8.11.1: Batteries

Learning Objectives

• Know the components of a battery: anode, cathode, salt bridge
• Know where electrons are flowing as energy is transferred from a battery.

As we discussed in the previous chapter, a charged particle will move through an electric field similar to a ball rolling down a hill. We can see the ball moving and the hill and understand how it works, as we have seen similar phenomena several times. But why does an electric potential work the way that it does? Potential differences can be caused by a variety of factors, some of which will be explored later in this text. For now we will discuss the functioning of a battery.

There are some chemical reactions which involve the transfer of electrons in order for the reaction to take place. Remember that energy is stored in chemical bonds and is often released from those bonds as chemical reactions take place. It turns out that when electrons are transferred in a chemical reaction, there is an enormous amount of energy that is also released in these reactions. (We will explore chemical reactions in more detail later in this text, but for now consider a combustion reaction as an example of a reaction in which electrons are transferred as the reaction takes place.)

One way to create power from a chemical reaction is to extract useful work from the heat produced as was discussed earlier in this text. But what if we could take those individual electrons that are being transferred and make them do some work in the process? It turns out that we can do this, and in fact this is exactly what occurs inside of a battery. A battery is simply a chemical reaction divided into sections: one section where the electrons are lost and another section where the electrons are gained. These sections are commonly referred to as half-cells. These half-cells are then separated from each other. If a material that allows the flow of electrical energy is used to connect these two half cells, some of that electrical energy may be transferred to do useful work. Once the reaction is complete, there is no longer a potential difference to drive the flow of electrons and the battery ceases to function.

Figure \(\PageIndex{1}\): A voltaic cell for demonstration purposes. By Alksub at the English Wikipedia, CC BY-SA 3.0, (Source)

Figure \(\PageIndex{1}\) illustrates a schematic illustration of the function of a battery (though this specific type of battery would not be very functional for any application in which it might move around). Most batteries function because of electrodes, which allow for the flow of electrical energy from the battery to do work and also sometimes function as reactants or products of the reaction. Each electrode is placed in a solution within a half cell. The electrode may react by dissolving into the solution and releasing electrical energy in the process. Or the electrical energy transferred into an electrode might cause a reaction within the solution. The electrode that the electrons flow away from within a battery is called the anode. The electrode that the electrons flow to on the other side of the battery is called the cathode. In addition to the two half cells, each battery also contains a salt bridge which connects the half cells. The function of the salt bridge it to prevent the accumulation of charge at either of the electrodes within the battery.

MISCONCEPTION ALERT: CONVENTIONAL CURRENT AND FLOW OF ELECTRONS

When we discuss the function of the anode and cathode related to the flow of electricity, we have been considering these from the perspective of the flow of electrons. It turns out that flowing electricity was understood prior to the knowledge of electrons and a convention was created by Benjamin Franklin opposite of what we now understand. As we discussed previously, the convention in studying electricity is to consider the current as flowing from the positive charge. We have seen this with the diagrams of electric field lines. Conventional Current is the flow of positive charge. However, the electrons are flowing in the opposite direction because they have a negative charge.

Of course the batteries that we use do not have beakers filled with solutions. These would be impractical, as any movement would cause them to spill. However, they do function on the same principle: there are two half reactions which are separated from each other, and the electrons that flow from one of these reactions to the other are available to do work. These batteries can be different sizes and shapes, and in some cases a battery is multiple batteries strung together. But in all cases, batteries function on a similar principle.

Section Summary

• A battery is a chemical reaction separated into two parts.
• Electricity can flow from one part of the battery to the other in order to complete this reaction.
• Electrons flow from the anode to the cathode while conventional current flows from the cathode to the anode.

Glossary

anode

The electrode that electrons flow away from within a battery.

cathode

The electrode into which electrons flow when they return to a battery.

conventional current

The flow of positive charge.

electrode

A portion of a battery capable of transferring an electrical charge to a device outside of the battery, or accepting an electrical charge from a device outside of the battery. It is also sometimes engage in a chemical reaction related to the function of the battery.

salt bridge

connects the anode and cathode half-cells and provides charge neutrality within the battery.