SI Units - notes

Some comments on standards. We generally use SI units in physics. To inform you:

Mass is measured based on a kilogram (kg) standard.
Length (or displacement or position) is based on a meter (m) standard.
Time is based on a second (s) standard.

How do we get these standards?

Length - meter (m)

- originally 1 ten-millionth the distance from north pole (of Earth) to equator
- then a distance between two fine lines engraved on a platinum-iridium bar
- (1960): 1,650,763.73 wavelengths of a particular orange-red light emitted by atoms of Kr-86 in a gas discharge tube
- (1983, current standard): the length of path traveled by light during a time interval of 1/299,792,458 seconds

That is, the speed of light is 299,792,458 m/s. This is the fastest speed that exists. Why this is is quite a subtle thing. Short answer: the only things that can travel that fast aren't "things" at all, but rather massless electromagnetic radiation. Low-mass things (particles) can travel in excess of 99% the speed of light.

Long answer: See relativity.

Time - second (s)

- Originally, the time for a pendulum (1-m long) to swing from one side of path to other
- Later, a fraction of mean solar day
- (1967): the time taken by 9,192,631,770 vibrations of a specific wavelength of light emitted by a cesium-133 atom

Mass - kilogram (kg)

- originally based on the mass of a cubic decimeter of water
- standard of mass is now the platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris
- secondary standards are based on this
- 1 u (atomic mass unit, or AMU) = 1.6605402 x 10^-27 kg
- so, the Carbon-12 atom is 12 u in mass

Volume - liter (l)

- volume occupied by a mass of 1 kg of pure water at certain conditions
- 1.000028 decimeters cubed
- ml is approximately 1 cc

Temperature - kelvin (K)

- 1/273.16 of the thermodynamic temperature of the triple point of water (1 K = 1 degree C)
- degrees C + 273.15
- 0 K = absolute zero

For further reading:

http://en.wikipedia.org/wiki/SI_units

http://en.wikipedia.org/wiki/Metric_system#History

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In addition, we spoke about the spherocity of the Earth and how we know its size. I've written about this previously. Please see the blog entries below:

http://howdoweknowthat.blogspot.com/2009/07/how-do-we-know-that-earth-is-spherical.html

http://howdoweknowthat.blogspot.com/2009/07/so-how-big-is-earth.html

Homework related to the mini-projects

Try to find an explanation for how your device works (or is supposed to work).  This means, you should be looking up what pertains to your device:

- how do speakers work
- how do microphones work
- how do motors work
- how do telegraphs work

Or whatever makes sense.  Bring some kind of explanation to class next time and we can talk it through so it makes sense to you.  A picture might be really helpful, too!

Keep in mind that you'll be teaching the class a little about your device, so you should understand as much of the basic operation as possible.  You'll have time to compare notes with others who built the same type of device.

Thanks!

Project ideas

Projects?

Speaker

Microphone

Motor

(Generator)

Guitar pickup

Telegraph

Circuit?  Amplifier circuit?  LED blinking circuit?

Arduino?

And in honor of Clara Rockmore's birthday:

https://www.youtube.com/watch?v=pSzTPGlNa5U

(Also see today's Google doodle...)

If you missed class today (Monday)...

Brainstorm a list of things about magnetism:

- what you know (or think you know)

- what you don't know and/or might want to know

Also, look into "magnetite" - what is it, etc.?

Write down your list and bring it to Wednesday's class.  Thanks!