Charging by induction is a fundamental concept in static electricity, explaining how neutral objects can acquire a charge without direct contact. This section details the mechanisms of temporary and permanent charging, illustrated with examples such as the interaction between a negatively charged balloon and a neutral wall. Practical applications, including electrostatic speakers and forensic techniques like electrostatic lifting, are also discussed. Ideal for students studying physics or preparing for exams in static electricity, this content provides clear explanations and diagrams to enhance understanding.

Key Points

  • Explains the process of charging by induction, including temporary and permanent methods.
  • Illustrates examples such as the interaction between charged balloons and neutral objects.
  • Covers practical applications like electrostatic speakers and forensic techniques.
  • Includes diagrams to support understanding of static electricity concepts.
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11.6
Charging by Induction
You have learned that objects can be charged by conduction when they
come in contact with a charged object. However, the same charged object
can also be used to charge a neutral object without contact. Th is process is
called charging by induction. Objects can be temporarily or permanently
charged by induction.
Charging Objects Temporarily by Induction
Recall from Section 11.1 that when a charged object is brought near a
neutral object it causes (induces) the electrons to shift in position, resulting
in an uneven distribution of charges. Th is will only be temporary as the
electrons will move back to their original positions once the charged object
is taken away.
Figure 1 shows a negatively charged balloon that is brought near a neutral
wall. Th e electrons in the balloon repel the electrons in the wall, causing
an induced charge separation in the wall (the electrons in the wall move
away from the balloon). Th is creates a positive charge on the surface of the
wall, which the negatively charged balloon is attracted to. Th e result is that
the balloon moves toward the wall. Th e wall remains neutral because it still
contains the same number of positive charges as negative charges.
charging by induction charging a
neutral object by bringing another
charged object close to, but not
touching, the neutral object
An everyday example of charging by induction occurs with the buildup
of dust on the screen of a television or computer monitor (Figure 2). When
a computer monitor or television screen is turned on it begins to build up a
charge. When a neutral dust particle comes near the screen, the charge on
the screen induces an opposite charge on the near side of the dust particle
and a charge, similar to that on the screen, on the far side. Th e result is that
the dust is attracted to the screen.
–
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electrons in
the wall move
away from
balloon
attraction
this side
of the wall
is negatively
charged
this side of the wall
is positively charged
neutral wall
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Figure 1 A negatively charged balloon (a) is brought near a neutral wall, causing an induced
charge separation in the wall. (b) The wall becomes temporarily charged by induction.
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–
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+
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negatively
charged balloon
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+
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neutral wall
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–
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negatively
charged balloon
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neutral wall
(a)
(b)
Figure 2 After being charged by
induction, the dust is attracted to
the computer screen.
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486 Chapter 11 • Static Electricity
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6646_Sci9_ch11_pp478-501.indd 486 8/3/09 2:20:39 PM
Charging Objects Permanently by Induction
An object can be permanently charged by induction by grounding the
object. For example, consider a negatively charged ebonite rod and a neutral
pith ball. When the rod is brought near but not touching the pith ball, the
electrons in the pith ball are repelled by the electrons in the rod. As a result,
the side of the pith ball closest to the rod becomes temporarily positively
charged, while the side farthest from the rod becomes temporarily negatively
charged (Figure 3(a)). If you then ground the negatively charged side with
your hand, some of the electrons travel from the pith ball into your hand,
and the pith ball is left with a positive charge. You could also remove the
electrons by connecting a conducting wire to the ground (Figure 3(b)).
When the conducting wire is disconnected from the pith ball, the pith ball
is left with a permanent positive charge (Figure 3(c)). For the charge to
be permanent, the ground must be disconnected or removed before the
charged object is removed.
Charging by induction always results in two objects with opposite
charges. Th e object that induces the charge keeps its original charge, while
the object whose charge was induced receives the opposite charge.
To learn more about charging by
induction,
GO TO NELSON SCIENCE
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(a) (b) (c)
Figure 3 (a) When a negatively charged ebonite rod is brought near a neutral pith ball, the
electrons in the pith ball are repelled and it becomes temporarily negatively charged on its right
side. (b) Attaching a ground wire to the pith ball conducts the repelled electrons on the right side
into the ground. (c) After removing the ground wire, the pith ball remains permanently positively
charged.
SKILLS: Predicting, Observing, Communicating
You have read about charge interactions between solid objects.
In this activity, you will observe charge interactions between a
solid and a liquid.
Equipment and Materials: faucet; balloon
1. Blow up the balloon.
2. Run a gentle stream of water from a faucet. Place the balloon
beside, but not touching, the stream of water. Record your
observations in a diagram.
3. Rub the balloon against your hair to charge it.
4. Predict what you think will happen when you bring the charged
balloon near, but not touching, the stream of water. Test your
predictions and use a diagram to record your observations.
5. Try moving the charged balloon to the other side of the
stream of water. Does the same thing happen?
A. Does your observation from step 2 prove that both the
balloon and the water are neutral? Explain.
T/I
B. Use your knowledge of electrons to explain your observations
in step 4.
T/I
C. Predict whether it is possible to bend water away from a
charged object. Explain your reasoning.
T/I
TTRY THIS
BENDING WATER
SKILLS HANDBOOK
3.B.3., 3.B.6., 3.B.9.
11.6 Charging by Induction 487
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Electrostatic Speakers
You have probably already heard the results of one application of charging by
induction—that of electrostatic loudspeakers (Figure 5). Th ese speakers are
constructed of three thin layers. Th e outer two layers, called stators, are fi xed
in place and are made of a porous material. Th e inner layer is a fl exible fi lm
called the diaphragm. In order to produce sound, the diaphragm must vibrate.
Th is is accomplished using the principles of induction and the Law of Electric
Charges. First, the inner surface is given a permanent electrical charge. Th en
an audio transformer is used to induce opposite charges in the two outer
plates. Th is causes the diaphragm to move—as it is simultaneously attracted to
one outer plate and repelled by the other (Figure 6(a)). Th e audio transformer
then rapidly induces the static charges in the outer plates to reverse themselves.
Th is causes the diaphragm to now move toward the opposite outer plate
(Figure 6(b)). Th is reversing of charges on the outer plates happens repeatedly
and at variable frequencies causing the diaphragm to rapidly vibrate back and
forth between the plates, producing sound waves in the air.
Electrostatic speakers have the advantage of being extremely thin and
light weight. A disadvantage is their poor bass response.
Figure 6 (a) In an electrostatic speaker, an audio transformer induces opposite electrical charges
on two outer plates (stators) and then (b) reverses them. This process is repeated rapidly causing
a fl exible and charged inner membrane (diaphragm) to vibrate back and forth between the plates,
producing sound waves in the air.
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audio
transformer
+++++++
– – – – –– –
– – – –– – ––
+++++++
– –
– –
diaphragm stator
sound waves in air
+
+
+
++
+
+
+
(a)
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+++++++
– –
– –
+++++++
– – ––––– –
– – –– – –
audio
transformer
+
+
+
++
+
+
+
+
sound waves in air
(b)
Technological Applications of Charging
by Induction
Charging by induction has many useful applications, including forensics and
air-cleaning technologies.
Electrostatic Lifting Apparatus
Footprints are oft en left behind at crime scenes. Investigators can use this
important evidence to help determine who was present at the time of the
crime. But how can you make a copy of a footprint if it is very diffi cult to
see? Investigators use an electrostatic lift ing apparatus (ESLA) (Figure 4).
Special fi lm or foil is placed over the footprint. Th e black side of the fi lm is
placed over the footprint. Th e fi lm is then electrostatically charged. Th e dust
and dirt particles from the footprint are attracted to the black side of the
fi lm. Th e dust particles “jump” off the fl oor onto the black fi lm, revealing the
details of the footprint. Now investigators have a copy of the footprint on the
fi lm that they can take to a laboratory to analyze.
To learn more about being a
forensic investigator,
GO TO NELSON SCIENCE
Figure 4 An electrostatic lifting
apparatus is used in forensics to create
a copy of a footprint from a crime scene.
Figure 5 Electrostatic loudspeakers
operate on the principles of induction.
488 Chapter 11 • Static Electricity
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FAQs

