physics / midyear.texon commit add midyear cheatsheet, add ch8-9 notes for waves (e57f9b6)
   1\documentclass[a4paper]{article}
   2\usepackage{multicol}
   3\usepackage[cm]{fullpage}
   4\usepackage{amsmath}
   5\usepackage{amssymb}
   6\setlength{\parindent}{0cm}
   7\usepackage[nodisplayskipstretch]{setspace}
   8\setstretch{1.3}
   9\usepackage{graphicx}
  10\usepackage{wrapfig}
  11\usepackage{enumitem}
  12\setitemize{noitemsep,topsep=0pt,parsep=0pt,partopsep=0pt,leftmargin=5pt}
  13
  14
  15\begin{document}
  16
  17\pagenumbering{gobble}
  18\begin{multicols}{3}
  19{\huge Physics}\hfill Andrew Lorimer\hspace{2em}
  20
  21\section{Motion}
  22  \subsection*{Unit conversion}
  23  $\operatorname{m/s} \times 3.6 = \operatorname{km/h}$
  24
  25  \subsection*{Inclined planes}
  26  $F = m g \sin\theta - F_{frict} = m a$
  27
  28  \subsection*{Banked tracks}
  29  \includegraphics[height=4cm]{/mnt/andrew/graphics/banked-track.png}
  30  $\theta = \tan^{-1} {{v^2} \over rg}$ (also for objects on string)
  31
  32  $\Sigma F$ always acts towards centre, but not necessarily horizontally
  33
  34  $\Sigma F = {{mv^2} \over r} = mg \tan \theta$
  35
  36  Design speed $v = \sqrt{gr\tan\theta}$
  37
  38  \subsection*{Work and energy}
  39  $W=Fx=\Delta \Sigma E$ (work)
  40
  41  $E_K = {1 \over 2}mv^2$ (kinetic)
  42
  43  $E_G = mgh$ (potential)
  44
  45  $\Sigma E = {1 \over 2} mv^2 + mgh$ (energy transfer)
  46
  47  \subsection*{Horizontal motion}
  48
  49  $v = {{2 \pi r} \over T}$
  50
  51  $f = {1 \over T}, \quad T = {1 \over f}$
  52
  53  $a_{centrip} = {v^2 \over r} = {{4 \pi^2 r} \over T^2}$
  54
  55  $\Sigma F$ towards centre, $v$ tangential
  56
  57  $F_{centrip} = {{mv^2} \over r} = {{4 \pi^2 rm} \over T^2}$
  58
  59  \includegraphics[height=4cm]{/mnt/andrew/graphics/circ-forces.png}
  60
  61  \subsection*{Vertical circular motion}
  62  $T =$ tension, e.g. circular pendulum
  63
  64  $T+mg = {{mv^2}\over r}$ at highest point
  65  $T-mg = {{mv^2} \over r}$ at lowest point
  66
  67  \subsection*{Projectile motion}
  68  \begin{itemize}
  69  \item{horizontal component of velocity is constant if no air resistance}
  70
  71  \item{vertical component affected by gravity: $a_y = -g$}
  72\end{itemize}
  73
  74$v=\sqrt{v^2_x + v^2_y}$ (vector addition)
  75
  76$h={{u^2\sin \theta ^2}\over 2g}$ (max height)
  77
  78$y=ut \sin \theta-{1 \over 2}gt^2$ (time of flight)
  79
  80$d={v^2 \over g}sin \theta$ (horizontal range)
  81  \includegraphics[height=3.2cm]{/mnt/andrew/graphics/projectile-motion.png}
  82
  83  \subsection*{Pulley-mass system}
  84
  85  $a = {{m_2g} \over {m_1 + m_2}}$ where $m_2$ is suspended
  86
  87  \subsection*{Graphs}
  88  \begin{itemize}
  89    \item{Force-time: $A=\Delta \rho$}
  90    \item{Force-disp: $A=W$}
  91    \item{Force-ext: $m=k,\quad A=E_{spr}$}
  92    \item{Force-dist: $A=\Delta \operatorname{gpe}$}
  93    \item{Field-dist: $A=\Delta \operatorname{gpe} / \operatorname{kg}$}
  94  \end{itemize}
  95
  96  \subsection*{Hooke's law}
  97
  98  $F=-kx$
  99
 100  $E_{elastic} = {1 \over 2}kx^2$
 101
 102  \subsection*{Motion equations}
 103
 104
 105\begin{tabular}{ l r }
 106  $v=u+at$ & $x$ \\
 107  $x = {1 \over 2}(v+u)t$ & $a$ \\
 108  $x=ut+{1 \over 2}at^2$ & $v$ \\
 109  $x=vt-{1 \over 2}at^2$ & $u$ \\
 110  $v^2=u^2+2ax$ & $t$ \\
 111\end{tabular}
 112
 113\subsection*{Momentum}
 114
 115$\rho = mv$
 116
 117$\operatorname{impulse} = \Delta \rho, \quad F \Delta t = m \Delta v$
 118
 119Momentum is conserved.
