charge on a proton

• Atoms normally have an equal number of protons and electrons and thus have zero charge

• Normal matter thus has equal numbers of protons and electrons and is electrically neutral

• If the number of electrons is not equal to the number of protons a body will have a net charge

– Electrons are easier to detach from a body than protons thus usually

» a negatively charged body has an excess of electrons

» a positively charged body has a deficit of electrons

Electric Charge

• Electric Charge

• Like charges repel

• Unlike charges attract

• Gravity only attracts

• There is only one type of mass

• There are two types of charge

Electrostatics holds the universe together!!!

Electrostatics in Matter

Force in Model Basis of Attraction Example

Atom Opposite Charges H

Ionic Crystal

Opposite Charges NaCl

Covalent Bond

Molecules –

shared e- pair

H - H

Metal Metal cations and delocalized electrons

Au

–

+

Conductors and Insulators

• Consider how charge is carried on macroscopic objects.

– Insulators: In these materials the charges (electrons) ARE NOT FREE TO MOVE. Plastics, glass, and other “bad conductors of electricity” are good examples of insulators.

– Conductors: In these materials, the electrons ARE FREE TO MOVE. Metals are good examples of conductors.

– Semi-conductors: the charges are not free to move normally but small changes can free them and turn them into conductors. Silicon is a semi-conductor and is the basis of all modern electronics

Qualitative: Electroscope• The Phenomena

• Charge electroscope with a PVC rod which has been rubbed with a cloth. Leaves separate.

» Bring acrylic rod (rubbed with celophane) close to top of electroscope. Observe leaves approach each other.

• Now repeat experiment, but charge with acrylic rod. Leaves still separate.

» Now PVC rod causes leaves to approach each other.

» Acrylic rod causes leaves to separate.

• Explanation?• There exist two kinds of charge

• PVC rod gains electrons from the cloth, negative charge

• Acrylic rod loses electrons to the cellophane, positive charge

• Unlike charges attract; like charges repel.

Charging electroscopes

• Inside a conductor charges (electrons) are free to move

• The electroscope is made

out of conductors– conducting main electrode

– 2 conducting gold leaves

• Add some negative charge (electrons)

– the leaves repel each other - -

More about conductors and charging electroscopes(continued)

• Add some positive charge to negatively charged leaves (subtract electrons)

– the leaves no longer repel

- -

• Add some more positive charge (subtract more electrons

– Leaves now positively charged, repel each other

+ +

Summary and Next Lecture

• Apart from gravity all the forces in the normal world are produced by electrical charges

• Charges come in two varieties– negative and positive (electrons and protons)

– negative charge on a normal body means extra electrons

– positive charge means a deficit of electrons

• Next Lecture - Forces between charges

• Read Fishbane Chapter 21

• Don’t forget you need a transmitter