Grade 9 Academic Science – Unit 4 Physics (Electricity)
Static Electricity - Contact and Induction
Section 11.2 Pages 472-477
Section 11.4 Pages 480-482
Section 11.6 Pages 486-489
Charging by Contact
 Charging Objects by Friction
 The transfer of electrons between two neutral objects
(…made from different materials) that occurs when
they are rubbed together or come in contract (touch).
In other words, an electric charge is passed or
transferred from one object to the other object. If the
object attracts electrons, the object becomes
negatively charged. The object giving away
electrons becomes positively charged.

Electrostatic Series (…or triboelectric series…) is a
list of materials in order of increasing tendency to
gain electrons (see Figure). Air and human skin give
away electrons to become positively charged, while
silicon and Teflon strongly attracted electrons to
become negatively charged.
To use the Electrostatic Series, see
Sample Problem 1 on Page 473 of
your text. Practice: You have a
rubber balloon. You rub the balloon with some cotton. What is
the charge of each material?
TASK – Complete “Try This – Charging by Friction” on Page 473.

Charging Objects by Conduction
 Charging an object by contact with a charged object. That is, two objects with
different electric charges come in contact and electrons move from one object to
the other object.
 An electrically neutral object is charged by
conduction when a charged object comes
into contact with it. For example when a
rod (that has an excess of electrons)
touches a neutral ball the charge
distributes itself over both objects. When
they are separated, the ball will now be
electrically charged.
 NOTE: Charging by Conduction does not require a charged and neutral object.
Two charged objects can transfer electrons by conduction. In this case,
electrons ALWAYS move from the object with the greater negative charge to the
object with the smaller negative charge. Why?
 See Figure 5 on Page 474 of your textbook.

Grounding
 Objects with an excessive charge (+ or -) can have the excess charge remove by
grounding.
 Grounding is removing an excess charge by transferring electrons between an
object and a large neutral object (e.g., Earth).


If an object is positively charged, electrons travel from the ground into the object
until the object is neutral.
If the object is negatively charged, electrons travel from the object to the ground
until the object is neutral.
Electrostatic Precipitators (…or scrubbers…) (See Page 476)
 When generating electricity
by burning hydrocarbons
(e.g., coal, gas, oil), small
particles of soot, dust and
other substances are
released.
 To reduce emissions, an
electrostatic precipitator can
be used. (1) Smoke
containing particles passes
by negatively charged plates
and, via conduction, the
smoke (…and the particles…)
adds electrons and becomes
negatively charged. (2) The
smoke and particles pass by
positively charged plates, and
the negative particles are
attracted to the opposite
charge on the plates. (3) The
particles fall onto a collection plate for removal and “clean air” is released. Almost 99% of
the particles in a smokestack can be removed by electrostatic precipitators
Questions
 Page 477, Questions 1-3, 5-6
Conductors
 Materials that permit electrons to flow freely from atom to atom and molecule to molecule.
An object made of a conducting material will permit charge to be transferred across the
entire surface of the object.
 Familiar conductors are copper and aluminum.
 Graphite and silicon are SEMI-CONDUCTORS because electrons can move through
these substances but not effectively (see Table 1 on Page 480)
Insulators
 Materials that impede the free flow of electrons from atom to atom and molecule to
molecule. If charge is transferred to an insulator at a given location, the excess charge
will remain at the initial location of charging. The particles of the insulator do not permit
the free flow of electrons; subsequently, charge is seldom distributed evenly across the
surface of an insulator.
 Good insulators are oil, fur, pure water, ebonite, paper, wool and rubber.
Laser Printers
 Laser printers use conductors and insulators AND the Law of Electric Charges.
 A laser printer is a drum made of positively charged photoconductor (….a class of
conductor that conducts electrons only when a light shines on it – otherwise, the drum
remains an insulator). The light is a laser. The laser light “draws” the image across the
drum causing these sections of the drum to become negatively charged. The drum is
rolled across positively charged toner particles that are attracted to the negative charge.
Next, the drum is rolled across a piece of paper that has been given a larger negative
charge than the drum. Consequently, the toner particles are attracted to the paper. The
paper is heated by a fuser (over 200OC) that melts the toner particles to the paper.
Questions
 Page 482, Questions 1, 3-4, 7
Charging by Induction
 Charging a neutral object by bringing another charged object close to, but not touching,
the neutral object
 Charging by Induction always results in two objects with opposite charges. Moreover, the
object that induces the charge keeps its original charge, while the object whose charge
was induced receives the opposite charge
 Charging Objects Temporarily
 When a charged object is brought near a neutral object, the neutral object
becomes charged when the electrons shift position. This is temporary. When
the charged object is removed, the electrons shift back to their original position
(see Figure 1 on Page 486).
 Do activity “Try This – Bending Water” on Page 487
 Charging Objects Permanently
 An object can be permanently charged by induction by grounding the object.

As shown above, a negatively charged rod is brought close to an electrically neutral
ball. The electrons on the ball are repelled and move to the opposite side of the ball.
By touching the negative side of the ball, the electrons are then "earthed" off. In other
words the electrons flow from the ball leaving it with a net positive charge. The rod is
then removed leaving a permanently positively charged ball.
Questions
 Page 489, Questions 2-3, 6
In the context of electricity, polarization is the process of separating opposite charges within an
object. The positive charge becomes separated from the negative charge.
1. The diagram depicts the distribution of
charge over the surface of two spheres.
One sphere is copper and the other
sphere is rubber. Label which is which
and support your answer with one reason.
2. Which of the following materials are likely to exhibit more conductive properties than
insulating properties? Provide one reason for your answers.
a. Rubber
b. Aluminum
c. Silver
d. Plastic
e. Wet human skin
3. Suppose that a conducting sphere is charged positively by some method. The charge is
initially deposited on the left side of the sphere. Yet because the object is conductive, the
charge spreads uniformly throughout the surface of the sphere. The uniform distribution
of charge is explained by the fact that ____.
a. The charged atoms at the location of charge move throughout the surface of
the sphere
b. The excess protons move from the location of charge to the rest of the
sphere
c. Excess electrons from the rest of the sphere are attracted towards the
excess protons
Induction charging is a method used to charge an object without actually touching the object to
any other charged object.
Check Your Understanding
1. 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).
2. 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
3. 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?
The presence of different atoms in objects provides different objects with different electrical
properties. One such property is known as electron affinity. Simply put, the property of electron
affinity refers to the attraction which a material has for electrons. If atoms of a material have a
high electron affinity, the material will have a relatively high attraction for electrons. Electron
affinity leads to charging by contact.
ANSWERS
Contact and Induction
1. The diagram depicts the distribution of
charge over the surface of two spheres.
One sphere is copper and the other
sphere is rubber. Label which is which
and support your answer with one reason.
2. Which of the following materials are likely
to exhibit more conductive properties than insulating properties? Provide one reason for
your answers.
a. Rubber
b. Aluminum
c. Silver
d. Plastic
e. Wet human skin
3. Suppose that a conducting sphere is charged positively by some method. The charge is
initially deposited on the left side of the sphere. Yet because the object is conductive, the
charge spreads uniformly throughout the surface of the sphere. The uniform distribution
of charge is explained by the fact that ____.
a. The charged atoms at the location of charge move throughout the surface of the
sphere
b. The excess protons move from the location of charge to the rest of the sphere
c. Excess electrons from the rest of the sphere are attracted towards the
excess protons
Check Your Understanding
2. 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).
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.
3. 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
4. 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?
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 + charge (since it lost electrons) and the right
side will have an excess of - charge (since it gained the electrons).