But to throw a little at you for the questions listed, I think you need to take in 1 aspect of what the instructor is throwing at you too. This is electrical theory and basing off numbers and not actually reality. In theory you CAN have voltage without current if there is infinite resistance. IE: Open air between the 2 contacts such as your battery terminals. A prime example of this is capacitors. They use electrodes folder around each other as plates with a super thin insulator between them which makes them build up a charge. The stronger the capacitor the higher micro farads or farads they are rated. I=V/R with V of 9 and R= Infinite gives you 0 or near 0 current.
For your next question on opens and shorts. A switch acts as an open, it breaks the circuit. So you are correct in calling a switch, cut or break in a wire an open. Now a short is different in the way that, it is a non intended connection to the circuit, generally to ground but can also be phase to phase or phase to neutral. Now what happens is this makes an infinite currant to the circuit which would overload the breaker/fuse or even just the wiring itself, causing the wire to melt or equipment to cook itself. So your formula you used as an example is correct. Now as for the 9 volt battery killing you...... remember that your body has resistance, so now that 9v will be divided by the natural resistance of your body. Also the path that current takes when going through your body would determine the damage to your body. Hence why when I do work on switches and outlets live in peoples houses I only use 1 hand and touch nothing with my other hand. Shortest path between the fingers in my same hand won't bring it through vitals. Not telling people to just go do field tests of this, please don't. I am an electrician and have a very healthy respect for this stuff.
Another thing to keep in mind. It only takes .043 Amps to disturb the beating of your heart and all. The average human body resistance has been stated, an average, to be about 500 ohms. So that being said, following the right path V=IR V=.043x500=21.5V is all it would take to kill you. In the most ideal circumstances (Path and all).
Last edited by Blue64; 10-04-2017 at 12:51 AM.