How does earth ground complete the circuit?

Question

I just started reading a book on electronics and currently reading some concepts of earth ground. I came across a section that mentions practices about grounding as follows:

The ground terminal of the supply is tied to case of the instrument, which in turn is wired to the mains earth ground system. A common mistake for a novice to make is to attempt to power a load, such as a lamp, using the positive and ground terminals of the supply. This, however, doesn't complete a current return path to the energy source (supply), so no current will flow from the source; hence, the load current will be zero. The correct procedure, of course, is to either connect the load between the positive and negative terminals directly, thus creating a floating load, or using a jumper wire between ground and negative supply, create a grounded load.

Quote from Chapter 2, Theory of Practical Electronics for Inventors 3rd Edition by Paul Scherz

My question is:
If the circuit doesn't complete just using positive and ground terminals of the supply, how does it complete then in the case when the ground is actually used correctly (in case of a accidental shock etc.)? When the hot wire comes in contact with the chasis, the electrons flow from the less resistive path (instead of passing from the body) to the ground. Why can't they flow then when one end is connected to a positive terminal and the other to a ground terminal?

Answer 1

Where is that quote from? It is self-contradictory, and pretty much just plain wrong.

Current will flow if a load is connected between a generic power supply positive and negative, no ground connection is needed, unless you have some special purpose power supply.

A ground connection is often used for safety, especially in line non-isolated supplies, or as a means to reduce noise. Even so, not all supplies pass the ground on to the output terminals or even make it available (for common examples, think wall-warts and such with just a two pole output). Many supplies don't even have an incoming ground terminal from the mains (small switchers, and again, wall-warts).

I was going to post this as a comment, but it seemed to cover the question.

Edit as per comment below and corrected text.

Well, that does change things. Generally, a DC power supply has no output tied to ground, or connected to the supply electrically at all, so connecting a load between an output and earth ground would result in no current flow. Supplies that have an output set of terminals and a separate ground can often be configured (by jumping ground to one of the output terminals) as positive ground or negative ground if desired.

Answer 2

I read your description, over and again but fail to understand what it said!

After reading again with "Quote from Chapter 2, Theory of Practical Electronics for Inventors 3rd Edition by Paul Scherz" what I understand is auther was describing 'Electronics' device, which mean its possibly describing power load connection of some device which operated on DC power.

If you connect any two phase AC power operated load with live wire from AC supply and other terminal directly with proper earth ground ( conducting material burried on ground), it will operate without any problem. Allthough it might create high power consumtion reading on bill, or trip circuit breacker of common home installation to prevent overload / error conndition etc.

DC power operated device must have backward path for current to same source. To make electric current flow ether in or out from a DC power source, you have to provide opposite path to same source. Connecting to diffrent source (Earth as negative for this case) there will be no 'voltage/potential difference' between two point and no current will flow. Connection with earth ground insted of source own negative terminal will act same like connecting one positive terminal from one battery and other negative terminal from other different battery.