A container filled with air at room temperature and atmospheric pressure is irradiated with X-rays that ionize a small portion of the molecules. Negative ions are O2 molecules that have "captured" an electron. The size of the container is 10 × 10 × 2 cm; two walls of 10 × 10 cm are made of metal, and the rest are made of insulating material. A voltage of 1000 V is applied between the conducting walls, causing a current of 1.5 μA. Assuming the number of positive and negative singly charged ions is the same, estimate the fraction of ionized gas molecules. The free path of ions is 10−7 m.