Backticks vs braces in Bash

They behave slightly differently in a specific case:

$ echo "`echo \"test\" `"
test

$ echo "$(echo \"test\" )"
"test"

So backticks silently remove the double quotes.

The `` is called Command Substitution and is equivalent to $() (parenthesis), while you are using ${} (curly braces).

So all of these expressions are equal and mean "interpret the command placed inside":

joulesFinal=`echo $joules2 \* $cpu | bc`
joulesFinal=$(echo $joules2 \* $cpu | bc)
#            v                          v
#      ( instead of {                   v
#                                 ) instead of }

While ${} expressions are used for variable substitution.

Note, though, that backticks are deprecated, while $() is POSIX compatible, so you should prefer the latter.

---

From man bash:

Command substitution allows the output of a command to replace the command name. There are two forms:

          $(command)
   or
          `command`

Also, `` are more difficult to handle, you cannot nest them for example. See comments below and also Why is $(...) preferred over ... (backticks)?.