I'm trying to teach myself EM using Griffiths. However, the number of actual Physics problems in the book is limited, and since it takes a lot of practice to master a skill, I wanted to know if there are some resources from which I can practice ED from. For example, there are not many problems on solving Laplace's equation. Is there a book that I can refer to besides Griffiths for problems? I have Purcell but there isn't much about solving PDEs using separation of variables in Purcell.
What books are recommended for an advanced undergraduate course in electrodynamics?
When can I make opportunity attacks? I'm a bit confused on how extra damage works for critical hits. Let's take the following example: A thief with sneak attack rolls a crit. I'm under the impression he rolls double his normal attack damage dice AND double his Sneak Attack damage dice. Is this accurate? Another example, I finished DM-ing my first campaign recently. The Giant Spider enemy monster has an melee attack that causes a constitution saving throw. If the player fails, the enemy rolls an additional 2D8 of damage. In this situation, if there was a critical hit, and the player failed their saving throw, would I also roll double dice on the additional damage? Or is that considered a separate attack? How can I install Oracle 11g R2? The title itself is more than self explanatory. I simply need a way to install Oracle's 11g R2 in the latest version of Ubuntu, I'm using a 32-bit OS, and I need to know how to install the x86 version. I also have a 64 bit desktop and I need the install the x86_64 version. Can anyone help me? How do wizards cast spells which appear on their spell list when it has a divine focus component? What does a wizard do to cast a spell which contains a DF component? Take  for example,  Components V, S, DF    A divine focus component is an item of spiritual significance. The divine focus for a cleric or a paladin is a holy symbol appropriate to the character's faith. The divine focus for a druid or a ranger is a sprig of holly, or some other sacred plant. If the Components line includes F/DF or M/DF, the arcane version of the spell has a focus component or a material component (the abbreviation before the slash) and the divine version has a divine focus component (the abbreviation after the slash).  So do wizards have holy/unholy symbols in their spell components pouch or what? Change desktop background to a solid color (e.g. pitch black) in GNOME 3 I just installed Ubuntu 16.04 with GNOME and I would like to set the background color to black. If I right click in the background I can select a wallpaper or a photo or a color. But there are only a few colors available. How can I open a palette and select the #000000 color ? In the two following snippets, is the first one safe or must you do the second one?  By safe I mean is each thread guaranteed to call the method on the Foo from the same loop iteration in which the thread was created?  Or must you copy the reference to a new variable "local" to each iteration of the loop?  var threads = new List&lt;Thread&gt;(); foreach (Foo f in ListOfFoo) {           Thread thread = new Thread(() =&gt; f.DoSomething());     threads.Add(thread);     thread.Start(); }   -  var threads = new List&lt;Thread&gt;(); foreach (Foo f in ListOfFoo) {           Foo f2 = f;     Thread thread = new Thread(() =&gt; f2.DoSomething());     threads.Add(thread);     thread.Start(); }   Update: As pointed out in Jon Skeet's answer, this doesn't have anything specifically to do with threading. Could I use a 1/2TB USB 2/3 external HDD with an XBox 360? I'm considering getting an XBox 360 (plus Kinect). I need storage for other devices as well (movies, PC games, whatever an Ouya might need), so I'm considering getting a big external HDD. Can I store downloaded games (e.g. Fez) on such a device? Getting started with ArcObjects? What is the best route or place to get started with ArcObjects if one is not a developer and not aiming to become a developer?  I've been using gis professionally for a long time, almost two decades, arcinfo/arcgis for most of that; I'm pretty good. I'm learning software development, and even have a modestly successful small python application used in a public project; I'm not good! I don't want to become a full fledged software developer but I keep running into things I just can't do without programming (). In the arcgis world this puts me pretty squarely in the python camp, which is fine by me since I like python, however python does not have straightforward access to ArcObjects. (Python and ArcObjects , but it's an unsupported route. This question is about following a beaten path.)  I need to get started, but where?  to get started with the  but $1500/yr is definitely not in my budget, and sounds like using a sledge hammer to swat mosquitos. And which language .NET, Visual Basic, or Visual C++?    UPDATE: Thank you everyone for the wonderful answers. In light of them I realise  I unecessarily narrowed the scope of my question be pre-supposing "ArcObjects" is the direction I need to go in. A more open ended formulation is more along the lines of:     I keep running into problems I just   can't solve with arcgis and python   alone. What else can I learn/use to   solve problems like X? I've no   interest or intention of becoming a   software developer. I just need to do   a couple things which aren't exposed   to the arcgis python modules. Trying to figure what language this PDA (pushdown automata) accepts I have the following PDA and I can't figure our what words it accepts, would like to get some help with figuring this out.    