Galvanic cells.

Step 1 of 5

A cell’s voltage

When zinc is dipped in a solution of copper ions, the electrons go straight from the zinc to the ions, and the energy turns into heat. A galvanic cell separates the two halves of the reaction, so the electrons have to travel through a wire, where they can be used. The electrode where oxidation happens is the anode, the negative terminal; the one where reduction happens is the cathode, the positive terminal.

How strongly each half-cell “pulls” electrons is measured by its standard reduction potential, $E^\circ$⁠, against a hydrogen electrode, in 1 mol/L solutions at 25 °C. The cell’s voltage with no current flowing, its electromotive force (emf, $\varepsilon$⁠), is the difference:

$E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$$E^\circ_{\text{cell}} =$$E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$⁠

Step 2 of 5

Choosing the metals

Metal	$E^\circ$⁠
Magnesium	−2.37 V
Zinc	−0.76 V
Iron	−0.44 V
Lead	−0.13 V
Copper	+0.34 V
Silver	+0.80 V

The metal in the right half-cell is always copper. Load resistance is fixed at 500 Ω, nearly an open circuit, and Left metal is free.

Step 3 of 5

The poles swap

Which electrode is the anode is not fixed in advance: it is always the metal with the lower potential of the two. Show ions and electrons is On, and Left metal is now fixed at zinc.

Step 4 of 5

Voltage under load

Like any source of voltage, the cell has internal resistance: the ions have to travel through the solutions and the salt bridge, and that costs voltage. In the Scene’s cell the internal resistance is 10 Ω. When a current $I$⁠ flows, the voltage at the terminals is less than the emf: $V = \varepsilon - Ir$$V =$$\varepsilon - Ir$⁠. Now Load resistance is 20 Ω.

Step 5 of 5

The salt bridge

In the zinc beaker positive ions are made all the time, and in the copper beaker positive ions are used up. The salt bridge, a tube of salt solution, closes the circuit inside the cell. Show ions and electrons is On: watch what moves through it.