In order for a spark to occur, it is necessary to ionize the air, which is not usually a conductor. One possible model for the
process by which air becomes ionized is this: if a sufficiently strong electric field were applied to an atom, it would be
possible to pull an outer electron out of the atom, leaving a positively charged ion and a free electron. We will estimate the
strength of the electric field required to pull an electron out of an atom.
(a) Consider the interaction of a single outer electron in a nitrogen atom with the “atomic core” (all the other charged
particles in the atom—the protons in the nucleus and all the other electrons). What is the net charge of the atomic
core?
(b) If the radius of the atom is approximately , what is the magnitude of the electric field due to the
atomic core at the location of the outer electron?
(c) What is the magnitude of the electric field you would have to apply in order to pull the outer electron out of the
atom?
(d) It is observed experimentally that an applied electric field of V/m is sufficient to cause a spark in air. What
is the ratio of the electric field you calculated to the observed field needed to start a spark?
(e) What should we conclude about this model?
(1) Using more significant figures would not improve the agreement.
(2) We need to think of a different physical explanation of how air gets ionized.
(3) If our calculation used more accurate values the numbers would probably agree.
(4) Since the discrepancy is so large, this model must be wrong.