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Name: Laura O.
Status: N/A
Age: 14
Location: N/A
Country: N/A
Date: 3/1/2004


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)?"


Replies:
A voltmeter has two leads and allows you to measure the difference in potential (voltage) between two points. If you try to measure the potential of a point by placing one lead of a voltmeter at that point, you will not get a reading. Only differences in potential are meaningful. Just like gravitational potential energy. If you lift a five pound weight three feet, you increase its potential energy by 15 ft-lb. This is true no matter what its potential energy is defined to be before it was raised.

It is, nonetheless, often convenient to define a certain level, perhaps floor level, to be the height at which the weight has zero potential energy. You are free to define the zero to be anywhere since only the change is useful.

Similarly, you can define the electrical zero of potential to be where you like. It is usually most convenient to define the chassis of an electronic circuit to be at zero potential and that is then called "ground". When power is transmitted to your home, One wire is connected to the earth and is considered to be at zero electrical potential and is called "ground" (for obvious reasons).

If you touch one pin of an electrical outlet and, at the same time, a metal faucet, you will feel nothing if that pin is the grounded one (faucets make a good electrical connection to ground through the pipe bringing water into your house). If you then touch the other "hot" wire and faucet, you will get a severe electric shock due to the difference in potential. DO NOT try this at home! If you have a voltmeter it is instructive to measure the potential difference between a faucet (or any grounded metal object) and the pins of an electrical outlet -- one at a time.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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