What "d" means...

[Michael]

Even though I got this out of an electronics book, i'm sure this is used in mechanical engineering.

The author gives current as: delta Q/ delta t (delta meaning the greek triangle thing....).

Then suddenly reduces it to dQ/dt, and uses the d through out the book (I can still understand what he usually means though; I'm pretty far in it).

What does the "d" mean? why not just use the delta?

[cwarner7_11]

"d" and delta are very similar, and sometimes used interchangeably. dQ/dt means "the change in Q with respect to time", that is, how Q changes with time. The delta symbol most commonly is used to define discrete steps, where the d symbol would indicate a continuous process, although this distinction is not necessarily universal. If you were to plot Q on the vertical axis, and t on the horizontal axis, dQ/dt would be the slope of the line. This is pretty basic differential calculus terminology.