Classical Physics - Continued Electricity & Magnetism : Classical Physics - Continued Electricity & Magnetism Faraday Electric Fields Magnetic Fields Coulomb’s Law Franklin Ampere’s Law Two signs of
electric charge No
magnetic charge
Announcements : Announcements Today:
Go over exam 1
Start Electricity and Magnetism (Hobson Ch 8)
Wednesday, Oct. 6
HW 5 (web survey) due
Discuss survey results
Continue Electromagnetism and Waves
Reminder: Term paper is still in your future. You should start thinking about a topic.
Exam 1 : Exam 1 Scores
4 A
14 B
6 C
2 D
This exam is 15% of your final grade.
I strongly encourage people with scores below 60 to come and see me.
I can help. I know you can do better. Go over: 2,14,17,24,25,30,35
Timeline : Timeline See Timeline description of lives of various scientists on WWW pages. 0 1000 2000 -1000 Asia, Egypt
Mesopotamia Aristotle Euclid Galileo
Kepler Newton “Modern”
Physics Greece, Rome Middle Ages Ptolomy Copernicus Renaissance Al-Khawarizmi Fibanacci Plato Erastosthenes
Aristarchus 1900 1800 1700 1600 Faraday Maxwell Franklin Coulomb Volta Ampere
The state of science around 1700 : The state of science around 1700 The Newtonian Paradigm (or World View):
What is the world made of?
Particles - objects that have mass.
What Happens in the World?
Forces cause particles to move in accordance with Newton’s Laws of Motion.
Determinism
Everything is determined !
Universe acts like a giant machine, a giant clock with every part obeying precise laws
(The second law of thermodynamics was not yet formulated)
What forces do we know about? : What forces do we know about? Force of gravity:
Force = G m1 m2 / R2
Force on mass m near surface of earth F = G m mearth / Rearth2 = m g g = G mearth / Rearth2
Force of due to a spring:
Force F = - K x
Force of due to contact
What transmits the force? This was an open question in the 1700’s. What do you think?
Other? F x
What was not understood around 1700? : What was not understood around 1700? What is ordinary matter around us?
Made up of particles? Atoms?
What is light?
Newton viewed light as particles?
Huygens, others said light acts as waves.
What is the source of energy for the sun?
What is electricity, magnetism?
Other?
Metals and Insulators : Metals and Insulators Two types of materials
Metals conduct electricity
Electric charge moves through material
Examples: Copper, Gold
Insulators do not conduct electricity
Electric charges stay can be induced by contact friction,but do not move through material unless there is “breakdown” that leads to sparks and lightening.
Examples: Glass rod, Rubber, paper, air
Known long ago in Greece
Benjamin Franklin 1706 - 1790 : Benjamin Franklin 1706 - 1790 Regarded as the First Great American Scientist
10th of 17 children, Left school at age of 10 Learned Craft of Publishing as apprentice
Became Businessman, Author, Inventor, Scientist, Statesman
Discoveries on electricity were in advance of European Scientists.
Famous kite experiments established electrical nature of lightening
Invented “Lightning Rod” at time when Europe had medieval view of lightening - rang church bells to ward off lightening
Statesman representing the Colonies and the United States in Europe
Electrostatics : Electrostatics A kind of force.
It causes masses to accelerate (move)
Does it fit in the Newtonian picture?
Examples:
Experiments with insulating “Pith Balls”
Experiments using Van de Graaf generator.
Can you think of other examples?
Explanation?
There exist two kinds of charge. (Ben Franklin, 1751)
Unlike charges attract Like charges repel
Forces between Charges - Demo : Forces between Charges - Demo Charge can be transferred between insulators and then stays in place. Causes electrical forces.
Forces between Charges : Forces between Charges How do we describe these forces within Newtonian system?
Need expression for forces in terms of the positions of charges
Forces Law Coulomb (1785) Inverse square law: F = K q1q2 / R2 , q1 , q2 = charge (plus or minus) Like gravity, except electric force can be attractive or repulsive
Forces between Charges : Forces between Charges Electrostatic forces MUCH stronger than gravity:
Electrostatic: FE = K q1q2 / R2
Gravity: FG = G m1m2 / R2
In meter- Kg - second system: K = 9.0 x 109 G = 6.67 x 10-11
Force between two protons at distance of 1 m: Charge = 1.6 x 10-19 Coulomb Mass = 1.6 x 10-27 Kg FE = 2.3 x 10-28 Newtons; FG = 1.8 x 10-64 Newtons Factor of 1036
How can Gravitational Forces ever be important? : How can Gravitational Forces ever be important? Electrostatic forces are zero between two neutral objects (equal amounts of positive and negative)
Gravitational forces always have the same sign (attractive) and never cancel out
Force between sun and earth (both nearly neutral) is mainly gravitational
Magnetic Forces : Magnetic Forces Yet another kind of force
Known since prehistory – lodestone compasses used in China thousands of years ago
Bar Magnets: Two poles (North & South)
Force description: Like poles repel; Unlike poles attract.
Explanation?
Is there a magnetic charge (analogous to electric charge) ?
Try to isolate the charges: cut the magnet in half:
In fact, no experiment to date (and there have been many attempts) has shown evidence for the existence of magnetic charge. We believe the source of the magnetic force is not a new kind of charge, but is due to motion of electric charge.
Magnetic Forces due to Electric Current : Magnetic Forces due to Electric Current Current is charges in motion
Causes force on magnet
Example: Compass near wire with current wire current Side View Top View
Magnetic Forces due to Electric Current Loop : Magnetic Forces due to Electric Current Loop Current in a loop causes magnetic forces just like a magnet
The Field Concept : The Field Concept Michael Faraday (1791 - 1867) had the idea that forces between bodies were cause by
Fields that fill all space and act on the bodies
Electric Field E due to positive charge
Faraday (building upon Ampere’s work) discovered the interdependence of Electric & Magnetic Fields:
A moving or changing electric field generates a magnetic field and a moving or changing magnetic field generates an electric field. +
The Field Concept (continued) : The Field Concept (continued) Electric and Magnetic Fields at a given point in space determine the force on a “test” electric charge if it were placed at that point in space.
Electric field E causes force F = qE (q = charge)
Magnetic field B causes force F = qvB on charge moving with speed v perpendicular to B
This “harmless” equation has the seeds of trouble for the Newtonian picture!
Magnetic force depends on the speed of the particle.
Force is meant to be an absolute quantity. The laws of physics are supposed to be the same for two different observers (people or instruments) even if they are moving at constant v with respect to one another --- Remember Galileo, Newton !
But speed is not the same to the different observers! What is going on? Will this lead to a breakdown of the ideas of Galileo and Newton?
Demo E-M Cannon : Demo E-M Cannon ~ v Connect solenoid to a source of alternating voltage. The flux through the area to axis of solenoid therefore changes in time. A conducting ring placed on top of the solenoid will have a current induced in it opposing this change. There will then be a force on the ring since it contains a current which is circulating in the presence of a magnetic field.
Fields are real! side view B B B F F top view
Summary : Summary New Forces and ideas in the Newtonian World!
Electric Charge:
Property of particles... Determines new Coulomb Force: F = K q1q2 / R2
Fields – new idea in Newtonian physics:
Extend through space
Electric Fields: created by electric charges
Magnetic Fields: created by electric charges in motion
Principles used in electric generators, ….
Electric Fields and Magnetic Fields are not independent of each other. A changing magnetic field generates an electric field and a changing electric field generates a magnetic field. Maxwell realized the full significance of this interrelationship --- next time.