The toaster is modelled as a resistor. The man is another resistor. They are in parallel. Most of the current goes through the toaster because of its small resistance, less goes through the man. The man's resistance is conditional. It is high if he is wearing well-insulated shoes; it is low if he's barefoot on a flooded kitchen floor that is in contact with the ground. In the latter case he is a small valued resistor and may well die of electrocution.

 

I don't blame you for being confused. The author of that book seems to be trying to deliberately confuse you. For one, don't take 'the path of least resistance' dogma to the extreme. If two paths are in parallel with a voltage source, they both carry current; it's just that the smaller valued resistor will carry more current than the other one.

As for the diagram, do you see that person at the top? His foot shows a ground symbol. That means he's not standing on a fiberglass ladder, but rather an aluminum ladder. I mean that ground symbol implies a path to ground and that means he'll be shocked. The artist should not have put that symbol on that guy's foot.

One last thing. Few people on this thread seem to be talking to each other. Try clicking on Display Modes then on Threaded Mode. You'll see a chart that shows who is talking to who. Once you've done it you'll see what I mean. It's a real improvement.

 

That's right. The term came about because the power company connected one lead to the ground and sent the other over a wire while the user hooked one post of his service entrance to the ground and used the wire as a hot. The term stuck even though its usage changed. There is no earth ground in a transistor radio for instance. The radio has a common node that all other voltages are referenced to and it came to be called ground. A more exact term is common.

I think that Ben Franklin and Andy Crosse knew about earths or grounds, long before any power company existed on the Hudson or anywhere else.

I also find your explanation of what a ground is somewhat lacking. What about rail referenced inputs to op amps which also have ground connections for instance?

 

A typical person has somewhere around 300K ohms of resistance. Of course humidity and perspiration play a part in this. But given the much higher resistance in the human model, much less current is flowing through that branch than the toaster. But since they are parallel paths, both have the ability to do work with more current flowing through the heater elements of the toaster because of less resistance. Both branches have the same amount of voltage across them. Simply using ohms law should paint the entire picture of what's happening. Don't mistake what I am saying, just because there is much less current flowing through you, it still has the potential to be lethal! You're not a heater element but a fluidic sack. You'll conduct just nicely.

 

You need to use threaded mode in order to see who exactly you are talking to. Click on Display Modes, then on Threaded Mode. This gives you a chart that clears the situation up immediately. Try it, look at it; you'll immediately see what I mean.

Welcome to All About Circuits.
Our local custom is to use the quote button. Please enjoy your stay.

 

I don't blame you for being confused. The author of that book seems to be trying to deliberately confuse you.

I would certainly hope he's not "trying to deliberately" confuse anyone..lol. I would believe it to be just a simple illustrative error. One that has certainly confused me, but i would give the author the benefit of the doubt. He seems to have put much effort into that e-book. I haven't gone through even near the whole thing yet, but it looks to be a really great source of electrical info. I'm just taking it one step at a time so I know I have crawling mastered before I even think about walking. But I do appreciate your concern and not holding my confusion against me.

For one, don't take 'the path of least resistance' dogma to the extreme. If two paths are in parallel with a voltage source, they both carry current; it's just that the smaller valued resistor will carry more current than the other one.

Yes this fact has been brought to my attention by Bill Marsden, Mik3, beenthere, Rp1, studiot and others. I'm working on creating a strong basic foundation of electrical rules. Resistance always following the path of least resistance was something I recently learned and I have even more recently learned that although it is true, it is an incomplete statement. I appreciate this correction.

As for the diagram, do you see that person at the top? His foot shows a ground symbol. That means he's not standing on a fiberglass ladder, but rather an aluminum ladder. I mean that ground symbol implies a path to ground and that means he'll be shocked. The artist should not have put that symbol on that guy's foot.

Thank you, that certainly clears a few things up. I will let what you explained settle in my head for a bit and then try to summerize what's going on in the diagram to see if I do in fact follow what's going on.

One last thing. Few people on this thread seem to be talking to each other. Try clicking on Display Modes then on Threaded Mode. You'll see a chart that shows who is talking to who. Once you've done it you'll see what I mean. It's a real improvement.

I apologize, I tried to view the forum in threaded mode and I think I see what your talking about. It seems like evertime I reply on the thread, it's as though i'm replying directly to the last person who posted. I don't know how to reply to where it appears as though it's a general reply directed at everyone...i don't even know if what I just said makes any sense. What I post is for everyone in general to read and that seems to be how it appears in the current linear mode. That's the only way I know how to submit replies.

Anyhow thanks for clearing up that diagram issue.

 

The first diagram is actually wrong. Without a current path there is no current, no shock.

The next two are pretty much on track.

The point the AAC eBook makes though, is while the first diagram no one is shocked the danger increases with an accidental ground, such as a tree. Then you don't know which line is dangerous.

 

If the diagram pictures are converted to let...

in this diagram...

Then the diagram can be translated as...

Then, further simplifying the branches of the circuit to...

If I am looking at the original diagram correctly where both men and the tree are connected to earth, then I'd assume this version of the diagram should be the same exact circuit as the 2 men, the tree, and the toaster above.

