Hello Forum,
a question about the Shock current path tutorial:

in 3rd figure where the person is not shocked and the bird is not shocked the person is grounded....
Surely current can flow in a path with no potential difference. Current in fact flows in the wire (which is assume to be an equipotential conductor). Why does the current not split and some current go through the ground to the person, and then into the resistor?

IS it because the path through ground has an enormous resistance compared to the wire?

In the 2nd figure the person gets shocked....the only difference is a potential different now. The path from the current is still through earth which offers a larger resistance compared to the wire.....

thanks
fisico

 

You should include links when you are referring to a page.
The tutorial is here:
http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

in 3rd figure where the person is not shocked and the bird is not shocked the person is grounded....

This image:

Surely current can flow in a path with no potential difference.

If the voltage between two arbitrary points in a conductor is zero, there will be no measurable current flow between the two points. You need a difference in voltage for current to flow from one point to another.

Current in fact flows in the wire (which is assume to be an equipotential conductor). Why does the current not split and some current go through the ground to the person, and then into the resistor?

IS it because the path through ground has an enormous resistance compared to the wire?

In a schematic, all points that are indicated as ground, are electrically connected to each other, and they are all at 0v potential. They are considered as a single point.

In a real circuit, conductors have a small amount of resistance. If there is current flow through the load (the resistor), then there will be a difference in voltage between the lower connection of the resistor to the wire leading to ground, and ground itself. However, the difference will be relatively small by design of the circuit; less than a couple of volts. In a modern electrical home wiring system, you would not see any current in the ground wire, as it is for safety rather than power transfer. The neutral wire is the power return wire. The ground wire is there to protect humans.

In the 2nd figure the person gets shocked....the only difference is a potential different now. The path from the current is still through earth which offers a larger resistance compared to the wire.....

This is the image you are referring to:

What you are not considering is that there is a large voltage drop across the load. The voltage on the "hot" high side of the load is still dangerous, and although the resistance of dirt is higher than a copper wire, it is still surprisingly good due to the moisture and mineral content. It is all too easy to get a lethal shock that way.

 

Unless we are talking of superconductors, current flow always requires a potential difference to drive it, although the pds in some parts of a circuit may be very small. A real wire is thus never a true equipotential.

That said, in the first diagram you refer to the current does divide (inversely) according to the path resistances represented by the ground wire and the human body. Normally the wire resistance will be a matter of ohms or less, typically many thousands of times the body's resistance, except in very unfavourable conditions ( e.g. barefoot and standing in in sea-water). In less extreme conditions the human will not pass enough current to be shocked.

Another way to look at this is that the current multiplied by resistance of the grounded cable does not result in a big enough voltage at the ground end of the load to present a shock hazard.

In the case where the man is shocked, there is a large potential difference between the wire and ground, not just a little cable drop. Here there is plenty of potential to push current through his body resistance, and the poor fellow gets a belt.

Edit: Once again, somebody beats my geriatric typing, and with a clearer illustrated example...

 

In the first picture (person not shocked), current is not flowing from the ground to the person, since the person is at the same potential as the ground.

But current is surely flowing from ground to the lower terminal of the resistor through the wire even if there is no potential difference even along that wire....
It seems that both the wire from the negative terminal of the battery to the lower terminal of the resistor is an equipotential conductor...

thanks