In an electric car, on the other hand, the battery is the tank. The source of energy or power that it stores is made up of electrons, nearly massless elemental particles that can move around at the speed of light exactly in accordance with Einstein’s Theory of Relativity. In reality, though, the resistance inside the lines slows the power down somewhat, but it nevertheless travels much faster than either liquid or gaseous fuels. This difference explains why an electric motor can reach the desired speed in fractions of a second. As long as the voltage is constant, the mechanical energy generated is solely dependent on how many electrons it generates in the windings of the electric motor.