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LT72884 10-04-2017 12:01 AM

Electronics help please
I am in the middle of an electrical engineering class on the fundamental Theroms(not thermos) of AC/DC circuits. However, I am having some conceptual issues, no not the pregnancy kind; the other kind.. where your brain cant seem to figure things out haha.

Here is my first question. My professor keeps saying that you can have voltage with out current... HOOOOOOOOOOOWWWWW!!!!!!

The mere fact of any amount of electrons moving from one location to another(called a charge, just like when men yell CHARGE before a battle. they are moving from one place to another) creates current because there is flow and when said electrons move from one place to another, that is voltage. that movment called voltage creates current. If I hold a 9V battery in my hand, it has the potential to push 9 volts but it aint pushin a thing until there is some wire connected to it. so how does voltage exist if there is no current. V=IR if I=0 then V=0

ok next question. I do not understand opens and shorts. To me an open is like a light switch or the wire has been cut. A short is where the path for current has been shortened by adding a wire from one end to the other. But I thought with a short, it literally removes those components out, but I think I may be wrong.. First let me ask about this example. I take a 9V battery again and I run a wire across both terminals, I know it gets red hot very quickly. now, is this creating infinite current? I=V/R and if R is practicaly 0, then current will be infinite because of I=9/0 now if that is true, is that not enough amps to kill you?

ok thanks for reading and helping

The Flounder 10-04-2017 12:39 AM

First answer. Voltage is a difference in potential. If there is not path for electron flow then there is no current.

Your second question is not infinite current because there is some resistance to the wire. As the wire heats up that resistance also increases.

Blue64 10-04-2017 12:44 AM

Pmed you.

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).

LT72884 10-04-2017 01:00 AM


Originally Posted by The Flounder (Post 3421749)
First answer. Voltage is a difference in potential. If there is not path for electron flow then there is no current.

Your second question is not infinite current because there is some resistance to the wire. As the wire heats up that resistance also increases.

so I was on the right path but so many sources say you can have voltage with out current. My professor is included as one of the sources.

for the second part, I guess infinite is assuming 0 resistance of copper wire or whatever material. but lets say since the resistance of a thin copper wire is pretty dang small, or current jumps to high amounts.

I=9volts/0.001 ohms. current becomes well, 9000amps at this stage. am I on a correct path?

@ other poster. I am currently reading your post. thanks for the info from both of you guys

Blue64 10-04-2017 01:05 AM

Yes you are on the right path with the example you put in.

Just a reference for you in regards to wire resistance. It is low but all wire has resistance, in an AC circuit you also have eddy currents in the wire that hurts it. Here is the table from the code book we use for calculating voltage drops in wire and such for wire installs.

punkncat 10-04-2017 08:44 AM


Originally Posted by Blue64 (Post 3421754)
Here is the table from the code book we use for calculating voltage drops in wire and such for wire installs.

Pretty neat.

I do LV stuff, and lucky for us (most) of the manufacturers sites have a load/equipment calculator on board now. It was a complete PITA to figure this crap by hand.

CrowsFeast 10-04-2017 09:00 AM

I think you basically get the ideas now. The one thing you seem to be struggling with is voltage without current. In terms of the equation for an open circuit V = 0 x infinity. Well 0 x infinity does some weird mathematical stuff where depending on the situation it can be anything.

Remember that voltage is potential energy (keyword potential). Imagine a bucket of water with a closed valve on the bottom. The water has plenty of potential energy, you know it's pushing against the valve, it just can't go anywhere or do anything. That's your potential energy without current.

Grendel 10-04-2017 09:57 AM

...and now the water and pipe analogies begin.... ;)

To begin with the water and pipe analogies work for very basic analog circuits like simple Power Supplies and Lighting switch networks but when you move into more complicated circuits the analogies start to breakdown quickly. So don't become too dependent those type of analogies. Spent way to many man-hours with prospective engineering officers trying to get them to unlearn them from their basic electrical/electronic classes in college.

by the way the nominal resistance of a dry healthy human body is somewhere between 300 - 1000 ohms

20 gauge solid wire has ~ resistance of 10 mohm

NYKNIGHT01 10-04-2017 10:12 AM

Geeeeez. And I thought I'd never see the power triangle here!!!
Simply put. Think of this. A battery. Has voltage but where is it going? Current is the flow. Voltage is the power behind that flow.
Back to the water. The aforementioned bucket of water holds five gallons. But until u pour it. Put a hole in it or a tube for the water to go some where...the five gallons stay there. Voltage.
Put a hole in it. The five gals will flow out...a bigger hole. Faster flow. Current.
The relationship of the volts and the current create power. But volts and current as everyone has stated ...are different. .

nofxkid2005 10-04-2017 10:50 AM

When I was learning this stuff in welding school, I loosely related it to a Sheridan valve..

where voltage = reg pressure
current = valve chamber volume

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