Each cup, with its electrodes and electrolyte solution, is a simple electrochemical cell. The two electrodes are made of dissimilar materials (in this case, two different metals) with different chemical activities. A tug-of-war for electrons occurs between the two electrodes, resulting in a potential difference, or voltage. In the cells you’ve made, aluminum is the more active metal—atoms of aluminum lose their electrons more easily than do atoms of copper. The potential difference causes electrons lost by the atoms in the aluminum electrode to travel through the LED to the copper electrode, and this flow of electrons is the electric current that lights the LED.
If this flow of electrons continued, and nothing else happened, then fairly quickly there would be a buildup of electrons on the copper electrode and a shortage of electrons on the aluminum electrode.
Because electrons have a negative charge, this would result in the copper electrode becoming negatively charged and the aluminum electrode becoming positively charged. Additional electrons that tried to move from the aluminum to the copper would be repelled by the copper and attracted back to the aluminum, and electron flow would stop.
This is where the saltwater electrolyte solution comes into play. Table salt is sodium chloride, and when it’s dissolved in water, it forms positive sodium ions and negative chloride ions. The positive sodium ions are attracted to the negative copper electrode, where they participate in neutralizing the extra negative charge through chemical reactions. Likewise, the negative chloride ions are attracted to the positive aluminum electrode, where they participate in neutralizing the extra positive charge. Therefore, there’s a constant flow of charge from one electrode through the LED to the other electrode, and then through the electrolyte solution, forming a complete circuit.
The five cells make up a battery when they’re connected in series. (A battery is two or more electric cells that are joined together.) The five-cell battery has five times the voltage of each individual cell.
It takes a minimum voltage to light an LED. If you don’t have enough cells, you won’t provide the necessary voltage.
In any electrochemical cell, the greater the difference in the activity of the two materials making up the electrodes, the greater the strength (voltage) of the cell. Since chemical reactions are taking place at the electrodes, the larger the area of the electrodes, the greater the number of electrons that can be pulled per second, and the larger the current (measured in amperes, or amps).
The big idea here is the difference in the abilities of two materials to lose and gain electrons. This same idea is at the heart of the wide variety of batteries used for everything from flashlights to cell phones. The materials, size, and shape of these batteries may differ from those of this saltwater pentacell, but the general principle remains the same.