GENERAL COMMUNICATIONS WITH STUDENTS
1/25/00:
1) A number of you still have difficulty with a distinction I emphasised at our first class meeting - the difference between observed phenomenon and explanation of that phenomenon!  Electrical, magnetic, and gravitational interactions between bodies are observed "facts": compasses point N (magnetism), bodies fall (gravity), combs run through hair attract bits of paper (electricity).  Explanations of the observed forces differ, e.g., "action-at a-distance" ( a "one-step" explanation) or "field" (a "two-step" explanation). Both of these are "physics explanation" -they involve the idea of force to "overcome" inertia. You could also have non-physics explanations, e.g.: Aristotle (things "tend" toward their "natural place in the universe - no concept of inertia or force) ; religeous ("angels" move them); etc.
2) Since lines of force tell you the unique direction that a force would act IF a body were present, they may not cross each other since thatwould mean two different directions at that point.  Electrical lines of force go from positive to negative or to infinity - they do NOT go from positive to positive or from nagative to negative.  Similarly fro magnetic lines of force. Since gravity is always attractive, gravitational lines of force would go from body to body.
3) "Action-at -a-distance" was too "spooky" for physicists who were trying to get away from angels and Aristotle. They thought of physical forces as pushs or pulls, between bodies touching each other ; they were always looking for a "mechanism" by which one body influenced another.  They knew that masses influence (attract) each other (gravity), that charged bodies influence (attract or repell) each other (electrical force), and that magnets influence (attract or repell) each other (magnetism), but what was the "mechanism - angels???? Newton had no answer (he ruled out angels!) so just kept quite - his mathematical inverse-square-law worked, even if he couldn't explain it. The field explanation of Faraday and Maxwell got rid of the spookiness by providing a "connection" between the two interacting bodies; even if you couldn't "see" the field, you could confirm its existence indirectly via the laws of conservation of momentum and conservation of energy.  Of course, if you didn't believe in these laws then you didn't "see" the field.
4) A number of you missed some of the similarities and differences between gravitational, electrical, and magnetic phenomena. All of them take place between separated bodies with no obvious physical contact between them.  All three effects fall off with the inverse-square of the distance between the bodies. Gravity is only attractive whereas the other two can be either attractive or repulsive.  The entities effected by gravity and electrical forces - masses and charge - can appear as isolated bodies whereas  magnetic poles always occur in pairs.  Electrical and magnetic effects seem to be intertwined, both independent (so far as we now know) from gravitational effects.  All three can be "explained" either as "action-at -a-distance" or as field phenomena, with separate gravitational, electic, and magnetic fields, though the latter two are not independent of each other.
5)You all confuse sound waves and electromagnetic wave! They are NOT the same!  Sound is a vibration in matter - moving molecules, whereas EM waves can, and do, occur in vacuum. The sound that you get from your radio or TV is not coming from the broadcast station - it is produced in the loudspeaker of your set, travels through the air (at about 700 miles per hour) to your ears.  The EM waves trave through space ( at 186,000 miles per second) from the radio or TV station to your radio.  Your radio, or TV, converts the information carried by the EM wave into the sound (and picture) produced in your home set.
6) It is true that , as the speeding police car gets closer to you, its siren sounds louder (the waves have a bigger amplitude). But the more interesting fact is that the pitch is higher - the frequency is increased due to the Doppler effect; if the car were traveling away from you, the pitch would be lower.  Effect has nothing to do with distance, everything to do with velocity, of sound source. Same phenomenon occurs for all waves, e.g., also for EM waves.

Comments on IQs for Wednesday, January 26:

