diff --git a/_psets/11.md b/_psets/11.md index 53432acac255d3d98ebadf91d37aa91a4dc8fbe8..7803f3851263b9f29bdfd05580b0e9b15c057492 100644 --- a/_psets/11.md +++ b/_psets/11.md @@ -47,6 +47,77 @@ Typical parameters for a quartz resonator are $$C_e = 5 \si{pF}$$, $$C_m = 20 \s \si{mH}$$, $$R_m = 6 \si{\ohm}$$. Plot, and explain, the dependence of the reactance (imaginary part of the impedance), resistance (real part), and the phase angle of the impedance on the frequency. +The circuit in question is depicted in figure 14.4. There's a capacitance $$C_e$$ in parallel with +a series RLC circuit ($$R_m$$, $$L_m$$, and $$C_m$$). To solve this it's helpful to know the complex +impedances of the basic electrical components: + +$$ +\begin{align*} +Z_C &= \frac{1}{i \omega C} \\ +Z_L &= i \omega L \\ +Z_R &= R +\end{align*} +$$ + +Then we just combine these, using the sum for series connections, and the inverse of the sum of +inverses for parallel connections. So the total impedance $$Z$$ is + +$$ +\begin{align*} +\frac{1}{Z} &= \frac{1}{Z_{C_e}} + \frac{1}{Z_{C_m} + Z_{L_m} + Z_{R_m}} \\ +&= i \omega C_e + \frac{1}{1/(i \omega C_m) + i \omega L_m + R_m} \\ +&= i \omega C_e + \frac{\omega C_m}{-i + \omega C_m (R_m + i \omega L_m)} \\ +Z &= \frac{1}{i \omega C_e + \frac{\omega C_m}{\omega C_m (R_m + i \omega L_m) - i}} +\end{align*} +$$ + +Let's see what this looks like with the given values plugged in. All the x axes are in radians per +second. + +The resistance (real part) is very small except for a spike near $$\num{4e6} \si{rad/s}$$. + + + +Mathematica tells me the max occurs at + +$$ +\begin{align*} +\omega_\text{max} &= \frac{1}{L_m} \sqrt{\frac{2 C_e L_m + 2 C_m L_m -C_e C_m R_m^2}{2 C_e C_m}} \\ +&\approx \num{4.09e6} \si{rad/s} +\end{align*} +$$ + +But the first term in the numerator is a couple orders of magnitude larger than the others, so we +can approximate $$\omega_\text{max}$$ by dropping them. + +$$ +\begin{align*} +\omega_\text{max} &\approx \frac{1}{L_m} \sqrt{\frac{2 C_e L_m}{2 C_e C_m}} \\ +&= \frac{1}{\sqrt{C_m L_m}} \\ +&\approx \num{4.08e6} \si{rad/s} +\end{align*} +$$ + +The reactance decreases without bound as $$\omega$$ approaches zero, and tends toward 0 as +$$\omega$$ increases. It has a kink in about the same place the resistance has a spike. + + + +Finally the phase is almost always $$-\pi$$, except where it shoots just above one (radian), again +near $$\omega = 1/\sqrt{C_m L_m}$$. + + + +Let's zoom in on those spikes. + + + + + +$$ +\begin{align*} +\end{align*} +$$ ## (14.5) diff --git a/_sass/main.scss b/_sass/main.scss index 504e66300de0fda0793e34104310bce0dc6dc471..0dc99a8e1f09154553fa02853567d020a832cb2d 100644 --- a/_sass/main.scss +++ b/_sass/main.scss @@ -46,3 +46,7 @@ table, th, td { th, td { padding: 5px 10px; } + +img { + width: 100%; +} diff --git a/assets/img/pset11_im_full.jpg b/assets/img/pset11_im_full.jpg new file mode 100644 index 0000000000000000000000000000000000000000..dbbb893241a92bee71ab0cedd9f8edfc850eb434 Binary files /dev/null and b/assets/img/pset11_im_full.jpg differ diff --git a/assets/img/pset11_im_zoom.jpg b/assets/img/pset11_im_zoom.jpg new file mode 100644 index 0000000000000000000000000000000000000000..8fb821716faa778ff9435230646647de006d6970 Binary files /dev/null and b/assets/img/pset11_im_zoom.jpg differ diff --git a/assets/img/pset11_phase_full.jpg b/assets/img/pset11_phase_full.jpg new file mode 100644 index 0000000000000000000000000000000000000000..74604b027c2e43bd53efa98927d2678826c8f17e Binary files /dev/null and b/assets/img/pset11_phase_full.jpg differ diff --git a/assets/img/pset11_phase_zoom.jpg b/assets/img/pset11_phase_zoom.jpg new file mode 100644 index 0000000000000000000000000000000000000000..72c975a5011a6c452c040f037b6fe93082ae95dc Binary files /dev/null and b/assets/img/pset11_phase_zoom.jpg differ diff --git a/assets/img/pset11_re_full.jpg b/assets/img/pset11_re_full.jpg new file mode 100644 index 0000000000000000000000000000000000000000..305d9874a8a3afce63f74e5d63d5ca094d22e6cf Binary files /dev/null and b/assets/img/pset11_re_full.jpg differ diff --git a/assets/img/pset11_re_zoom.jpg b/assets/img/pset11_re_zoom.jpg new file mode 100644 index 0000000000000000000000000000000000000000..39eec70c0d5d3dcd256c2ee80a13b842d34c592e Binary files /dev/null and b/assets/img/pset11_re_zoom.jpg differ