Of course it only accepts if that stack is empty in the accepting state. How do I install Firefox 52 ESR on 16.04? This  asks how to install FF32 bit while keeping the 64 bit.  I want the much more simple thing: is there a canonical way to install firefox 52 ESR as "default" firefox in my 16.04?  I found this  ... but there are no packages; probably no wonder given the fact that the linked blog article dates back to 2012.  So, 2017, Ubuntu 16.04; how to install FF 52 ESR (as I simply need the java plugin to work for me).  ( my current solution: I downloaded the TAR file from mozilla, extracted that to /opt; and then changed /usr/bin/firefox to be a link to that ESR version of firefox. works, but ugly ) How to capture text from the operating system in a practical and simple way? Sometimes we can't select the text to copy &amp; paste, such as dialog boxes.  Sometimes it's just not practical, such as 100 files inside a folder.  Is there any practical and simple way to capture those text from the system into a text file?  I'd wish to know both in OS X and Windows. Maybe it just can't be done today... At the end of a letter, is it correct to omit "I am" in this sentence?     I am looking forward to hearing from you   Then it will become:     Looking forward to hearing from you Prove $\frac{1}{2}\sum_p \frac{1}{p^2}+\frac{1}{3}\sum_p \frac{1}{p^3}+\cdots$ converges Isn't $HOME supposed to be the location of the user's home directory? I'm having trouble with a web application I'm writing: The web application, which runs as user nobody, needs to launch an inkscape process to manipulate some SVG files. Because this runs as user nobody, there is no home directory. What I am doing is creating a temporary directory under /tmp and trying to set that as the home directory via export HOME=/tmp/someUniqueId. I then set a few things in that "temporary" home directory, like a symlink .fonts to the folder for the font files to use.  I know this works for finding the ~/.fonts directory. But when I launch Inkscape, all I get is a message that Inkscape could not create the .inkscape directory:    Doesn't $HOME set the location of my home directory? Why is Inkscape still trying to access / as my home directory? How do I show that all continuous periodic functions are bounded and uniform continuous? A function $f:\mathbb{R}\to \mathbb{R}$ is periodic if there exits $p&gt;0$ such that $f(x+P)=f(x)$ for all $x\in \mathbb{R}$. Show that every continuous periodic function is bounded and uniformly continuous.   For boundedness, I first tried to show that since the a periodic function is continuous, it is continuous for the closed interval $[x_0,x_0+P]$. I know that there is a theorem saying that if it is continuous on a closed interval, then it is bounded. However, I'm not allowed to state that theorem directly. Should I just aim for a contradiction by supposing f is not bounded on the interval stated above? How to pick a specific Pantone color in Photoshop? What about finding a Pantone that is close to a specific CMYK color? How would I select a specific Pantone color in Photoshop, e.g. PMS 5625?  Also, how do you pick a Pantone colour that is similar to a specific CMYK color, e.g. C61, M41, Y58, K16 green?    In this case Pantone 5615C looks closest to the CMYK color. Is sum of two orthogonal matrices singular? I am trying to solve following problem.      Let $A, B \in \mathbb{R}^{n\times n}$ be an orthogonal matrices and $\det(A) = -\det(B)$. How can it be proven that $A+B$ is singular?   I could start with implication:  $\det(A)=-\det(B) \Rightarrow B$ is created from $A$ by swapping two lines or columns.   But I am not sure if this implication is correct. Posting and answering questions you have already found the answer to Create a page for useful links? Four tasks in parallel... how do I do that? I have a bunch of PNG images on a directory. I have an application called pngout that I run to compress these images. This application is called by a script I did. The problem is that this script does one at a time, something like this:  FILES=(./*.png) for f in  "${FILES[@]}" do         echo "Processing $f file..."         # take action on each file. $f store current file name         ./pngout -s0 $f R${f/\.\//} done   Processing just one file at a time, takes a lot of time. After running this app, I see that the CPU is just 10%. So I discovered that I can divide these files in 4 batches, put each batch in a directory and fire 4, from four terminal windows, four processes, so I have four instances of my script, at the same time, processing those images and the job takes 1/4 of the time.  The second problem is that I lost time dividing the images and batches and copying the script to four directories, open 4 terminal windows, bla bla...  How do that with one script, without having to divide anything?  I mean two things: first how do I from a bash script, fire a process to the background ? (just add &amp; to the end?) Second: how do I stop sending tasks to the background after sending the fourth tasks and put the script to wait until the tasks end? I mean, just sending a new task to the background as one tasks end, keeping always 4 tasks in parallel? if I do not do that the loop will fire zillions of tasks to the background and the CPU will clog.