 120
 121$\Sigma E_{K \operatorname{before}} = \Sigma E_{K \operatorname{after}}$ if elastic
 122
 123\section{Relativity}
 124
 125\subsection*{Postulates}
 1261. Laws of physics are constant in all intertial reference frames
 127
 1282. Speed of light $c$ is the same to all observers (Michelson-Morley)
 129
 130$\therefore , t$ must dilate as speed changes
 131
 132{\bf Inertial reference frame} - $a=0$
 133
 134{\bf Proper time $t_0$ $\vert$ length $l_0$} - measured by observer in same frame as events
 135
 136\subsection*{Lorentz factor}
 137
 138$$\gamma = {1 \over {\sqrt{1-{v^2 \over c^2}}}}$$
 139
 140$t=t_0 \gamma$ ($t$ longer in moving frame)
 141
 142$l={l_0 \over \gamma}$ ($l$ contracts $\parallel v$: shorter in moving frame)
 143
 144$m=m_0 \gamma$ (mass dilation)
 145
 146$$v = c\sqrt{1-{1 \over \gamma^2}}$$
 147
 148\subsection*{Energy and work}
 149
 150$E_0 = mc^2$ (rest)
 151
 152$E_{total} = E_K + E_{rest} = \gamma mc^2$
 153
 154$E_K = (\gamma - 1)mc^2$
 155
 156$W = \Delta E = \Delta mc^2$
 157
 158\subsection*{Relativistic momentum}
 159
 160$$\rho = {mv \over \sqrt{1-{v^2 \over c^2}}}= {\gamma mv} = {\gamma \rho_0}$$
 161
 162$\rho \rightarrow \infty$ as $v \rightarrow c$
 163
 164$v=c$ is impossible (requires $E=\infty$)
 165
 166$$v={\rho \over {m\sqrt{1+{p^2 \over {m^2 c^2}}}}}$$
 167
 168\subsection*{Fusion and fission}
 169
 170$1 \operatorname{eV} = 1.6 \times 10^{-19} \operatorname{J}$
 171
 172e- accelerated with $x$ V is given $x$ eV
 173\subsection*{High-altitude muons}
 174\begin{itemize}
 175  {\item $t$ dilation - more muons reach Earth than expected}
 176  {\item normal half-life is $2.2 \operatorname{\mu s}$ in stationary frame}
 177  {\item at $v \approx c$, muons observed from Earth have halflife $> 2.2 \operatorname{\mu s}$}
 178  {\item slower time - more time to travel, so muons reach surface}
 179\end{itemize}
 180
 181\section{Fields and power}
 182
 183
 184\subsection*{Non-contact forces}
 185\begin{itemize}
 186  {\item electric fields (dipoles \& monopoles)}
 187  {\item magnetic fields (dipoles only)}
 188  {\item gravitational fields (monopoles only)}
 189\end{itemize}
 190
 191\begin{itemize}
 192\item monopoles: field lines radiate towards central object
 193\item dipoles - field lines $+ \rightarrow -$ or $\operatorname{N} \rightarrow \operatorname{S}$ (opposite in solenoid)
 194\item closer field lines means larger force
 195\item dot means out of page, cross means into page
 196\end{itemize}
 197
 198\subsection*{Gravity}
 199\[
 200F_g=G{{m_1m_2}\over r^2}\tag{grav. force}
 201\]
 202
 203\[
 204g={F_g \over m}=G{M_{\operatorname{planet}} \over r^2}\tag{grav. acc.}
 205\]
 206
 207\[
 208E_g = mg \Delta h\tag{gpe}
 209\]
 210
 211\[
 212W = \Delta E_g = Fx\tag{work}
