<span>While
modern humans first evolved approximately 200,000 years ago,
the age of
the Earth, the Sun, and the rest of the solar system is believed
to be approximately 4.6 billion years.
That makes the Earth, the Sun, and the rest of the solar system
something like 23 thousand times as old as the human species !
</span>
The answer is going to be c
Answer:
Frictional Force: Frictional force is the force caused by the relative motion of two surfaces that come into contact with each other.
Answer:
![d=1.84\ mm](https://tex.z-dn.net/?f=d%3D1.84%5C%20mm)
Explanation:
<u>Capacitance</u>
A two parallel-plate capacitor has a capacitance of
![\displaystyle C=\frac{\epsilon_o A}{d}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20C%3D%5Cfrac%7B%5Cepsilon_o%20A%7D%7Bd%7D)
where
![\epsilon_o=8.85\cdot 10^{-12}\ F/m](https://tex.z-dn.net/?f=%5Cepsilon_o%3D8.85%5Ccdot%2010%5E%7B-12%7D%5C%20F%2Fm)
A = area of the plates = ![\pi r^2](https://tex.z-dn.net/?f=%5Cpi%20r%5E2)
d = separation of the plates
![\displaystyle d=\frac{\epsilon_o A}{C}=\frac{\epsilon_o \pi r^2}{C}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20d%3D%5Cfrac%7B%5Cepsilon_o%20A%7D%7BC%7D%3D%5Cfrac%7B%5Cepsilon_o%20%5Cpi%20r%5E2%7D%7BC%7D)
We need to compute C. We'll use the circuit parameters for that. The reactance of a capacitor is given by
![\displaystyle X_c=\frac{1}{wC}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20X_c%3D%5Cfrac%7B1%7D%7BwC%7D)
where w is the angular frequency
![w=2\pi f=2\pi \cdot 36000=226194.67\ rad/s](https://tex.z-dn.net/?f=w%3D2%5Cpi%20f%3D2%5Cpi%20%5Ccdot%2036000%3D226194.67%5C%20rad%2Fs)
Solving for C
![\displaystyle C=\frac{1}{wX_c}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20C%3D%5Cfrac%7B1%7D%7BwX_c%7D)
The reactance can be found knowing the total impedance of the circuit:
![Z^2=R^2+X_c^2](https://tex.z-dn.net/?f=Z%5E2%3DR%5E2%2BX_c%5E2)
Where R is the resistance,
. Solving for Xc
![X_c^2=Z^2-R^2](https://tex.z-dn.net/?f=X_c%5E2%3DZ%5E2-R%5E2)
The magnitude of the impedance is computed as the ratio of the rms voltage and rms current
![\displaystyle Z=\frac{V}{I}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20Z%3D%5Cfrac%7BV%7D%7BI%7D)
The rms current is the peak current Ip divided by
, thus
![\displaystyle Z=\frac{\sqrt{2}V}{I_p}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20Z%3D%5Cfrac%7B%5Csqrt%7B2%7DV%7D%7BI_p%7D)
![I_p=0.65\ mA/1000=0.00065\ A](https://tex.z-dn.net/?f=I_p%3D0.65%5C%20mA%2F1000%3D0.00065%5C%20A)
Now collect formulas
![\displaystyle X_c^2=Z^2-R^2=\left(\frac{\sqrt{2}V}{I_p}\right)^2-R^2](https://tex.z-dn.net/?f=%5Cdisplaystyle%20X_c%5E2%3DZ%5E2-R%5E2%3D%5Cleft%28%5Cfrac%7B%5Csqrt%7B2%7DV%7D%7BI_p%7D%5Cright%29%5E2-R%5E2)
Or, equivalently
![\displaystyle X_c=\sqrt{\frac{2V^2}{I_p^2}-R^2}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20X_c%3D%5Csqrt%7B%5Cfrac%7B2V%5E2%7D%7BI_p%5E2%7D-R%5E2%7D)
![\displaystyle X_c=\sqrt{\frac{2\cdot 18^2}{0.00065^2}-15000^2}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20X_c%3D%5Csqrt%7B%5Cfrac%7B2%5Ccdot%2018%5E2%7D%7B0.00065%5E2%7D-15000%5E2%7D)
![X_c=36176.34\ \Omega](https://tex.z-dn.net/?f=X_c%3D36176.34%5C%20%5COmega)
The capacitance is now
![\displaystyle C=\frac{1}{226194.67\cdot 36176.34}=1.22\cdot 10^{-10}\ F](https://tex.z-dn.net/?f=%5Cdisplaystyle%20C%3D%5Cfrac%7B1%7D%7B226194.67%5Ccdot%2036176.34%7D%3D1.22%5Ccdot%2010%5E%7B-10%7D%5C%20F)
The radius of the plates is
![r=18\ cm/2=9 \ cm = 0.09 \ m](https://tex.z-dn.net/?f=r%3D18%5C%20cm%2F2%3D9%20%5C%20cm%20%3D%200.09%20%5C%20m)
The separation between the plates is
![\displaystyle d=\frac{8.85\cdot 10^{-12} \cdot \pi\cdot 0.09^2}{1.22\cdot 10^{-10}}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20d%3D%5Cfrac%7B8.85%5Ccdot%2010%5E%7B-12%7D%20%5Ccdot%20%5Cpi%5Ccdot%200.09%5E2%7D%7B1.22%5Ccdot%2010%5E%7B-10%7D%7D)
![d=0.00184\ m](https://tex.z-dn.net/?f=d%3D0.00184%5C%20m)
![\boxed{d=1.84\ mm}](https://tex.z-dn.net/?f=%5Cboxed%7Bd%3D1.84%5C%20mm%7D)