Grounding - the Removal of a Charge
The previous three sections of Lesson 2 discussed the three
common methods of charging - charging by friction, charging by induction, and charging by
conduction. A discussion of charging would not be
complete without a discussion of uncharging. Objects with an excess of charge - either positive or negative - can
have this charge removed by a
process known as grounding. Grounding is the
process of removingthe excess charge on an object by means of
the transfer of electrons between it and another object of substantial size.
When a charged object is grounded, the excess charge is balanced by the
transfer of electrons between the charged object and a ground. A ground is simply
an object that serves as a seemingly infinite reservoir of electrons; the
ground is capable of transferring electrons to or receiving electrons from a
charged object in order to neutralize that object. In this last section of
Lesson 2, the process of grounding will be discussed.
Grounding
a Negatively Charged Object
To begin our discussion of grounding, we will consider the
grounding of a negatively charged electroscope. Any negatively charged object
has an excess of electrons. If it is to have its charge
removed, then it
will have to lose its excess electrons. Once the excess electrons are removed
from the object, there will be equal numbers of protons and electrons within
the object and it will have a balance of charge. To remove the excess of
electrons from a negatively charged electroscope, the electroscope will have to
be connected by a conducting pathway to another object that is capable of
receiving those electrons. The other object is the ground. In typical
electrostatic experiments and demonstrations, this is simply done by touching
the electroscope with one's hand. Upon contact, the excess electrons leave the
electroscope and enter the person who touches it. These excess electrons
subsequently spread about the surface of the person.
This process of grounding works because excess electrons find
each other repulsive. As is always the case, repulsive affects between
like-charged electrons forces them to look for a means of spatially separating
themselves from each other. This spatial separation is achieved by moving to a
larger object that allows a greater surface area over which to spread. Because
of the relative size of a person compared to a typical electroscope, the excess
electrons (nearly all of them) are capable of reducing the repulsive forces by
moving into the person (i.e., the ground). Like contact
charging discussed earlier, grounding is simply another
example of charge sharing between two objects. The extent to which an object is
willing to share excess charge is proportional to its size. So an effective
ground is simply an object with significant enough size to share the
overwhelming majority of excess charge.
Grounding
a Positively Charged Object
The previous discussion describes the grounding of a
negatively charged electroscope. Electrons were transferred from the 
electroscope to the ground. But what if the electroscope is positively
charged? How does electron transfer allow an object with an excess of protons
to become neutralized? To explore these questions, we will consider the
grounding of a positively charged electroscope. A positively charged
electroscope must gain electrons in order to acquire an equal number of protons
and electrons. By gaining electrons from the ground, the electroscope will have a balance of charge and therefore be
neutral. Thus, the grounding of a positively charged electroscope involves the
transfer of electrons from the ground into the
electroscope. This process works because excess positive charge on the
electroscope attracts electrons from the ground (in this case, a person). While
this may disrupt any balance of charge present on the person, the significantly
larger size of the person allows for the excess charge to distance itself
further from each other. As in the case of grounding a negatively charged
electroscope, the grounding of a positively charged electroscope involves
charge sharing. The excess positive charge is shared between the electroscope
and the ground. And once again, the extent to which an object is willing to
share excess charge is proportional to its size. The person is an effective
ground because it has enough size to share the overwhelming majority of excess
positive charge.
The
Need for a Conducting Pathway
Any object can be grounded provided that the charged atoms of
that object have a conducting pathway between the atoms and the ground. A
common lab activity involves taping two straws to a charged aluminum plate. One straw is covered with aluminum foil and the other straw is bare plastic.
When the aluminum-covered straw is touched,
the aluminum plate loses its charge. It is
grounded by means of the movement of electrons from the ground to the aluminum plate. When the plastic straw is touched,
grounding does not occur. The plastic serves as an insulator and prevents the
flow of electrons from the ground to the aluminum plate.
Grounding requires a conducting pathway between the ground and the object to be
grounded. Electrons will travel along that pathway.
Lesson 2 of this unit of The Physics Classroom has been
focused on methods of charging and discharging objects. One principle that
continually arose was the relationship between force and distance.
Check Your Understanding
Use your understanding of charge to answer the following
questions. When finished, click the button to view the answers.
1. A positively charged pop can is touched by a person
standing on the ground. The pop can subsequently becomes neutral.
The pop can becomes neutral during this process because ______.
a. electrons pass
from the pop can to the person (ground)
b. electrons pass
from the person (ground) to the pop can
c. protons pass
from the pop can to the person (ground)
d. protons pass
from the person (ground) to the pop can
Answer: B
Protons do NOT
move during electrostatic activities, so choices c and d can be ruled out. To
ground a positively charged object, electrons must be added to it in order
neutralize its excess positive charge. So electrons must move from the ground
into the pop can.
2. A physics student, standing on the ground, touches an
uncharged plastic baseball bat to a negatively charged electroscope. This will
cause ___.
a. the
electroscope to be grounded as electrons flow out of the electroscope.
b. the
electroscope to be grounded as electrons flow into the electroscope.
c. the
electroscope to be grounded as protons flow out of the electroscope.
d. the
electroscope to be grounded as protons flow into the electroscope.
e. the baseball
bat to acquire an excess of protons.
f. absolutely
nothing (or very little) to happen since the plastic bat does not conduct.
Answer: F
In order to
ground an electroscope, electrons must have a conducting pathway between the
ground and the object. In this case, a piece of plastic is part of the pathway
connecting the ground (the student) and the charged object. Since plastic is an
insulator, electrons are incapable of moving through the baseball bat. Grounding
does not occur in this instance. Were there a conducting pathway available,
choice a would be the proper choice.
3. TRUE or FALSE:
An
object that becomes grounded gains neutrons during the grounding process.
Answer: False
Neutrons are
positioned in the nucleus of an atom. And like protons, neutrons are never
transferred in electrostatic experiments. They are bound in the nucleus and
cannot escape by ordinary electrostatic methods.