Have you got the power?
What is a battery? A battery is a device which performs work by supplying electricity to a load. The battery has the potential to move charge (usually electrons) in one terminal and out the other. This potential is sometimes called the voltage. But, a battery can only supply a finite amount of charge at any one instant in time. The motion of charge past a reference is called the current. The battery supplies charge and the energy to push this charge, but it is the load (e.g., a light bulb) working with the battery which will draw a certain amount of charge from the battery at any instant in time. If the load tries to draw more charge than the battery can supply, then the potential for the battery to move charge (voltage) decreases.
There are a large number of different types of batteries. We shall categorize these different types as being either Primary Voltaic Cells (irreversible cells) or Secondary Voltaic Cells (reversible cells). Irreversible means that the battery cannot be recharged to its original state by the application of an external voltage supplying direct current of electricity to flow in the opposite direction of battery discharge. Reversible implies that the battery may be recharged with the appropriate application of an external voltage source. Not all types of batteries fit these two categories exactly but this does cover a large range of sources. For example, the Daniell cell (sometimes called the gravity cell) and the dry cell (World War II torpedo battery, wrist watch battery, Rubin-Mallory cell for hearing aids, Leclanche dry cell) and the fuel cell (electrode materials are usually in the form of gases) are typical Primary Voltaic Cells. The lead storage battery (car battery) and the Edison storage battery are typical Secondary Voltaic Cells. The voltaic cells (of both types) contain an electrolyte medium between the positive and negative terminals inside the battery. The solar battery cell does not quite fit either categories of the Voltaic type. Its source of energy comes from an external source like the sun in Earth’s solar system. The sun’s electromagnetic radiation (light) is transformed into electricity. Unfortunately, I cannot depend on the light from the stars to fuel my spaceship. Intergalactic space travel sometimes takes me far from the nearest star. Consequently, the star’s light is too small to supply energy for my spaceship.
What are electrolytes? Electrolytes are liquid mediums that conduct electricity. Typical electrolytes are:
- orange juice
- Nevada water
- salty water
- sports drinks
- lemonade...
How can common solid salt crystals dissolve in water to make an electrolyte? Salt crystals are composed of a sodium atom and a chloride atom. Under the right conditions, these atoms combine as a result of an electrostatic force between the atoms. The sodium atom (11 protons and 11 electrons) has one electron that is loosely bound to the atom compared to the remaining 10 electrons. This means that it may be willing to share this electron with another element such as chloride under the right conditions. Chloride (17 protons and 17 electrons) has 17 tightly bound electrons with room to share one more electron in a loose manner. A field of study called quantum mechanics states that electrons in atoms are located in discrete energy levels. That is right, an electron cannot have an energy value midway between two energy levels. The nuclei (central portion of the atom containing protons and neutrons) of atoms are surrounded by electrons in a series of these energy levels sometimes called shells. These shells may be imagined as follows. Consider three spheres, a ping pong ball inside a baseball inside a basketball, all sharing the same center. The shell may be thought of as the surfaces of these three spheres. The closer the shell is to the nucleus, the stronger the attraction between the electrons (negative charge) and the protons (positive charge) in this atom, and the higher the energy level. Recall, that opposite charges attract (electromagnetic force) and this attractive force increases as the distance between charges decreases. The outer most energy level of chloride and sodium can potentially contain eight electrons. So, when chloride and sodium combine to become a molecule, the one electron in sodium and the seven electrons in chloride combines to fill their respective outer most energy levels. Both atoms share their outer most electrons and, of course, forms salt. When immersed in water, the electrostatic force binding the atoms together as a molecule is weakened because of the special electrical properties of the water molecule (dipole properties to be explained later). By stirring or slightly heating the salt water mixture (dissolving the salt), it is energetically favorable for the sodium chloride bond to split leaving behind the weakly bound electron in the outer most energy shell of the sodium atom attached to the chloride atom. The sodium atom minus an electron is a positive sodium ion and the chloride atom with an extra electron is a negative chloride ion. These ions floating freely in the water medium can be manipulated with the electromagnetic force and therefore forms what we call an electrolyte. Electrolytes provide a vehicle for electricity (current) to flow between two electrodes (two dissimilar metals immersed in the electrolyte) in the battery usually called the positive and negative terminals of the battery. For batteries, the positive terminal is call the cathode, and the negative terminal is call the anode. The anode is the electrode through which electrons are removed from the electrolytic liquid causing what chemist refer to as an oxidation reaction. The cathode is the electrode from which electrons enter the electrolytic solution which chemists have coined as a reduction reaction.
Did you know that all materials have a so called free energy?
Cool, we get something for free.
Well, not really.
Then, what is free energy?
