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Electricity is a form of energy resulting from the existence of charged particles called ions. Examples of electrically driven processes are all around you; for example the screen on which you are reading this relies on electrical energy to produce photons of light. These small packets of light energy are received by photoreceptors in your eyes which initiate a cascade of events, entirely reliant on the flow of ions, to send this information to your brain. Ions may be atoms or molecules with an excess or deficiency of electrons; they experience an attractive or repulsive force when close to other charged particles. An electric circuit can be considered simply as the flow of electrons in a loop (a current), by using ions as vehicles. It is important to realise that electrons are fundamental building blocks of atoms. They are already in the wires of electrical devices! Only when the electrons flow around the circuit can their energy be used to do work.
But what causes electrons to flow? If your screen is plugged into a mains power socket in the wall, then the screen’s electrons are being pushed through the circuit using energy derived from a power station. It is quite incredible to consider that when your device is plugged into the mains, an enormous electrical circuit many miles in diameter is completed. When this happens electrons are able to flow and this movement is converted into whatever form of energy your device outputs. If however, you are reading this article from a mobile device, the energy powering your screen originates in a battery.
Batteries have the same effect as power stations; they provide the energy to push electrons around a circuit. You will have noticed that batteries have two terminals marked positive and negative. When the circuit is completed, a series of chemical reactions inside the battery cause electrons to move from the negative terminal; through the appliance connected; to the positive terminal.
The external terminals of the battery can be thought of as electron bridges that link the connected appliance to important internal components called electrodes – the actual collectors or emitters of electrons. The positive terminal connects to an electron collecting cathode whilst the negative terminal connects to an electron emitting anode. A liquid electrolyte surrounds the electrodes and allows charged particles to ferry electrons between them.
We have now covered the function and anatomy of the battery, but what happens in the electrodes that forces the electrons to flow around the circuit? A chemical reaction termed oxidation occurs at the anode. This involves the anode reacting with ions from the electrolyte to form a new compound and one or more excess electrons. Whilst this is happening, a chemical reaction termed reduction occurs at the cathode. This involves the cathode reacting with ions from the electrolyte to form a new compound and requires electrons, which are received from the anode if the electrodes part of a completed circuit. Put simply: the anode produces electrons which flow to, and are used by, the cathode when the terminals are connected. The electrons will continue to flow until one of the electrodes depletes its store of required chemicals, as one reaction cannot occur without the other.
Metal atoms bond with other metal atoms via ‘metallic bonding’. This bonding can be envisioned as a solid arrangement of metal atoms with some of their electrons (valence electrons) dissociated and able to move throughout the solid structure. This is why metals conduct electricity –their atoms don’t hold on to all their electrons within the solid structure.
Water, H2O, is a polar molecule. The oxygen atom is slightly negative and each hydrogen is slightly positive. When a strip of metal is placed in water, metal atoms on the surface of the strip are attracted to the oxygen in water some are pulled into solution. The important point to recognise here is that the metal atom pulled into solution does not bring all of its electrons (the ones able to move through the metallic structure) with it. This leaves with metal strip with a surplus of electrons and a negative charge. Metal strips composed of different metals will lose their atoms into water more easily than others, meaning that some will be more negatively charged in water than others. If two different metal strips in water are externally connected by a wire, a current will flow from the more negative metal. Using zinc and copper in this arrangement causes a current to flow from the zinc to the copper. The copper strip becomes negatively charged as it gains electrons from the zinc strip, but has the same number of copper atoms as before. This battery quickly stops working as the electromotive forces balance out.
Adding acid to the water in the arrangement described produces a longer lasting battery. Acids are just molecules that release positively charged hydrogen ions when in solution. Vinegar contains acetic acid and splits into hydrogen ions (+ve) and acetate ions (-ve) in solution. The acetate ions bond with the zinc ions in the solution to form zinc acetate. The electron deficient hydrogen ions in the solution are attracted to the negatively charged copper strip with its surplus of electrons. Two hydrogen ions each remove an electron from the copper strip to become neutral atoms, which then join up to form hydrogen gas, H2; hydrogen atoms are most stable in pairs. This allows an acid battery to last much longer than a water battery as electrons are removed from the copper strip by the hydrogen gas the battery releases. This attracts more electrons to the copper strip and the process continues until the zinc has been totally dissolved or the acid is used up. If you make your own acid battery, you will notice that the zinc strip becomes coated with copper. This happens when copper ions grab surplus electrons in the zinc strip before they flow through the wire to the copper strip.
You can create the two batteries described above with commonly purchased materials. Copper and zinc plated nails can be found in most hardware stores (along with link wire to connect these two electrodes) and vinegar or lemon juice can be added to the water to make the acid battery.