Monday, August 15, 2011

Einstein notes

Worth a read (from an Einstein scholar and former prof of mine):


Notes from class:

And then there was Einstein…

Albert Einstein 1879-1955
http://www.aip.org/history/einstein/index.html

What’s happening around the turn of the 20th century? Physics was set to explode with 30 brilliant years of excitement and unprecedented activity

X-rays – Roentgen
Radioactivity – Becquerel, Marie & Pierre Curie
Blackbody radiation (and the quantum discontinuity) – Planck

1905/6 – Einstein publishes 6 major papers:

a) “On the electrodynamics of moving bodies”
b) “Does the inertia of a body depend upon its energy content?”
c) “On a heuristic point of view about the creation and conversion of light”
d) “On the theory of the Brownian movement”
e) “On the movement of small particles suspended in stationary liquid demanded by the molecular-kinetic theory of heat”
f) “A new determination of molecular dimensions”

What are these about anyway?

a. Special relativity (SR)
b. E = m c2 (actually, L = m c2)
c. Photoelectric effect, light quanta, fluorescence
d. Same as title
e. Brownian motion agan
f. Avagadro’s number, etc.

Now, these are interesting (and very different fields of study), but is this why we revere Uncle Al? Not necessarily. Others (Poincare, Lorentz) were working on what would become SR. Planck had introduced the quantum discontinuity (E = h f) and quantum mechanics (QM) would have many contributors. The photoelectric effect had also several investigators (Lenard, et.al.).

Mostly, Einstein’s legend grows because of General Relativity (GR), which appears 1912-1915 and later and on which he worked largely alone with pad and pen. He forced us to re-examine how we see ourselves in the universe; indeed, how we think of gravitation. All of this around the time his marriage was falling apart (he married young after fathering 1 illegitimate child) and he began an affair with his cousin (whom he would later marry). Also, between 1902 and 1909, Einstein held a modest post in Bern, Switzerland as a Patent Clerk. By 1914, he would be director of the Kaiser Wilhelm Institute (later Max Planck Institute).

Special Relativity

Spaceship – Inside an inertial reference frame (constant velocity), you can’t tell whether or not you’re moving (“Principle of Relativity”)


Biographical notes

1879 – born in Ulm, Germany
1884 – receives first compass
1895 – attempts to gain entrance to Swiss Polytechnic (and finish high school early), but is rejected
1896 – begins Federal Polytechnic (ETH) in Zurich, Switzerland
1898 – meets Mileva Maric
1900 – graduates from ETH

1901 – Einstein becomes Swiss citizen and moves to Bern; Mileva becomes pregnant
1902 – Lieserl born (put up for adoption); Hermann dies
1903 – Albert and Mileva marry
1904 – Hans Albert born
1905 – Einstein’s “Annus Mirabilus”, his miracle year; Ph.D. (Zurich)

1919 – divorces Mileva (having lived apart for 5 years); marries Elsa; GR verified
1921 – awarded the Nobel Prize in Physics "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect".
1933 – settles in Princeton, NJ
1936 – Elsa dies
1939 – E. writes FDR

1940 – E. becomes American citizen
1949 – Mileva dies
1955 – E. dies

http://www.aip.org/history/einstein/index.html
http://www.albert-einstein.org/
http://einstein.stanford.edu/
http://en.wikipedia.org/wiki/Einstein

General Relativity

By 1907, E. wanted to advance the SR theory to include non-inertial (accelerated) frames of reference. Around this time, E. has the “happiest thought of my life”. In a uniformly accelerated spaceship, a stationary thing (ball, etc.) would appear to be falling (accelerating) down – it would be indistinguishable from normally accelerated motion. Light, too, follows this idea. Two clocks at different ends of an accelerated spaceship would be out of sync. Gravity is the result of the curvature of space and time?

Why does the Earth follow the Sun? Gravity – the very presence of the Sun causes Earth to veer from its otherwise straight (Newtonian inertial) path. With the Sun, it takes an elliptical path as its natural motion. Getting to this point, and showing that mass alters space and time is the real genius.

There is a breakdown of the observed geometry (Euclidean). E. must use non-Euclidean geometry (Gauss, surfaces, infinitesimal geometry) to consider the behavior of things (rods, etc.) on surfaces. He also considers the shortest distance between 2 points on a sphere (geodesic, great circle). He obtains the mathematical advice of his friend Marcel Grossman and studies (at great length) tensor calculus, differential geometry, Riemann and Minkowski math … it’s all puzzle solving. Soon, the principle of equivalence emerges. Eventually, the gravitational field equations appear (to show how matter “produces” gravity. GR was mostly worked out by 1913

Wednesday, August 10, 2011

Skepticism 101 FYI

Related to our brief foray yesterday into all things skeptical.