Inside an ideal bulk material, atoms and molecules line up in such a manner that they are in equilibrium with the electrostatic forces acting upon them from neighboring atoms and molecules. At the surface of a medium, there is no continuation of bulk medium so the atoms and molecules on the surface are bound to the surface in a manner that is different than its neighbors inside the bulk. The electrostatic forces generated by these unusual surface bonds are not completely cancelled by neighboring atoms of the bulk since they do not exist on the one side. The surface atoms are polarized. That is, the electron cloud around the nucleus of a surface atom is pulled enough in one direction by neighboring molecules such that the cloud no longer terminates the electrostatic field components of the atom. Of course, the far reaching electrostatic fields of the atoms or molecules inside the bulk near the surface also contribute to the total weak electrostatic force outside of the medium. This electrostatic force is sometimes called a Coulomb force (free energy) and is responsible for this so called free energy. Typically, this force is weak, but it has the ability to attract other materials until an equilibrium is reached. [A lowest energy state is reached.] When exposed to air, the surface attracts some of the air molecules causing the surface to oxidize. Did you know that this so called free energy can be observed in your own home? Check it out yourself. Pour water into a glass cup. Look at the top edge where the water meets the glass and the air. Is the water on the glass surface at a different level as compared to the height of the water at the center of the cup? The difference in height is due to the free energy of the surface molecules in the glass attracting the water molecules. How can a water molecule be attracted to the surface? Good question, two hydrogen atoms are bound to an oxygen atom in such a way that the water molecule has its own field sometimes called a dipole field.
The water molecule is said to be polarized, therefore it is not really neutral on a molecular level. Remember, electric and magnetic fields can not attract or repel a neutral charge.
Okay... Back to our battery. Because the metals are dissimilar, the difference in the free energy of the metals generates a potential difference between the electrodes. This potential difference is sometimes referred to as voltage.
Now we must consider the electrical properties of metals and good conductors. The essence of the metal is the atoms and molecules bound together fixed in the material. This gives the solid properties of the material. Atoms and molecules are composed of a nucleus (protons and neutrons) immersed in an electron cloud. The electron cloud is shared among neighboring nuclei. At very, very, very cold temperatures, the electron clouds are bound to the nucleus or nuclei. As the temperature is raised, say to room temperature, some of the electrons have enough energy to break away from the nucleus and move freely, conduct, in the metal. The motion due to temperature is random. This means over a long period of time, the electron will not have moved far from its original position. As a potential difference (or voltage) is applied to the metal, the electrons will tend to drift in a particular direction far from its original position. The electrons in the metal "feel" this potential difference because the potential difference creates an electric field which extends from the source to attract or repel charge. If a potential difference is applied from some point in space to the metal, [Yes, like from the positive terminal of a battery to a negative termninal when nothing is attached to the terminals.] some of the electrons in the metals will rush to an appropriate surface of the metal. For most cases, the electrons do not have enough energy to leave the metal surface (work function). It is interesting to note that this electric field is present both inside the battery between our dissimilar metals in the electrolyte as well as outside of the battery.
So why doesn't the electrolyte short out the battery?
Good question. Let us consider what happens when clean electrodes are placed in water. First, the electrons in the metal do not have enough energy to just leave the electrode and enter the electrolyte on their own. This has to do with the binding energies of the metal at its surface and is sometimes modeled in terms of what is called a work function. Further, the metal electrode is neutral. That is, there is an equal number of positive and negative charges in the metal electrode. (Remember, the binding energy is due to the way in which the electron cloud is distributed about the nucleus in the surface atoms of the material.) Second, the electrolyte contains positive and negative ions. These ions adjust to the electric field generated by the free energy in such a way that one electrode is shielded from the other. Further, those ions attracted to the surface of the electrodes tend to coat the electrode surface forming a weak resistive barrier. Don’t forget that the water molecule is also polar and is obviously attracted to the electrodes as well. Equilibrium is eventually established. In the picture below, the + terminal electrode has a higher more positive free energy level compared to the - terminal electrode. The charge distribution about the electrodes may differ depending on electrode material, temperature, motion of fluid, etc. The shielding effect is shown.
When a load is placed across the battery electrodes, the difference in the free energy of the electrodes generates the potential difference (voltage) required to push the conducting electrons in the negative electrode through the load to the positive electrode giving rise to a current. This path is easier for the electrons to follow as compared to moving through the electrolyte. As the negative electrode looses electrons and the positive electrode gains electrons, the localized field in the electrolyte changes. Excess electrons in the positive electrode move towards the electrode suface due to an imbalance of the number of positive and negative charges in the metal. These charges have enough energy to overcome both the binding energy of the metal in suolution and the resistive barrier so to conduct in the electrolyte solution. Over time, the resistive barrier builds up decreasing the amount of current flow through the electrolyte. As a result, the battery discharges loosing its ability to maintain the voltage required to drive a current through the load. At this point, the battery has to be recharged or replaced. The ability to move a lot of charge from one electrode to another through a load is dependent on how fast the ions diffuse or conduct current in the electrolyte between the electrodes and the ion concentration in the electrolyte. The amount of charge stored is dependent on the electrode area and the types of dissimilar metals.
I use batteries for a temporary source of energy in my spaceship and home - when other electric sources fail or are not available.
