I'm thinking you're asking about heating pads that you plug into the wall. If this is not what you meant, please e-mail SciLine back and we'll re-answer the question for you.

For the heating pads that you plug into the wall, electricity is the source of heat. Heating pads are made with electrical components that have resistance in them. This means that electrons have a hard time traveling through around the wires within the heating pad.

Heating pads work the same way as incandescent light bulbs do, somewhat. Electrons are supplied by alternative current (AC) from your wall outlet. In an AC circuit (like in the light bulb and a heating pad), electrons tend to travel towards the surface of the wire (as opposed to its core). This phenomena is called the skin effect and you can read about it here:

Electrons start to concentrate more at the outside than at the inside of the light bulb filament (or resistive wire in the case of a heating pad). What's known as eddy current are set up, where the electrons are traveling rapidly from the outside of the wire (where they want to be) to the center of the wire (where there is a lower concentration of electrons). This round and round motion makes electrons loose their energy. In the case of a light bulb, that energy is released in the form of light and heat. In the case of a heating pad, the energy is released as heat.

Great question! I learned a lot in researching the answer (to what I hope you were asking!).

The process is called Exothermic chemical reaction.

Explanation:
The contents inside stick-on heating pads are made up of iron powder, charcoal/salt, and are activated when exposed to the air.The iron reacts with oxygen in the air and gives off heat; the charcoal and salt help speed up reaction.