1) (E,p263) In general, the sounds from the two speakers of a stereo don't have the same wavelength and are not "coherent" = coordinated = definite relations between their respective crests, etc.  So there is no interference in general.  If there were some instants when conditions for interference existed, and the leads were interchanged, constructive interference would become destructive and vuce-versa.
 (#6, p.263) Generally, time of travel is distance traveled divided by speed of travel Thus:
(a) Light-3km/300,000 km/s (3km/ 3*105 km/s) = 1*10-5 s = 0.00001 seconds, certainly not very noticeable.
(b) Sound-3km/0.340km/s = 3,000m/340 m/s =8.82 seconds, a very noticeable time.  Thus you see the flash long before you hear it.
 (#7, p.263)(a) The fundemental note on a string only has nodes at the ends of string, 1 meter apart in this case.  Since the wavelength is twice the distance between nodes, the wavelength is 2 meters.  (b) Since the wave velocity is the product of the frequency and the wavelength, the speed is 440Hz*2m = 880 m/s (since 1 Hz = 1 s-1).
2) If you will look at Fig 7-10, page 89 of the March text, you will see that the wave length of microwaves is greater than that of infra-red, the frequency of microwaves less than that of infr-red. Your stove gives off infra-red radiation, your microwave oven obviously uses microwaves.  The fact that the latter, in some cases, cooks faster than the former has nothing to do with which frequency is greater but with which frequency is more in resonance with the molecular vibration frequencies of the food being cooked.
 
 



Communication with Students
4 February, 2000


 










A number of you had difficulty with today’s IQ.  Here are some answers:
a)
Jane says Joe is 100miles*cosine(10o) = 98.4miles North, 100miles*sine(10o) = 17.4miles east of her.
Joe says Jane is 100 miles South, 0 miles east of him.
They both agree that they are a distance of 100 miles = Square root [(98.4)2 + (17.4)2] apart.
b) For this problem, gamma = 3.20.
Thus, while Jane thinks it takes 30 minutes for her breakfast, Joe thinks it takes 30*3.20 = 96.1 minutes.
While Jane thinks it takes 15 minutes to walk back, Joe thinks it takes 15*3.20 = 48 minutes.
While Jane thinks she has walked a distance of 0.5 km, Joe thinks she has walked 0.5/3.20= 0.16km.
They will agree on “proper times”(rest times) and “proper lengths” ( rest times).  For example, J ane thinks she is at rest while eating: delta x = 0, delta t = 30 minutes = 1800seconds.
For Joe, delta t’ = 96.1 minutes = 5.77*103 seconds and during this time she has moved (with her space ship) a distance of delta x’ = 0.95c*96.1 minutes = 1.64*1012 meters (distamce = speed *time) .  If you calculate Joe’s space -time interval, you get
(1.64*1012 meters)2  - (c*5.77*103 seconds)2 = - 3.0*1023 meters squared.
Jane’s space-time interval is just - (c*1800 seconds)2 = - 2.91023 which is the same number, given rounding errors.  Thus the two space-time intervals are both the same, time-like, both proper times.
Try doing the same kind of calculation for her walk - its somewhat more complicated, but again you get an agreed upon space-time interval, this time, space-like.

7 February 2000

BRINGING TOGETHER SEEMINGLY OPPOSING CONCEPTS
SOMETIMES LEADS TO INTERESTING NEW UNDERSTANDINGS
 
 

Concept         Very different concept       Matchmaker                New Understanding
_____________________________________________________________________
Terrestrial Physics  Celestial Physics                     Galileo                               “New” Physics

Falling Apple       Falling Moon                         Newton                                 Universal Gravitation

Inter-particle forces -Action-at-a-distance            Faraday                                Field lines

Electricity            Motion                                    Oersted                                 Magnetism

Electricity            Magnetism                              Maxwell                                Electromagnetic Fields

Galilean Relativity- Maxwell’s Equations           Einstein                                 Special Relativity

Space                 Time                                       Einstein                                  Space-Time

Lorentz Contraction -Time Dilation                  Einstein                                   Same Phenomenon-
                                                                                                                        Different Explanations

Mass                  Energy                                  Einstein                                   Creation of matter and    the universe

Cons. of Energy -Cons. of Momentum           Einstein                                   Cons.of 4-Momentum
                                                                                                                        Rest mass as invarient

Special Relativity -Gravitation                        Einstein                                     General Relativity

General Relativity- Hubble Expansion           Gamow et al                              “Big Bang”

Particles           Waves                                 Schrodinger,etc                           Quantum Mechanics

Quantum Mechanics -Special Relativity       Dirac, et al                                   Quantum Field Theory

Electromagnetism -Weak Interactions          Gell Man et al                               Electroweak Theory

Strong Interactions -Electroweak Theory      Gell Man et al                              “Standard Theory”

General Relativity-Quantum Mechanics       many                                           String Theory?