Good books, sites, etc.

by Michael Shermer:

Why people believe weird things
The believing brain
The science of good and evil
Science friction
Why Darwin matters

Skeptic Magazine



James "The Amazing" Randi

Flim Flam
Conjuring
The Faith Healers
An encyclopedia of claims, frauds and hoaxes of the occult and supernatural


Skeptical Inquirer


Skeptic's Dictionary



Richard Feynman - "Cargo Cult Science" essay


Martin Gardner

Fads and fallacies in the name of science

Carl Sagan

The demon-haunted world

Richard Dawkins

Climbing mount improbable

Schick/Vaughn

How to think about weird things

Other good essays and sites:



Tuesday, August 9, 2011

More problems, in prep for final.

Final topics:

circuit basics (series, parallel, power, current, voltage, resistance)

combination circuits

magnetism, electromagnetism, electromagnetic induction

special theory of relativity

>

1. Find the charge that "flows" past a given point, if a 12-V battery is in series with a 3-ohm resistor.

2. Consider 4 identical resistors, connected to a power supply. Two are in series, and these are connected to two that are in parallel. If each resistor is a light bulb, which one(s) are brightest? (Hint: Consider the equation for power, particularly P = i^2 R)

3. In the above problem, if one of the series "bulbs" are removed, what happens to the brightness of all of them? If one of the parallel bulbs are removed, what happens to the brightness of all of them?

4. Review the meaning and subtleties of magnetic fields, especially what compasses do in a magnetic field.

5. Consider a coil (100 turns) of wire, with a radius of 0.1-m. A magnetic field increases from 0-20 T in 5 seconds, directly onto the coil. What voltage is induced?

6. Distinguish between motors and generators.

7. Distinguish between speakers and microphones.

8. Imagine two twins: Earthy and Spacey. Earthy stays home, while Spacey travels at 0.5c to a star system 5 light-years away. How long will this trip last according to each twin? There are two answers for this question. Also, leave the distance (5 light-years) in light-years (or c-years). This makes the time work out to years, and in general, makes the math much easier. No conversions to do.

Monday, August 8, 2011

Problems a-plenty

Consider 4 resistors, each 10 ohms. Two are in series - these are followed by the other two resistors which are in parallel. They are connected to a 20 volt battery. Find all currents and voltages.

Draw the magnetic field around: a bar magnet, a current carrying wire, and a coil with current.

A magnetic field is directed into the page. A 0.3 meter long wire carries 2 amperes of current to the left. How strong is the magnetic field if the force on the wire is 5 newtons? Also, which way is the force?

Consider the previous problem. If a proton had been shot to the right in this field, what would the path look like? Draw.

Define and give units for all recently defined quantities.

Tuesday, August 2, 2011

Circuit info

Some information about circuits, in general.

voltage (V) = energy(work)/charge

V = W/q

The unit is the volt (joule/coulomb).


current (I) = charge/time

I = q/t

The unit is the ampere (coulomb/second).


Resistance = voltage/current

R = V/I

The unit is the ohm (volt/ampere).


The last relationship is often referred to as Ohm's Law, typically written as:

V = I R

Furthermore, power (used or radiated) in a circuit can be expressed by:

P = I V = I^2 R = V^2 / R

The unit is the joule/second, also called a watt (W).



Series circuit reminders

In a simple series circuit:

The current is the same in each resistor
The voltages ("over" each resistor) add to the total voltage (battery) available
The total resistance of the circuit is equal to the sum of the individual resistances -
Rs = R1 + R2 + R3 + .....


Parallel circuit reminders

In a simple parallel circuit:

The voltage is the same over each resistor
The currents ("through" each resistor) add to the total current (battery) available
The total resistance of the circuit is equal to the inverse of the sum of the inverted individual resistances. That is -

1/Rp = 1/R1 + 1/R2 + 1/R3 + ......


Combination circuits

Solving combination circuits (series and parallel together) is not too difficult, IF you break the circuit down to a simpler one first.

Determine what resistors are in series (and add them appropriately) and what resistors are in parallel (and add them appropriately). You'll have a simpler circuit that should be able to be simplified even further. We will examine this in class with several examples. Here is one to consider:

Imagine having 2 resistors (10 ohms and 20 ohms) in series with each other. These two are in series with a pair of resistor (3 ohms and 6 ohms) in parallel with each other. The combinaton is powered by a 12-V battery. What is the total resistance of this circuit?

The 2 series resistors make 30 ohms. The two parallel resistors make 2 ohms (do the math). The total combination makes 32 ohms.

The next step would be to find the total current. Take the total voltage (12-V) and divide it by the total resistance (32 ohms). This will give you the battery current. And since the battery is directly in series with the 10 and 20 ohm resistors, THEY TOO have that same current.

More to see in class this evening.

Exam 2 Practice

Test 2 is tomorrow (Wednesday, 8/3). Play with these problems for some practice:

1. Determine the charge due to a cluster of one million electrons.

2. If the one million electron cluster (from above) were 0.5-m from a half-million cluster of protons, what force would exist between them? (Calculate this.)

3. Consider a convex mirror (f = -0.25 m). A 10-cm tall object is 0.5-m in front of it. Find:
a. the location, type (R or V), magnification, and orientation (up or down) of the image

4. Consider a red laser (632nm) hitting a diffraction grating (1000 lines/mm). Find the separation (on a wall, 1-m away) between the central image and the n=1 image.