What makes something qualify as a ground?

Question

I'm curious to what makes something qualify as a ground? I do some electrical work on my car. And the way I understand it, is that the chassis of the car is used as ground because it's such a big metal piece that the chassis doesn't build up enough charges to be considered charged. Therefore it's a 'zero potential'.

But how big does the metal object have to be, compared to the (e.g.) battery it's connected to. In other words, what makes the metal object qualify as a ground?

The reason for asking this question is because in many instructions I am told to "connect the other wire to a ground", and I keep asking myself: Well, what is a ground, and what is not?

Any large current sink with a low net charge – Voltage Spike May 13 '17 at 02:13 — Voltage Spike, May 13 '17 at 02:13

score 0 · Answer 1 · answered May 13 '17 at 00:47

Before you ask what ground is, you have to know what voltage is. Voltage is the potential difference between two points. For example you would say that the second node is 12 volts referenced to the first point, meaning that the potential of the second point is 12 volts higher than the first. The first point, to which you reference all other voltages, is ground.

Think of it this way: Your house is your starting position. Let's say the grocery store is 5 miles from your house, the school is 2 miles from your house, the bowling alley is 10 miles from your house, and the restaurant is 15 miles form your house. All of these distances are with respect to the location of your home. So, how far is your home from your home? 0 miles.

Think of each location as a different electrical node, each of which has a different voltage (in this analogy, distance) with respect to your ground (your reference, or your home).

The same goes for an electrical ground. Ground is a reference point. Nothing more, nothing less. it is simply the point to which you reference all other voltages in the circuit. It can have a voltage potential of 1000V with respect to a different point, but if you pick it as your ground, then it is your 0 volt reference. How much of a potential difference is between your reference point and the same reference point? 0 volts. This is ground.

I understand potential, what I don't understand, is what makes a metal object qualify as a ground, or in other words, what are the requirements of a metal object to be used as a ground. — Emil, May 14 '17 at 21:16
Ok so if I have, lets say a 10x10x10 cm piece of metal. Would I be able to use a multimeter and put the negative wire to the piece of metal and the positive wire to the positive side of a battery, and measure a potential difference? If not, why? If yes, would this work for an even smaller piece of metal? — Emil, May 14 '17 at 21:25
Unless the negative side of the battery was connected to the piece of metal, you would not be able to measure the potential difference between the two because the 12 volts is referenced to the negative terminal of the battery, not to the piece of metal. If you connect the piece of metal to the negative side of the battery, then they both become the same node and you will be able to measure 12 volts between the metal and the positive side of the battery. You cannot measure potential difference between two objects in completely different networks. They must have the same reference. — DerStrom8, May 15 '17 at 11:43

score 0 · Answer 2 · answered May 13 '17 at 00:48

I'm curious to what makes something qualify as a ground?

Ground is some universal conductor having potential being shared between entities. If you stand nearby another person, you share literal ground with him or her. If you connect two circuits, they must know what their grounds are so that they qualify signals between them properly.

So here we come that you need to not only consider ground, but ground in couple with another conductor which makes potential, electrons moving, and thus electrical current, and thus make circuits work.

chassis of the car is used as ground because it's such a big metal piece

It is referred as ground because other entities, including driver, share it.

Therefore it's a 'zero potential'.

Potential is relative. To have potential you need another wire.

But how big does the metal object have to be, compared to the (e.g.) battery it's connected to.

It must not be big, it mist be accessible. There're circuits which have negative wire as ground, some have positive wire as ground.

what makes the metal object qualify as a ground?

Designer of the electronic device, and regulating authorities (safety and legal documentation).

Well, what is a ground, and what is not?

There're special symbols for ground in circuit design; if you do not have circuit or unsure, it would be fairly logical to ask: "where is the ground?" :)

What makes something qualify as a ground?

2 Answers2