Charging by Induction
The previous section of Lesson 2 discussed
the process of charging an object by friction or rubbing. Friction charging is
a very common method of charging an object. However, it is not the only process
by which objects become charged. In this section of Lesson 2, the charging by
induction method will be discussed. Induction charging is a method used to charge
an object without actually touching the object to any other charged object. An
understanding of charging by induction requires an understanding of the nature
of a conductor and an understanding of the polarization process. If you are not
already comfortable with these topics, you might want to familiarize yourself
them prior to reading further.
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Charging
a Two-Sphere System Using a Negatively Charged Object
One common demonstration performed in a physics classroom
involves the induction charging of two metal spheres. The metal spheres are
supported by insulating stands so that any charge acquired by the spheres
cannot travel to the ground. The spheres are
placed side by side (see diagram i. below) so as
to form a two-sphere system. Being made of metal (a conductor), electrons are
free to move between the spheres - from sphere A to sphere B and vice versa. If
a rubber balloon is charged negatively (perhaps by rubbing it with animal fur)
and brought near the spheres, electrons within the two-sphere system will be
induced to move away from the balloon. This is simply the principle that like
charges repel. Being charged negatively, the electrons are repelled by the
negatively charged balloon. And being present in a conductor, they are free to
move about the surface of the conductor. Subsequently, there is a mass
migration of electrons from sphere A to sphere B. This electron migration causes
the two-sphere system to be polarized (see diagram ii. below). Overall, the
two-sphere system is electrically neutral. Yet the movement of electrons out of
sphere A and into sphere B separates the negative charge from the positive
charge. Looking at the spheres individually, it would be accurate to say that
sphere A has an overall positive charge and sphere B has an overall negative
charge. Once the two-sphere system is polarized, sphere B is physically
separated from sphere A using the insulating stand. Having been pulled further
from the balloon, the negative charge likely redistributes itself uniformly
about sphere B (see diagram iii. below). Meanwhile, the excess positive charge
on sphere A remains located near the negatively
charged balloon, consistent with the principle that opposite charges attract.
As the balloon is pulled away, there is a uniform distribution of charge about
the surface of both spheres (see diagram iv. below). This distribution occurs
as the remaining electrons in sphere A move
across the surface of the sphere until the excess positive charge is uniformly
distributed. (This distribution of positive charge on a conductor was discussed
in detail earlier in Lesson 1.)
The
Law of Conservation of Charge
The law of conservation of charge is easily
observed in the induction charging process. Considering the example above, one
can look at the two spheres as a system. Prior to the charging process, the
overall charge of the system was zero. There were equal numbers of protons and
electrons within the two spheres. In diagram ii. above,
electrons were induced into moving from sphere A to sphere B. At this point,
the individual spheres become charged. The quantity of positive charge on
sphere A equals the quantity of negative charge on sphere B. If sphere A has
1000 units of positive charge, then sphere B has 1000 units of negative charge.
Determining the overall charge of the system is easy arithmetic; it is simply
the sum of the charges on the individual spheres.
Overall Charge of Two Spheres = +1000 units + (-1000 units) =
0 units
The overall charge on the system of two objects is the same
after the charging process as it was before the charging process. Charge is
neither created nor destroyed during this charging process; it is simply
transferred from one object to the other object in the form of electrons.
Charging
a Two-Sphere System Using a Positively Charged Object
The above examples show how a negatively charged balloon is
used to polarize a two-sphere system and ultimately charge the spheres by
induction. But what would happen to sphere A and sphere B if a positively
charged object was used to first polarize the two-sphere system? How would the
outcome be different and how would the electron movement be altered?
Consider the graphic below in which a positively charged
balloon is brought near Sphere A. The presence of the positive charge induces a
mass migration of electrons from sphere B towards (and into) sphere A. This
movement is induced by the simple principle that opposites attract. Negatively
charged electrons throughout the two-sphere system are attracted to the
positively charged balloon. This movement of electrons from sphere B to
sphere A leaves sphere B with an overall
positive charge and sphere A with an overall negative charge. The two-sphere
system has been polarized. With the positively charged balloon still held
nearby, sphere B is physically separated from sphere A. The excess positive
charge is uniformly distributed across the surface of sphere B. The excess
negative charge on sphere A remains crowded
towards the left side of the sphere, positioning itself close to the balloon.
