When I first began learning about basic electrical components I had a hard time distinguishing between inductors, capacitors, and batteries because they all appear to do the same thing: store energy. As the inner workings of these devices became less opaque, however, myriad differences came into view. To help the beginner avoid some of my initial confusion I sat down to write a brief treatment of all three.
Though inductors, capacitors, and batteries do indeed store energy their means of doing so vary tremendously. This has implications for how quickly they can be charged, how quickly they can discharge, when and where they are most appropriately used, what future developments we can expect, etc.
Inductors store energy electromagnetically. Though there is some controversy over the specific mechanics of energy storage in an inductor, most seem to agree that it relies on the magnetic field that is created when current runs through the inductor wire. As current increases the magnetic field increases, opposing the change in the current and absorbing energy in the process. When current levels off the magnetic field just sits there, holding on to its energy stores and not hassling the electrons as they flow through. But when current begins decreasing the magnetic field begins to collapse, and its energy goes towards keeping the electrons flowing. Thus the energy stored in the initial buildup of current is discharged when current begins to slow down.
Capacitors store energy electrostatically. A basic capacitor is two conductive plates separated by an insulator, like air or micah. When current begins to flow onto one of these plates there is a build up of electrons and a resulting negative charge. On the other plate, electrons are drawn away both by the repulsive force of the electrons on the first plate and the attractive force of the positive terminal of the voltage source. As this is happening the orbits of the electrons of the atoms in the insulator separating the two plates begin to warp, spending more time near the positively-charged plate. Energy is thus stored in the field between the plates in a way similar to how energy is stored in a compressed spring.
Batteries store energy electrochemically. As I’ve written before the simplest kind of battery consists of two electrodes made of different materials immersed in an electrolyte bath. The electrodes must be made such that one is more likely to give up electrons than the other. When a load is attached to the battery electrons flow from the ‘negative’ terminal through the load to the ‘conductor’. Unlike inductors and capacitors batteries bring all their charge to the circuit in the beginning.