What is a voltaic cell?

A voltaic cell is an electrochemical cell consisting of a cathode and an anode connected via a salt bridge and a voltmeter:

The salt bridge completes the circuit and is typically soaked with a strong electrolyte like #"KNO"_3#.

Such an electrochemical cell, just as with electrolytic cells, have the electrons flowing from the anode into the cathode, as reduction always occurs at the cathode and oxidation always occurs at the anode.

In the above cell example, the following spontaneous reaction occurs:

#2"Ag"^(+)(aq) + "Cu"(s) -> "Cu"^(2+)(aq) + 2"Ag"(s)#

The silver is at the cathode, and the copper at the anode.

Using a standard reduction potential table you can find in the Appendix of your textbook, with #E_(ca)^@# and #E_(an)^@# both being standard reduction potentials,

#E_(cell)^@ = E_(ca)^@ - E_(an)^@#

#= "0.80 V" - "0.34 V"#

#= +"0.46 V"#

#=> DeltaG^@ = -nFE_(cell)^@ < 0# at #25^@ "C"#.

Notice that unlike with electrolytic cells, the cathode is the #bb((+))# terminal and the anode is the #bb((-))# terminal in a voltaic cell, because voltaic cells involve spontaneous reactions, and electrons naturally flow from the negatively-charged anode towards the positively-charged cathode.