1. The problem statement, all variables and given/known data Approximately how large an electric field would be required to cause a spark in a gas at standard temperature and pressure if the ionization energy for the molecule of this gas is 1.5 times the ionization energy for air molecules, and the cross-sectional area of the molecules is 0.8 times the cross-sectional area of air molecules? 2. Relevant equations eEd = U U = the amount of energy required to remove one outer electron from an oxygen or nitrogen molecule. (Air) e = 1.6E19 C d = mean free path 3. The attempt at a solution I used the equation eEd = U, and tried to solve for E. I first determined U. In the book it gives U explicitly as 2.4E-18J. Since the question asks what happens when the ionization energy is 1.5 times the regular ionization energy for air molecules, i took (2.4E-18J)(1.5) = 3.45E-18 J Next, I found d. Since one mole of gas occupies 22.4 liters, that converts to 3.72E-26m^3 per mole of gas. I then used the equation d = (vol)/(pi*r^2). The approximate radius of one air molecule is said to be 1.5*10E-10m in the text. Therefore, since the question asks what happens when the cross-sectional area of the molecule is 0.8 times its usual area, I take (1.5*10E-10m)(0.8) = 1.2E-10m. After plugging in the volume and the new radius, d comes out to be 8.223E-7 m. Now I have all the numbers I need for: eEd = U E = (3.45E-18J)/[(1.6E-19C)(8.223E-7m). E = 2.62 E 7 N/C The problem is, that answer is wrong. The real answer is 3.6E6N/C (it is given in the back of the book). Please help, at this point I am unsure if I am even using the right equations. All I know for sure is that the equations HAVE TO have ionization energy and the cross-sectional area of a molecule in them, and you have to be able to solve for the E field. Thanks in advance for any help.