If the electron’s kinetic energy just before a collision is 2.0 × 10−18J 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 accelerate, hit a molecule, and kick out another electron. Then there will be four electrons. In other words, 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 bust of light.

a) The average distance an electron travels between collisions is 2.0 m. What acceleration must an electron have to gain 2.0 × 10−18???? of kinetic energy in this distance?

b) What force must act on an electron to give it the acceleration found in part a?

c) What strength electric field will exert this much force on an electron? This is the breakdown field strength.

d) Suppose a free electron in air is 1.0 cm away from a point charge. What minimum charge must this point charge have to cause a breakdown of the air in create a spark?