 213\]
 214
 215\subsection*{Satellites}
 216\[
 217v=\sqrt{GM \over r} = \sqrt{gr} = {{2 \pi r} \over T}
 218\]
 219
 220\[
 221T={\sqrt{4 \pi^2 r^2} \over {GM}}\tag{period}
 222\]
 223
 224\[
 225\sqrt[3]{{GMT^2}\over{4\pi^2}}\tag{radius}
 226\]
 227
 228
 229
 230\subsection*{Magnetic fields}
 231% \begin{itemize}
 232% \item field strength $B$ measured in tesla
 233% \item magnetic flux $\Phi$ measured in weber
 234% \item charge $q$ measured in coulombs
 235% \item emf $\mathcal{E}$ measured in volts
 236% \end{itemize}
 237
 238% \[
 239% {E_1 \over E_2}={r_1 \over r_2}^2
 240% \]
 241
 242\[
 243F=qvB\tag{force on moving charged particles}
 244\]
 245
 246if $B {\not \perp} A, \Phi \rightarrow 0$ \hspace{1em}, \hspace{1em} if $B \parallel A, \Phi = 0$
 247
 248
 249\includegraphics[height=2cm]{/mnt/andrew/graphics/field-lines.png}
 250
 251\subsection*{Electric fields}
 252
 253\begin{align*}
 254F=qE \tag{$E$ = strength} \\
 255W=q_{\operatorname{point}}\Delta V \tag{in field or points} \\
 256F=k{{q_1q_2}\over r^2}\tag{force between $q_{1,2}$} \\
 257E=k{Q \over r^2} \tag{$r=||EQ||$} \\
 258F=BInl \tag{force on a coil} \\
 259\Phi = B_{\perp}A\tag{magnetic flux} \\
 260\mathcal{E} = -N{{\Delta \Phi}\over{\Delta t}} \tag{induced emf} \\
 261{V_p \over V_s}={N_p \over N_s}={I_s \over I_p} \tag{xfmr coil ratios} \\
 262\end{align*}
 263
 264
 265\textbf{Lenz's law:}  ``$-n$'' in Faraday - emf opposes $\Delta \Phi$
 266
 267\textbf{Eddy currents:} counter movement within a field
 268
 269\textbf{Right hand grip:} thumb points to north or $I$
 270
 271\textbf{Right hand slap:} field, current, force are $\perp$
 272
 273\textbf{Flux-time graphs:} gradient $\times n = \operatorname{emf}$
 274
 275\textbf{Transformers:} core strengthens \& focuses $\Phi$
 276
 277% \columnbreak
 278
 279\subsection*{Power transmission}
 280
 281\begin{align*}
 282  V_{\operatorname{rms}}={V_{\operatorname{p\rightarrow p}}\over \sqrt{2}} \tag
 283  P_{\operatorname{loss}} = \Delta V I = I^2 R = {{\Delta V^2} \over R}
 284\end{align*}
 285
 286\begin{itemize}
 287  {\item Parallel - voltage is constant}
 288  {\item Series - voltage is shared within branch}
 289\end{itemize}
 290
 291\includegraphics[height=4cm]{/mnt/andrew/graphics/ac-generator.png}
 292
 293\subsection*{Motors}
 294% \begin{wrapfigure}{r}{-0.1\textwidth}
 295
 296\includegraphics[height=4cm]{/mnt/andrew/graphics/dc-motor-2.png}
 297\includegraphics[height=3cm]{/mnt/andrew/graphics/ac-motor.png} \\
 298% \end{wrapfigure}
 299\textbf{DC:} split ring (one ring split into two halves)
 300
 301% \begin{wrapfigure}{r}{0.3\textwidth}
 302
 303% \end{wrapfigure}
 304\textbf{AC:} slip ring (separate rings with constant contact)
 305
 306
 307\end{multicols}
 308\end{document}