# Questions about classsical physics units.

1. Jun 3, 2006

### scott1

I've been reading a book called Physics Demystified. I'am currently on part 1 Classical physics, chapter 6 units and constants. I have some questions about some defftions and units:

The meter
: It says that the meter is defined by the distance of light travels through a perfect vacum in 3.33564095 billonths of a second.
Why did they chose to base the meter off the amount of time light takes travel between a certin?
The candela: not too much of importent question but how exactally do you pronuce it?
The Radian: What exactally does plan angular measure mean? Can you please show mean an example like a picture or somthing?
The Steradian:What does soild angular measure mean
The Joule:I think I understand the joule well but it says. "If reduced to base units in SI, the joule can be expressed in terms of unit mass multiplied by unit distance squared per unit time sqaured:
$$1 J kg \cdot m^2/S^2$$
I don't think I understand that equation very well.
The Watt: this probally going to sound like a stupid question but I'am not sure if I ever properly understand what this means:power
The Coulamb: What exactally does electric charge quantity mean
The Ohm: I don't need to know what anything means about it but It says that the ohm is the standerd unit of electrical resientance. I thought the Ohm can be used on things other then electricity too?
The Farad: what exactally does capacitance mean?
The Henery: What exactally does inducatnce mean?
The Weber: What exactally does magnetic flux mean?
The Tesla:What exactaly does magnetic flux density mean
I probally will have more questions later. I do understand what they mean but I just want to make sure I understand what they mean properly.

Last edited: Jun 3, 2006
2. Jun 3, 2006

### dav2008

Shouldnt the book define all of those terms?

You can look all of them up on www.wikipedia.org or http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html I think it would be easier that way.

As far as the ohm, I don't know why they say it's a unit of electrical potential. The ohm is a unit of electrical resistance or impedance. The volt is a unit of electrical potential.

As far as the Joule, those are the units that are equivalent to a Joule. 1 Joule = $$1 \frac{kg\cdot m^2}{s^2}.$$ This can be easily verified by taking the fact that work(energy) is a force times a distance. The SI unit for force is a Newton which by F=ma you know is $$kg\cdot m/s^2$$. It follows that the SI units of the Joule are $$kg\cdot m^2 \cdot s^{-2}$$.

Note that in general, energy has dimensions of $$ML^2T^{-2}$$

Last edited: Jun 3, 2006
3. Jun 3, 2006

### LURCH

The meter was originally set to the length of a rod. All meter sticks were compared to this same rod to check them for accuracy. Obviously, this was a very innefficient system and needed revising.

After while, someone thought of finding a lightwave with a wavelength that would factor perfectly into the standard meter. I don't know what the exact color was, or how many multiples of this wave makes one meter, but this became the new standard, because the frequency could be reproduced anywhere, elliminating the need to send your meter sticks to Geenwich Vilage (or wherever the heck that stupid rod might be) to get it checked.

Sometime in the early Eighties, the meter was altered because the speed of light was 299,792,458.62??? meters per second (I'm don't remember what digits were in these decimal spaces, but it wasn't an integer). So, in the year I graduated highschool (1983), the length of a meter was changed ever so slightly so that lightspeed would be exactly 299,792,458 meters per second. This makes the definition of a meter "the distance light travels in 1/299,792,458 of a second".

Simple, no?!

4. Jun 3, 2006

### arildno

The Radian: What exactally does plan angular measure mean? Can you please show mean an example like a picture or somthing?
The radian is circle arc-length per unit radius for a particular sector of the disk.

The Steradian:What does soild angular measure mean
The steradian is the area of the sphere per unit squared radius for a particular rounded cone sector of the ball.

5. Jun 3, 2006

### scott1

Sorry I'll correct that I missed posted that.

6. Jun 3, 2006

### rbj

if you want to go to a very authoritative source on the history of SI units and why they were chosen the way they were, go to:

http://physics.nist.gov/cuu/Units/index.html

the original choice for the meter and second were simply historical accidents. there is nothing particularly special about their quantitative definition. the meter was such a length that 10 million of them reach from the North Pole to the equator along an arc of the earth's mean surface through Paris.

the second was 1/(24*60*60)th of the time of a mean solar day.

the gram was supposed to be the weight of 1 cm3 of pure H2O at maximum density, but they screwed that up a little since defining it. the kg is 1000 grams, of course, and has since become the unit weight for SI even if the gram was defined first.

the ampere is defined to be such a current that the permeability of free space comes out to be exactly $\mu_0 = 4 \pi \times 10^{-7}$ when measured in SI units.

probably Paris. that is where the present Kilogram prototype is kept. someday they will redefine the kg so that Planck's constant is set to an exact value. (or they might set the elementary charge to an exact value or, maybe, Advogadro's number to an exact value. they cannot do all 3 with a redefinition of one base unit like the kg, but they can do one and i think, when the smoke and dust clear, it will end up being Planck's constant that replaces the kg prototype in Paris just as c replaced the meter prototype.)

actually, when they changed the definition, there was really no instantaneous change in the actual known length of the meter (those last two digits ".62" are metrological crap - they could just as well be zero for as well as they could measure c in 1983) but it changed the evolution of the meter. perhaps now, with better instrumentation, the space between those two little scratch marks on the prototype meter (i'm sure it still exists in Paris somewhere) is measureably different than the current definition of the meter using c. but it wasn't when they made the switch in 1983.

same for the second in 1967. but now we know there is a difference, hence the need for "leap seconds" once in a while.

Last edited: Jun 3, 2006
7. Jun 5, 2006

### dephys

hello
this description in the school level books is highly ridiculous certainly to understand the working of the units u should know at least Bachelor level physics so dont be fumed if u cant understand
physics is always enjoying