Ask A Scientist Physics Archive

name         Laura O.
age          14

Question -   I am a grade 9 student and we were given definitions in class based on our unit 
Electricity and one of the definitions is "grounding." Can you please send me the definition 
that is a bit more understandable than "zero potential (whatever that is)?"
-----------------
Laura,

Please forgive me if I sound a little remedial with this explanation.  I just do not know how 
much you know about electricity.

Yes, that one used to confuse me too.  What does "potential" mean anyway?

For example:  Consider an automobile.  Like any other auto., this one has a battery.  It also 
has a ( + ) terminal and a ( - ) terminal.  Electrical current flows from the ( + ) terminal 
to the ( - ) terminal when a an electrical circuit is applied (car stereo, fans, pumps, etc...).
Why? Because there is a POTENTIAL VOLTAGE between the two terminals.  In a car it is about 
12.6 Volts.  This electrical potential is just like the potential energy of water falling 
from the top of Niagara Falls ( GROUND ).  You can also think of "GROUNDING" as the lowest 
point of voltage of a particular electrical system....Hence ... "zero potential".

Usually when someone "grounds something out" or "grounds an appliance" etc... they are 
referring to attaching the outer electrical frame of the appliance, or the "whatever" to an 
electrical wire that will, in the end, be tied in with the "electrical ground" of your house
or your car or whatever application it may be used in.  The term "grounding" is used very 
loosely among electricians and electrical engineers.

If I have done nothing but totally confused you, just try to remember that the act of 
GROUNDING is to electrically attach that which you are grounding to the lowest "electrical 
potential" ...mostly referred to as 0 Volt (zero) potential.  This is done sometimes just
to complete a circuit.  It is also done to some household appliances to prevent electrical 
shock.

I hope I have been of some help to you.

Thanks for the question and thanks for using NEWTON AAS.

-Darin Wagner
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People have decided to label the electrical voltage of the earth, the dirt under your feet as 0 
volts, and it is often called ground
potential.  You have to put the zero point somewhere, this is a good place to do it.

So all other voltages are relative to this. You have to understand that a "voltage" or 
electrical "potential" really means "How far am I from somewhere else that we call the 
reference".  Its like saying you are at an altitude of 100 feet. What you really mean is 
100 feet off of the 0 foot point, the ground.  When you say a wall outlet has a voltage of 
110 volts (AC but ignore this), you mean that the potential, the voltage, of this wire is 
110 volts (the correct unit) away from 0 volts, which again is assigned to the dirt under 
your feet.  You say "the voltage at one end of a battery is 6 volts relative to the other 
end".  You say "The wall voltage is 110 volts relative to the ground".  You say "I am 
standing 10 feet up, relative to the earth" -- same concept.

OK that being said, the subject of grounding comes up.  And it is often tied to electrical 
safety, but shows up other way too.  People like to make sure that nowhere in your house or 
a factory is there an exposed point that has a very high voltage (relative to what?  relative 
to ground!).  Because a person could touch that point, while their feet are electrically 
touching the ground, the dirt.  If they did, then as we just said, a large voltage would be 
across their body, driving a large current, and shocking them, hurting them.

The way this can actually be done, to keep you safe, is to make sure there is  good electrical
connection from the outside of the box, the handle of the tool, whatever the person touches, 
to the earth, to the dirt at your feet.  The box is then "grounded".

That is the basic idea.  The rest of what I will say is just interesting stuff, but you can 
ignore me if you wish!

Often the best ground in your house, the best point tied electrically to the earth, are your 
water pipes.  Some people for example might tie the pole of their TV antenna (they do not 
have cable!) to their water pipe, which in turn goes into the earth.  If you are an electric 
power company, you take grounding very seriously.  You just do not go drive a stake into the 
dirt, and call that ground.  You lay down a mesh of wire, all around your power plant, or 
your transmission lines.  You put in lots of metal stakes into the actual earth, trying to 
get the best possible "connection".  There are books on how to do all this, that electricians 
have to understand, national safety codes etc.  Oh, and of course lightning strikes, it is 
trying to send its electrical current also to this mysterious "ground".

Steve Ross
=====================================================
Electrical potentials (voltages) are always "differences". The term "ground" or "grounding" 
comes from the fact that the earth because of its size relative to other things you might 
have around your lab has a GIGANTIC capacity to accept (or donate) electrons in response to 
a difference in voltage of some other device -- hence the term "ground". So in most cases 
whatever is attached to the earth (ground) is assigned a potential (voltage) of zero -- but 
strictly speaking it is relative.

Vince Calder
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