Automotive Electricity 101 Question

[Mike Murrell]

Voltage - electrical pressure
Current - flow
Resistance - created by a consumer, break in path; resistor.

Resistance is fairly straightforward. It would seem that there's a relationship between voltage and current. One volt pushes one amp thru one ohm.

Familiar with the "water pipe" scenario used to explain the difference between voltage and amperage(current).

In automotive diagnosis, one sees that tests are required for any one of the aforementioned, depending on the device/situation. Again, an ohm(resistance) test would be carried out to find a break in a circuit. Easy enough.

I'm trying to get a better grasp on voltage vs. current. Some auto tests say to test for a given voltage. Others say test for a given amt. of current. Is there a fundamental concept/rule that decides which of the two one tests for?

Thanks for your time.