More than 200 years ago, Italy scientist volt invented the world's first battery. It was a zinc plate made of metal zinc and a silver plate made of metal silver into the salt water. At this time, the zinc plate would have a chemical reaction called the oxidation reaction with the salt water, releasing a tiny particle called the electron and running the electron. After the silver plate on the other side, a chemical reaction called the reduction reaction will come back to the atom, so that the electrons flow from one plate to another, producing a current. This redox reaction is called galvanic effect.
Chemical reactions generate electricity as well as heat. When an electric current passes through an object, it will not take away all the energy. Some of the energy will be released into heat and light. For example, the lightning we see in a rainstorm is generating electricity while producing electricity. Similarly, when the oxidation-reduction reaction occurs, it also generates heat.
Heat patch contains a black powder, which is a mixture of iron powder, activated carbon powder, inorganic salt and water. After carefully mixing these ingredients, wrap them carefully with gelatin and breathable cloth. Gelatin is airtight. When used, the gelatin bag can be torn apart so that the air outside will touch the iron powder in the cloth bag. Iron oxide and air and water will undergo oxidation-reduction reaction, which will release a lot of heat when forming iron oxide. These energies were originally stored in iron powder in the form of chemical energy, and some of them were converted into thermal energy through oxidation-reduction reaction.