What is charging by induction and how does it work?
Charging by induction is a process where a charged object influences the distribution of charges in a neutral object without direct contact. When a charged object is brought near a neutral object, it causes the electrons in the neutral object to shift, creating an uneven distribution of charge. This can result in a temporary charge separation, which disappears once the charged object is removed. For permanent charging, grounding the neutral object while the charged object is nearby allows electrons to flow away, leaving the object with a permanent charge.
What are the differences between temporary and permanent charging by induction?
Temporary charging by induction occurs when a charged object is brought near a neutral object, causing a shift in charge distribution that lasts only as long as the charged object is nearby. In contrast, permanent charging requires grounding the neutral object while the charged object is present, allowing electrons to leave or enter the neutral object, resulting in a lasting charge. This distinction is crucial for understanding how different applications of induction work in practical scenarios.
How do electrostatic speakers utilize charging by induction?
Electrostatic speakers operate on the principles of charging by induction, using a charged diaphragm that vibrates between two stators. The inner diaphragm is given a permanent charge, while the outer plates are alternately charged with opposite charges through an audio transformer. This rapid switching of charges causes the diaphragm to move back and forth, producing sound waves. The design allows for a lightweight and thin speaker, although it may have limitations in bass response.
What role does grounding play in permanent charging by induction?
Grounding is essential for achieving permanent charging by induction. When a charged object is brought near a neutral object, grounding allows excess electrons to flow away from the neutral object, resulting in a permanent charge. For instance, if a negatively charged rod is brought near a neutral pith ball and the negatively charged side is grounded, electrons will leave the pith ball, leaving it positively charged. This process must be completed before the charged object is removed to ensure the charge remains.
What is an example of charging by induction in everyday life?
A common example of charging by induction is the attraction of dust particles to a charged television or computer screen. When the screen is turned on, it builds up a charge that induces an opposite charge on the near side of neutral dust particles. This causes the dust to be attracted to the screen, demonstrating how induction can lead to observable effects in everyday situations.

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