why isn't positive terminal of a battery used for grounding?

Question

i'm really confused by the concept of both conventional current and electron flow. Since electrons flow from negative terminal to positive terminal, shouldn't positive terminal be used for grounding?

Answer 1

Obligatory XKCD

There is benefits of grounding what we reference as positive. Telephone systems work on -48V for a very good reason.

http://archive.digivation.net/2006/08/11/why-telephones-use-negative-forty-eight-volts-dc/

The negative polarity is much more elusive, but can be summed up in one word: corrosion. Thanks to a bit of research performed by Sir Humphry Davy for the British Navy, we have a technology known as “cathodic protection.” First developed to keep the copper hulls of British naval ships from corroding, this technology has been applied to protecting everything from oilrigs to gas pipelines to telephony cabinets. By keeping the cabinet frame at a more positive voltage than ground, corrosion is reduced and the life of the equipment is increased. Who would have guessed?

Answer 2

Ground is an arbitrarily chosen reference point in a circuit.

Voltages are not absolute, they are a measure of the difference in potential between two points, and as such we need some reference. This is what ground is. All other voltages are usually described in reference to this point in the circuit. A +5V supply would be 5V higher in potential than ground, whereas a -15V supply would be 15V below ground.

You could use the +ve terminal of a battery as the reference point. In fact some cars do in fact do this - it is so called a positive ground because the chassis of the car is wired to the +ve terminal.

In terms of why we use ground as the negative most point typically, some of this comes from history, when a certain someone who didn't know about electron flow decided that current flows from positive to negative - so called conventional current.

Furthermore, in modern electronics, the choice persisting is actually a good thing. NMOS transistors have a much higher electron mobility compared with PMOS transistors, so for circuits which use things like half or full H-bridges (DC-DC converters, motor drivers, etc.), it is actually much more efficient to use an NMOS topology as they can handle higher currents and have lower resistance.

Why does the choice of where ground is affect this? Well, NMOS transistors work best when their source is at the lowest potential. In practice it can be easiest to make the low side of this ground for calculation purposes, and as such the negative terminal is labelled ground. But again, this is only really for mathematical/calculation purposes.

So are their any other systems which don't use the most negative potential as ground? Indeed there are many. A large number of amplifier designs you will see have split supply rails - they have both a +ve and a -ve supply rail, with the ground point chosen to be in the middle. For example you might have a +15V supply, and a -15V supply, with ground being the 0V reference. This is again done purely for calculation reasons. There is no reason you could connect the negative supply as ground, the original ground would then be +15V, and the original +15V would become +30V.

TL;DR; It is due to a mixture of historical, conventional, and calculation purposes, and not for any physical purposes. The point used to indicate ground is arbitrary.

Answer 3

Grounding is more to do with what is the most convenient point to tie together that also happens to be common with one side of the power supply.

Vacuum tubes naturally favored the negative side since the heaters and cathodes should be close together in voltage (in some early tubes they were one and the same) and the positive plate voltage is hundreds of volts higher. More importantly, the grids are referenced to cathode, not plate. This actually does have to do with the electrons being negatively charged, since you want the opposite polarity to attract them away from near ground.

If you apply a signal to a vacuum tube, NPN transistor, N-channel MOSFET or JFET, IGBT etc. it will be relative to the negative supply rail. If you do not use that as a common for the circuit input then the power supply ripple and noise will be added to the signal, which is very undesirable.

For a brief time, germanium PNP transistors were more common than NPN, which favored grounding the positive, but that turned around quickly since PNP transistors have an inherent disadvantage to NPN due to carrier mobility. Physics again.

In relatively modern times, there are a number of devices that are unobtanium in the complementary form- IGBT transistors, depletion mode power MOSFETs etc.

It also helps that the negative ground defacto standard has been adopted for wall wart power supplies- a decade or two ago, negative barrel types were seen far more often, which led to the occasional demise of a device when the wrong polarity, but right voltage, supply was connected.

One disadvantage of a negative ground is seen in automobiles where the chassis is used as a return for many different loads saving one of the high-current wires for each load. To switch the devices and retain that return path, a high side switch is used, and it has to use a PNP or P-channel type device if it's an electronic type.

Most logic chips use a negative ground, but some high speed parts (ECL types) use a positive ground and a -5.2V supply. This is because ECL chips are relatively immune to noise on the negative rail compared to the positive rail, due to the use of NPN BJTs in the chips. The reversal in logical choice is a result of emitter coupling rather than collector coupling used in RTL, DTL, TTL, etc.

Answer 4

Since electrons flow from negative terminal to positive terminal, shouldn't positive terminal be used for grounding?

What about a battery powered AM radio receiver? It doesn't require grounding for it to work. Grounding is for other reasons and nothing to do with polarity of the power source.

What about a hearing aid - is this grounded?

What about any AC appliance - the AC voltage rises positively then negatively at 50 or 60 times a second - how would you work out grounding rules based on your assumption?