Once the balloon is removed, electrons redistribute themselves about sphere A
until the excess negative charge is evenly distributed across the surface. In
the end, sphere A becomes charged negatively and sphere B becomes charged
positively.
This induction charging process can be used to charge a pair
of pop cans. It is a simple enough experiment to be repeated at home. Two pop
cans are mounted on Styrofoam cups using scotch tape. The cans are placed side-by-side
and a negatively charged rubber balloon (having been rubbed with animal fur) is
brought near to one of the cans. The presence of the negative charge near a can
induces electron movement from Can A to Can B (see diagram). Once the cans are
separated, the cans are charged. The type of charge on the cans can be tested
by seeing if they attract the negatively charged balloon or repel the
negatively charged balloon. Of course, we would expect that Can A (being positively charged) would attract the
negatively charged balloon and Can B (being negatively charged) should repel
the negatively charged balloon. During the process of induction charging, the
role of the balloon is to simply induce a movement of electrons from one can to
the other can. It is used to polarize the two-can system. The balloon never
does supply electrons to can A (unless your hear a
spark, indicating a lightning discharge from the balloon to the can).
The
Importance of a Ground in Induction Charging
In the charging by induction cases discussed above, the
ultimate charge on the object is never the result of electron movement from the
charged object to the originally neutral objects. The balloon never transfers
electrons to or receive electrons from the spheres; nor does the glass rod
transfer electrons to or receive electrons from the spheres. The neutral object
nearest the charged object (sphere A in these discussions) acquires its charge
from the object to which it is touched. In the above cases, the second sphere
is used to supply the electrons to sphere A or to receive electrons from sphere
A. The role of sphere B in the above examples is to serve as a supplier or
receiver of electrons in response to the object that is brought near sphere A.
In this sense, sphere B acts like a ground.
To further illustrate the importance of a ground, consider the induction charging of a single conducting sphere. Suppose
that a negatively charged rubber balloon is brought near a single sphere as
shown below (Diagram ii). The presence of the negative charge will induce
electron movement in the sphere. Since like charges repel, negative electrons
within the metal sphere will be repelled by the negatively charged balloon.
There will be a mass migration of electrons from the left side of the sphere to
the right side of the sphere causing charge within the sphere to become
polarized (Diagram ii). Once charge within the sphere has become polarized, the
sphere is touched. The touching of the sphere allows electrons to exit the sphere
and move through the hand to "the ground" (Diagram iii). It is at
this point that the sphere acquires a charge. With electrons having left the
sphere, the sphere acquires a positive charge (Diagram iv).
Once the balloon is moved away from the sphere, the excess positive charge
redistributes itself (by the movement of remaining electrons) such that the
positive charge is uniformly distributed about the sphere's surface.
There are several things to note about this example of
induction charging. First, observe that the third step of the process involves
the touching of the sphere by a person. The person serves the role of the
ground. If compared to the induction charging of a two-sphere system, the
person has simply replaced the second sphere (Sphere B). Electrons within the
sphere are repelled by the negative balloon and make an effort to distance
themselves from it in order to minimize the repulsive affects. (This distance
factor will be discussed in great detail in Lesson 3). While these electrons crowd to the right side of the sphere to
distance themselves from the negatively charged balloon, they encounter another
problem. In human terms, it could be said that the excess electrons on the
right side of the sphere not only find the balloon to be repulsive, they also find each other to be repulsive. They simply need more space to distance themselves from the balloon as
well as from each other. Quite regrettably for these electrons, they have run
out of real estate; they cannot go further than the boundary of the sphere. Too
many electrons in the same neighborhood is
not a good thing. And when the hand comes nearby, these negative electrons see
opportunity to find more real estate - a vast body of a human being into which
they can roam and subsequently distance themselves even further from each
other. It is in this sense, that the hand
and the body to which it is attached (assuming of course that the hand is
attached to a body) serve as a ground. Aground is simply
a large object that serves as an almost infinite source of electrons or sink
for electrons. A ground contains such vast space that it is the ideal object to
either receive electrons or supply electrons to whatever object needs to get
rid of them or receive them.