Now this seems like a very complicated circuit to me. It's not only a series circuit and it's not only a parallel circuit, it's some sort of complex series-parallel circuit. It's not very easy to grasp how this circuit calculates out. L seems to get the same voltage no matter what since nothing is connected in series with it. M1 and M2 seem to be connected in series. M1 and T seem to be connected in series. L and M2 seem to be connected in parallel. L and T also seem to be connected in parallel. But then T doesn't seem to be connected in parallel with M2 which looks strange. As of the moment, I can't make sense of it...but i'm going to ponder it for a few days and then come back to it.

 

You've got the basic concept down. Just remember it doesn't take much to be fatal. One of the guys is getting less than the other, but no shock is good.

 

Here is a real life example of what we are talking about with protective earthing.

http://www.vintage-radio.net/forum/showthread.php?p=197634#post197634

 

There seems to be some misconception about where the phrase "path of least resistance" comes from. The phrase is not used in the grounding section of the All About Circuits E-book. The only place the phrase is used in the e-book is in the "Ohm's Law" section of the Worksheets - and then only for clarification by example.

 

There seems to be some misconception about where the phrase "path of least resistance" comes from. The phrase is not used in the grounding section of the All About Circuits E-book. The only place the phrase is used in the e-book is in the "Ohm's Law" section of the Worksheets - and then only for clarification by example.

Right, I haven't read through that entire section of the e-book yet. I've been looking at the section that speaks about voltage drops and the section that speaks about grounds...and even there I don't recall any mention of current taking the path of least resistance either. I've heard that phrase elsewhere. That phrase is what I was told when someone was trying to explain the nature of current to me. I took it and ran with it. I've even heard others say it as though it was as exceptable as the world being flat...for example, I've heard an electrician say "people are like electricity, they take the path of least resistance...and such. So I just assumed that if there are 2 paths current can take, it will take the path with least resistance. What I believe I was neglecting was my new found understanding of parallel circuits in that just because there are two paths with different resistances, current is not stuck having to choose which path it takes. Instead, current will split up and then recombine when the branches recombine. I recently heard the term "Current In = Current Out" when discussing parallel circuits. I'm glad I found this site and forum. I feel i'm able to accomplish so much more when I can ask questions.

 

Welcome to All About Circuits.
Our local custom is to use the quote button. Please enjoy your stay.

Thanks for the welcome! Does using the quote button clear up the picture of who is talking to who when not using the threaded mode?

 

I just measured my body resistance with a DMM. You're right I had about 300K between my hands when holding the leads. But when I stuck one lead down my sock and held the other in a hand, I got 7 MegOhms! Each lead on my tough gave about 10 ohms and when I held both leads in one hand I got 235 ohms. So it really depends on where on the body you are measuring. To say 'the human body' must be qualified with 'where on the body.'

 

Franklin knew about earth ground, so you're right about the term not actually starting with power companies. His lightning rod, for example made use of a conductor being set up near a house and sunk into the ground so that it formed an easier path than the house. Nevertheless the term ground can cause confusion when dealing with electronic circuit boards that have absolutely no path to earth ground in many cases. Your op amp is an example. The circuit board uses a common point of reference and the literature usually calls it ground. But again, it is not earth ground. On a circuit board, the ground is usually a large area of copper connected to the negative supply voltage. All of the voltages are then referenced to this area and that area's voltage is called zero. A TV chassis, for example, is not ground. If you hook an earth ground to it, you'll fry some components. That chassis is a reference point.

 

@Learning Electrician.

I suggested you think about the term earth or ground and what it means.

this discussion has gone on far enough about resistance.

Earthing is not about resistance.

An earth is a body of sufficient current sourcing or sinking capacity that its potential (voltage) will not change regardless of the current flows occurring.

Thus earthing or grounding both sides (terminals) of a battery reduces both sides to the same (earth) potential.

The earth may have high, low or intermediate resistance and it is immaterial so long as its potential does not alter.

 

I just measured my body resistance with a DMM. You're right I had about 300K between my hands when holding the leads. But when I stuck one lead down my sock and held the other in a hand, I got 7 MegOhms! Each lead on my tough gave about 10 ohms and when I held both leads in one hand I got 235 ohms. So it really depends on where on the body you are measuring. To say 'the human body' must be qualified with 'where on the body.'

The where is important. If you wet your fingers and re-measured between your hands where you first got 300k, you would see a substantial drop in resistance.

That 300k was measured across your heart ... as the most direct path for the current. 1 mA, a perceptible current, would require 300V at 300k and a much lower potential when there is moisture on your hands.

That same 300 volt potential would let 30 Amps flow on your tongue ... according to your measurements.

When they talk about safety, they want you to measure any potential above 30 volts safely. Some go as far as requiring you to shut the equipment down, hook up the leads, then re-energize the equipment. Some prefer the one handed method ...

Just remember, the a$$ you save may be your own; so exercise due dilligence when making measurements.

 

Does using the quote button clear up the picture of who is talking to who when not using the threaded mode?

Yes, as exemplified by our interchange.

But when I stuck one lead down my sock and held the other in a hand, I got 7 MegOhms!

I suggest stepping barefoot on the lead for a more accurate measurement.