Well, because I just fried a brand new SN754410 H-Bridge chip. I have a 24 volt stepper motor hooked to an Adafruit motor shield. I'm giving it 24 volts from a 'remodeled' atx power supply. The Y axis did not fry, it worked okay, the X axis snap crackle popped almost immediately. As you can see from my original question, I have no idea how to use Ohm's law. I really need to know this and this is really the first time I've had a pressing reason to learn. I could just ask what resistor to use, but I need to learn how to do this. This is like the third time I've asked this and people always tell me, but without a real world project to try it on I don't get it. This is my chance to learn it with a real world project to make it stick.
Please, help me get this.