The second thing to note about the induction charging process
shown above is that the sphere acquires a charge opposite the balloon. This
will always be the observed case. If a negatively charged object is used to
charge a neutral object by induction, then the neutral object will acquire a
positive charge. And if a positively charged object is used to charge a neutral
object by induction, then the neutral object will acquire a negative charge. If
you understand the induction charging process, you can see why this would
always be the case. The charged object that is brought near will always repel
like charges and attract opposite charges. Either way, the object being charged
acquires a charge that is opposite the charge of the object used to induce the
charge. To further illustrate this, the diagram below shows how a positively
charged balloon will charge a sphere negatively by induction.
The Electrophorus
A commonly used lab activity that demonstrates the induction
charging method is the Electrophorus Lab. In this lab, a flat plate of foam is
rubbed with animal fur in order to impart a negative charge to the foam.
Electrons are transferred from the animal fur to the more electron-loving foam
(Diagram i.). An aluminum pie
plate is taped to a Styrofoam cup; the aluminum is
a conductor and the Styrofoam serves as an insulating handle. As the aluminum plate is brought near, electrons within
the aluminum are repelled by the negatively
charged foam plate. There is a mass migration of electrons to the rim of
the aluminum pie plate. At this point,
the aluminum pie plate is polarized, with
the negative charge located along the upper rim farthest from the foam plate
(Diagram ii.). The rim of the plate is then touched, providing a pathway from
the aluminum plate to the ground. Electrons along the rim are not only repelled by the negative foam
plate, they are also repelled by each other. So once touched, there is a mass
migration of electrons from the rim to the person touching the rim (Diagram
iii.). Being of much greater size than the aluminum pie
plate, the person provides more space for the mutually repulsive electrons. The
moment that electrons depart from the aluminum plate,
the aluminum can be considered a charged
object. Having lost electrons, the aluminum possesses
more protons than electrons and is therefore positively charged. Once the foam
plate is removed, the excess positive charge becomes distributed about the
surface of the aluminum plate in order to
minimize the overall repulsive forces between them (Diagram iv.).
The Electrophorus Lab further illustrates that when charging
a neutral object by induction, the charge imparted to the object is opposite
that of the object used to induce the charge. In this case, the foam plate was
negatively charged and the aluminum plate
became positively charged. The lab also illustrates that there is never a
transfer of electrons between the foam plate and the aluminum plate.
The aluminum plate becomes charged by a
transfer of electrons to the ground. Finally, one might note that the role of
the charged object in induction charging is to simply polarize the object being
charged. This polarization occurs as the negative foam plate repels electrons
from the near side, inducing them to move to the opposite side of the aluminum plate. The presence of the positive charge on
the bottom of the aluminumplate is the result of
the departure of electrons from that location. Protons did not move downwards
through the aluminum. The protons were always
there from the beginning; it's just that they have lost their electron
partners. Protons are fixed in place and incapable of
moving in any electrostatic experiment.
The Electroscope
Another common lab experience that illustrates the induction
charging method is the Electroscope Lab. In the Electroscope Lab, a positively
charged object such as an aluminum pie
plate is used to charge an electroscope by induction. An electroscope is a device
that is capable of detecting the presence of a charged object. It is often used
in electrostatic experiments and demonstrations in order to test for charge and
to deduce the type of charge present on an object. There are all kinds of
varieties and brands of electroscope from the gold leaf electroscope to the
needle electroscope.
