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What happens in a redox reaction? One element or molecule loses electrons. That's oxidation. The other gains electrons. It gets reduced. We learned we could separate them into two half-reactions. When we use a wire to connect the half-reactions, we generate a voltaic cell. The oxidation's electrons travel through the wire. They arrive at the cathode and reduce it. What causes the electrons to travel? The difference between the electrodes' potential energies. How do we measure it? As a voltage. We use the potential energy difference to do electrical work. But, there is something else we can gauge!
Voltage measures the difference in potential energy. But, it doesn't tell us anything about the quantity. Do we have one, ten, or a million electrons streaming through the wire? Also, how fast are they flowing? Is it one electron per second or a million? We need a new measurement! Thankfully we have current! The unit for current is the Ampere or Amp. It has the symbol .
Analogy for Voltage and Current
I remember learning the difference between voltage and current in high school physics. My teacher used a humorous analogy. Imagine a river. A shipping container spills thousands of yellow rubber ducks into it. As they travel down the stream, they approach a waterfall. Soon, rubber ducks are flying off. The voltage is the equivalent of how far they fall off the waterfall. There's a massive difference between a local stream and Niagra Falls. The current is how many rubber ducks fall per second. Again, in a local stream, the ducks might get backed up. Only a few can pass each second. In Niagra Falls, you could have thousands falling off every second.
Another way to distinguish voltage and current is intensity versus quantity. The voltage is how much force is applied. The current is how many times it happens per second.
Units for Current
We already said that the unit for current is the ampere. But, what is an ampere? One ampere equals one Coclomb per second. Remember, a Coulomb is a unit of charge. One electron has a charge of . So, if we know the current and time, we can calculate the number of transferred elections.
So, how do we calculate the current? We use Ohm's Law. It states that a circuit's voltage is proportional to resistance times current. Mathematically, that's . In Ohm's law, is voltage, is current, and is resistance.
Resistance in a Voltaic Cell
Are you wondering how a voltaic cell has a current? That's a great question! It shouldn't have any resistance. For example, we almost always assume a wire's resistance is zero. So, how can we apply Ohm's law? It must come from the half-cells. We call it internal resistance. What causes internal resistance?
In an electrochemical cell, electrostatic interactions generate resistance. One of the foundational aspects of a voltaic cell is the salt bridge. We have to maintain the charge balance. But, it takes time for the ions to diffuse. So, we can have a build-up of charge. The charge resists the current. Also, consider the electrolyte concentration. As the redox reaction happens, some of the ions precipitate. The precipitation reaction causes us to lose ion concentration. What happens when the concentrations become too low. Our solution becomes a poor electrolyte. So, the internal resistance increases. Finally, we know that metals are good conductors because of their sea of electrons. But, the movement of electrons is not instantaneous. The nuclei's protons attract the electrons. The attraction inhibits the electrons' motion. It takes time for charges to distribute throughout a metal. So, the electrode itself contributes some resistance. Moreso, the reaction only occurs where the electrode is in contact with the ion solution. So, the electrode's surface area also affects current flow.
So, what's a typical internal resistance? Energizer published the internal resistance of their batteries. Their cylindrical batteries have internal resistances between 150 and 300 milliohms. So, it's not an immense resistance. But, we can see that there would be a current.
