Electricity is the flow of electrons. For more than a century we have been finding interesting ways to use electrical energy. An electromagnetic power source must either store a surplus of electrons or be able to use some force--usually magnetism--to force electrons to move back and forth in wavelike motions. Electromagnetic energy usually refers to systems that transfer electrical power wirelessly. Electromagnetic energy power sources have both advantages and disadvantages.
Electromagnetic energy is clean. It is not polluting like oil and coal energy sources, nor do we have to destroy the environment to get the raw materials--electrons are everywhere. It has no radioactive components that can explode violently or produce dangerous radioactivity for thousands of years. It also is renewable--we will never run out of electrons or magnetism. Besides being clean and renewable, electricity is versatile. We already know hundreds of ways to use electricity to cool, to heat and to drive motors of all sizes to perform all kinds of work. Electricity can be made to work on extremely small scales, such as in microchips. For packing a lot of information-processing power into a low energy-consuming package there is no other power source that even comes close.
The wireless transmission of electrical power is an idea that goes back to at least the early part of the 20th century. Nikola Tesla (a contemporary of Thomas Edison) worked on the project and discovered the chief disadvantage: It is not easy to achieve. This challenge remains the major disadvantage. Even if it was easy, there is another disadvantage that worries many people: is it safe. Most researchers have concluded that Radio Frequency (RF) waves--the proposed means of transmission--are completely safe and that RF has no affect on living tissue. Not everybody agrees.
Electromagnetic power transmission is already a reality on a small scale. Joshua R. Smith, an Intel researcher in Seattle, has developed a device that collects power from ambient RF signals. These signals from radio and television broadcasts largely go to waste. The air is full of these signals. Only a small percent of the energy goes into activating the antennas of interested receivers--the rest goes into trees, houses, the ground or into outer space. Enough of this ambient energy already exists to power a large handheld calculator or an iPhone.