While there are different types of electroscopes, the basic
operation of each is the same. The electroscope typically consists of a
conducting plate or knob, a conducting base and either a pair of conducting
leaves or a conducting needle. Since the operating parts of an electroscope are
all conducting, electrons are capable of moving from the plate or knob on the
top of the electroscope to the needle or leaves in the bottom of the
electroscope. Objects are typically touched to or held nearby the plate or
knob, thus inducing the movement of electrons into the needle or the leaves (or
from the needle/leaves to the plate/knob). The gold leaves or needle of the
electroscope are the only mobile parts. Once an excess of electrons (or a
deficiency of electrons) is present in the needle or the gold leaves, there
will be a repulsive affect between like charges causing the leaves to repel
each other or the needle to be repelled by the base that it rests upon.
Whenever this movement of the leaves/needle is observed, one can deduce that an
excess of charge - either positive or negative - is present there. It is
important to note that the movement of the leaves and needle never directly
indicate the type of charge on the electroscope; it only indicates that the
electroscope is detecting a charge.
Suppose a needle electroscope is used to demonstrate
induction charging. An aluminum pie plate
is first charged positively by the process of induction (see discussion
above). The aluminum plate
is then held above the plate of the electroscope. Since the aluminum pie plate is not touched to the electroscope,
the charge on the aluminum plate is NOT
conducted to the electroscope. Nonetheless, the aluminum pie
plate does have an affect upon the
electrons in the electroscope. The pie plate induces electrons within the
electroscope to move. Since opposites attract, a countless number of negatively
charged electrons are drawn upwards towards the top of the electroscope. Having
lost numerous electrons, the bottom of the electroscope has a temporarily
induced positive charge. Having gained electrons, the top of the electroscope
has a temporarily induced negative charge (Diagram ii. below). At this point
the electroscope is polarized; however, the overall charge of the electroscope
is neutral. The charging step then occurs as the bottom of the electroscope is
touched to the ground. Upon touching the bottom of the electroscope, electrons
enter the electroscope from the ground. One explanation of their entry is that
they are drawn into the bottom of the electroscope by the presence of the
positive charge at the bottom of the electroscope. Since opposites attract,
electrons are drawn towards the bottom of the electroscope (Diagram iii.). As
electrons enter, the needle of the electroscope is observed to return to the
neutral position. This needle movement is the result of negative electrons neutralizing the
previously positively charged needle at the bottom of the electroscope. At this
point, the electroscope has an overall negative charge. The needle does not
indicate this charge because the excess of electrons is still concentrated in
the top plate of the electroscope; they are attracted to the positively chargedaluminum pie plate that is held above the electroscope
(Diagram iv.). Once the aluminum pie
plate is pulled away, the excess of electrons in the electroscope redistribute
themselves about the conducting parts of the electroscope. As they do, numerous
excess electrons enter the needle and the base upon which the needle rests. The
presence of excess negative charged in the needle and the base causes the
needle to deflect, indicating that the electroscope has been charged (Diagram
v.).
The above discussion provides one more illustration of the
fundamental principles regarding induction charging. These fundamental
principles have been illustrated in each example of induction charging
discussed on this page. The principles are:
· The
charged object is never touched to the object being charged by induction.
· The
charged object does not transfer electrons to or receive electrons from the
object being charged.
· The
charged object serves to polarize the object being charged.
· The object
being charged is touched by a ground; electrons are transferred between the
ground and the object being charged (either into the object or out of it).
· The object
being charged ultimately receives a charge that is opposite that of the charged
object that is used to polarize it.
Check Your Understanding
Use your understanding of charge to answer the following
questions. When finished, click the button to view the answers.
1. Two neutral conducting pop cans are touching each other. A
positively charged balloon is brought near one of the cans as shown below. The
cans are separated while the balloon is nearby, as shown. After the balloon is
removed the cans are brought back together. When touching again, can X is ____.
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a.
positively charged |
b.
negatively charged |
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c. neutral |
d.
impossible to tell |
Answer: c. Neutral
When the balloon
is near, electrons leave Can Y and enter Can X. Overall, the two cans are
neutral; yet as individual cans, Can X is negatively charged and Can Y is
positively charged. When the cans are touched again, the excess electrons in
Can Y return to Can X. Once more, the overall charge on the system of two cans
is zero - the system is neutral.
