You sometimes create a spark when you touch a doorknob after shuffling your feet on a carpet. why? the air always has a few free electrons that have been kicked out of atoms by cosmic rays. if an electric field is present, a free electron is accelerated until it collides with an air molecule. it will transfer its kinetic energy to the molecule, then accelerate, then collide, then accelerate, collide, and so on. if the electron’s kinetic energy just before a collision is 2.0×10−18 j or more, it has sufficient energy to kick an electron out of the molecule it hits. where there was one free electron, now there are two! each of these can then accelerate, hit a molecule, and kick out another electron. then there will be four free electrons. in other words, as shows below, a sufficiently strong electric field causes a "chain reaction" of electron production. this is called a breakdown of the air. the current of moving electrons is what gives you the shock, and a spark is generated when the electrons recombine with the positive ions and give off excess energy as a burst of light.