Electrical – What makes something qualify as a ground?


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?

Best Answer

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.