2. Two neutral conducting pop cans are touching each other. A positively
charged glass rod is brought near Can X as shown below. Which of the following
occur as the glass rod approaches Can X? List all that apply.
a. Electrons jump from the glass rod to can X.
b. Electrons jump from the glass rod to can Y.
c. Electrons jump from can X to the glass rod.
d. Electrons jump from can Y to the glass rod.
e. Protons jump from the glass rod to can X.
f. Protons jump from can X to the glass rod.
g. ... nonsense! None of these occur.
Answer: g
Since contact is
not made between the glass rod and Can X, there is no transfer of electrons
between them. And of course, there is never a transfer of protons in
electrostatic experiments. The glass rod simply induces the movement of
electrons from Can Y to Can X, causing Can X to acquire a negative charge and
Can Y to acquire a positive charge.
3. TRUE or FALSE?
Two
neutral conducting pop cans are touching each other. A negatively charged
balloon is brought near Can X as shown below. As the balloon approaches Can X,
there is a movement of electrons between the balloon and
can X (in one direction or the other).
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a. TRUE |
b. FALSE |
Answer: B - False
In induction
charging, there is never a transfer of electrons between the charged object
(the balloon) and the object being charged (Can X). The electron movement
happens between the object being charged (Can X) and the ground (Can Y). In this
case, electrons would leave Can X and enter Can Y.
4. A positively charged balloon is brought near a neutral
conducting sphere as shown below. While the balloon is near, the sphere is
touched (grounded).
At this point, there is a movement of electrons. Electrons
move ____ .
a. into the sphere
from the ground (hand)
b. out of the
sphere into the ground (hand)
c. into the sphere
from the balloon
d. out of the
sphere into the balloon
e. from the ground
through the sphere to the balloon
f. from the
balloon through the sphere to the ground
g. .... nonsense! Electrons do not move at all.
Answer: A
Since the
balloon is not contacted to the sphere, electrons do NOT move between the
balloon and the sphere (ruling out choices c, d, e, and f). The presence of the
positive balloon draws electrons from ground to the sphere. This is the
principle of opposites attract.
5. Suppose that a negatively charged balloon is
used to charge an electroscope by induction. The procedural steps are described
in the educational cartoon below. On the cartoon, draw the orientation of the
needle and indicate the location and type of any excess charge in steps ii. -
v. Explain in terms of electron movement what is happening in each step.
View Answer.
6. A negatively charged balloon is brought near a neutral
conducting sphere as shown below. As it approaches, charge within the sphere
will distribute itself in a very specific manner. Which one of the diagrams
below properly depicts the distribution of charge in the sphere?
Answer: C
Since the
balloon is charged negatively, electrons in the sphere will be repelled and
move from the left side to the right side of the sphere. As a result, the left
side of the sphere will have an excess of positive charge (since it lost
electrons) and the right side will have an excess of negative charge since it
gained the electrons).
7. A positively charged piece of Styrofoam is placed on the
table. A neutral aluminum pie plate is
brought near as shown below. While held above the Styrofoam, the aluminum plate is touched (grounded).
At this point, there is a movement of electrons. Electrons
move ____ .
a. out of
the aluminum plate into the ground (hand)
b. into the aluminum plate from the ground (hand)
c. into the aluminum plate from the Styrofoam
d. out of
the aluminum plate into the Styrofoam
e. from the ground
through the aluminum plate to the Styrofoam
f. from the
Styrofoam through the aluminum plate to the
ground
g. .... nonsense! Electrons do not move at all.
Answer: B
Since the foam
is not contacted to the aluminum plate,
electrons do NOT move between the foam and the aluminum (ruling
out choices c, d, e, and f). The presence of the positively charged foam plate
draws electrons from ground to the aluminum plate.
This is the principle of opposites attract.
