In physics, J (vector) is equal to the sigma sum of all k for qk * mk * Fk (vector), where q is the charge of the k particle, m is the number of k particles in a volume, and F is the flux of the k particles through the volume. The dot product of J and the area vector is equal to I, the current. A scalar J can be defined as current/area.

Also a source of boundless merriment in Physics 16, Winter 2001 term, at Dartmouth College. As the great Professor Gleiser was talking about J, saying stuff like "J is pointing in this direction" and "J is very large", a group of cohorts and I were holding back I myself am a very large J(ay). When the Professor stopped to ask what was so funny, we told him. From that point on, whenever he was about to do something with current/area value, he'd say something to me first, such as, "Jay, I need you to stay constant in time", or when he was differentiating "J", he wrote the standard dJ/dt on the board and remarked, "Hey, Jay, you've got a second job now, you're a DJ now!" I only wish it were that much fun when we got to relativity.