Register to reply 
Embarrassing questionby Galileo
Tags: embarrassing 
Share this thread: 
#1
Apr1805, 05:03 PM

Sci Advisor
HW Helper
P: 2,002

The magnetic constant, or permeability of vacuum [itex]\mu_0[/itex] is defined to be [itex]4\pi \times 10^{7} N/A^2[/itex].
The first time this constant comes up is usually in the BiotSavart law, however it is not an empirical constant. Why? In what quantity is it incorporated? Is it used to define the ampere (which defines the Coulomb)? Then, which came first? The Coulomb, [itex]\epsilon_0[/itex] or [itex]\mu_0[/itex]? I couldn't find anything about this... 


#2
Apr1805, 05:30 PM

Emeritus
Sci Advisor
PF Gold
P: 5,533

It is incorporated into the speed of light.
[tex] c=\frac{1}{\sqrt{\epsilon_0\mu_0}} [/tex] 


#3
Apr1805, 05:36 PM

Sci Advisor
HW Helper
P: 11,895

Incidentally,in SI,the Coulomb's law contains [tex] \frac{1}{4\pi\epsilon_{0}} [/tex] and the values are meant to yield EXACTLY [itex]c=3\cdot 10^{8} \mbox{m/s} [/itex],however we know that
EXACTLY [tex] c=299,792,458 \mbox{m/s} [/tex] Isn't this weird? Daniel. 


#4
Apr1805, 06:28 PM

Sci Advisor
HW Helper
PF Gold
P: 4,772

Embarrassing question



#5
Apr1805, 06:50 PM

Sci Advisor
HW Helper
P: 11,895

Yes,the Ampère is defined using the attraction/repulsion force between 2 infinite long parallel conductors situated in vacuum and,obviously,through which a constant current of 1 A flows...And it that force comes up this #.
Daniel. 


#6
Apr1805, 07:49 PM

P: 2,251

first (after the meter, kg, second) came the Ampere. it was defined to be such a current that when passed in two infinitely long and very thin parallel conductors in vacuum spaced apart by exactly 1 meter, induced a magnetic force on those conductors of exactly [itex] 2 \times 10^{7} [/itex] Newtons per meter. that is what set [itex] \mu_0 = 4 \pi \times 10^{7} N/A^2 [/itex] if [itex] \mu_0 [/itex] was anything different, that force per unit length would come out different than the defined value. then, of course, the Coulomb comes out to be an Amperesecond. there is nothing magical about these choices of units, they're quite anthropocentric and might not be used in 200 years. until 1983, the meter was defined to be the distance between the centers of two little scratch marks on a plantinumiridium bar in Paris (and got its original definition as 10,000,000 meters from the North pole to the equator) and the speed of light was measured to be 299792548 meters/second with some experimental error. at that time, then [tex] \epsilon_0 = \frac{1}{c^2 \mu_0} [/tex] also had experimental error. but in 1983 they changed the definition of the meter to be the distance that light in a vacuum travels in 1/299792548 seconds. that, plus the fact that [itex] \mu_0 [/itex] was defined, had the effect of defining [itex] \epsilon_0 [/itex]. someday reasonably soon, they may redefine the kilogram to effectively give Planck's constant [itex] \hbar [/itex] a defined value. r bj 


#7
Apr1805, 07:56 PM

Sci Advisor
HW Helper
P: 11,895

The meter was defined as the distance on a PtIr bar till 1961.
Daniel. 


#8
Apr1805, 08:07 PM

P: 2,251




#9
Apr1805, 09:00 PM

Sci Advisor
HW Helper
PF Gold
P: 4,772

[tex]\vec{B} = kI\int\frac{d\vec{l}\times \hat{r}}{r^2}[/tex] , we find the magnetic field produced at a distance of 1m from an infinitely long wire carrying a unit curent to be [tex]\vec{B} = 2k \hat{\phi}[/tex] But the force part is nebulous, for the TOTAL force is infinite (we have to integrate a constant from infty to +infty). It would work if it were the total force on 1m of wire: [tex]\vec{F} = I\int d\vec{l} \times \vec{B} = \int_0^1 2kdl = 2k[/tex] So [tex]k = 10^{7}[N/A^2] = \frac{4\pi \cdot 10^7}{4\pi}[N/A^2][/tex] Now define [itex]\mu_0 = 4\pi \times 10^7[/itex] and we get BiotSavart law as we know it. 


#11
Apr1905, 03:42 PM

P: 2,251

r bj 


Register to reply 
Related Discussions  
Most embarrassing moment in your life!  General Discussion  19  
What was your most embarrassing moment?  General Discussion  82  
Most embarrassing city name  General Discussion  27 