The amount of force that causes electrons to flow in a conductor is called electromotive force, which is abbreviated "emf". This force can be created in several ways. One way is a chemical reaction between different kinds of chemical solutions and metal. This chemical reaction produces many free electrons. This type of emf is called a cell. A group of cells together are called a battery.

The amount of current flow that can be produced by a cell is limited. As the electrons in the material that the cell is made from are depleted, the amount of emf is reduced proportionally. That is why we need to replace batteries in flashlights and other electronic devices. Some batteries are made up of materials that are not eaten away as the electrons are given up. The depleted electrons in these cells can be replaced with new electrons. These batteries are called rechargeable batteries and are the types of batteries we find in cars, in computers, and in many electronic devices.

The measuring unit of emf is called the volt. An ordinary "D" size flashlight cell battery generates an emf of about 1.5 volts. This is the force that pushes the electrons through the circuit or conductor. If we again compare electricity and current flow with water flow, the emf would be the amount of water in a mountain lake that supplies the water in the stream.

Most electric devices require much more energy or emf than a small flashlight cell or battery can create. Larger cells or batteries are necessary to provide enough emf to power electric devices. The ISS (International Space Station) is equipped with many batteries to supply its electrical power, and these batteries are rechargeable batteries that are charged by light energy from the sun. The light energy or rays from the sun are changed into electrical energy by a photovoltaic cell.