CHAPTER
11
Review
SUGGESTED ANSWERS
WHAT DO YOU REMEMBER?
1. (a)
The Law of Electric Charges says that like charges repel one other, opposite charges attract one another,
and a neutral object is attracted to an excess of either type of charge.
(b) Charging
by friction happens when two objects made of different materials are rubbed together. The
objects become oppositely charged because electrons move from the surface of one object onto the
surface of the other object.
2.
(c)
Charging by conduction happens when two objects that have different amounts of electric charge come
into contact. Electrons move from the object with the most negative charge to the object with the least
negative charge.
(d)
A conductor is a material through which electric charge can readily move.
(e)
An insulator is a material through which electric charge cannot move easily or at all.
(f)
Charging by induction occurs when a charge imbalance on one object causes a charge imbalance to
develop on part of another object held close to, but not touching, the first object.
In charging by friction, two surfaces acquire opposite charges when they touch. In temporarily charging by
induction, a charged object brought close to another object without touching, and it causes a separation
of positive and negative charges to develop on the second object; when the charged object is removed, the
negative charges on the second object move back to their original arrangement.
3. (a)
A pith ball electroscope has a small ball suspended on a thread. When neutral, the pith ball will be
attracted to a charged object due to an induced charge. If touched by a charged object, the pith ball will
take on a like charge and can indicate the charge on other objects according to whether it is attracted to
or repelled by these objects.
rod with excess
positive charge
⫹
⫹
⫹
⫹⫹
⫺⫺⫹
⫺
⫺⫹⫹⫹
neutral pith
ball
rod with excess
negative charge
⫺⫺
⫺
⫺
⫺
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⫹⫹
⫹
⫹⫹ ⫺
⫺ ⫹
⫺
⫺ ⫹
⫹⫹
neutral pith
ball
rod with excess
positive charge
⫹
⫹
⫹
⫹⫹⫹
pith ball with
excess positive
charge
NEL
rod with excess
positive charge
⫹
⫹⫹
pith ball with
excess positive
charge
⫹
⫹⫹⫹
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(b)
When neutral, the leaves of a metal leaf electroscope separate when a charged object is brought near the
plate or ball at the top because like charges are induced on the leaves. If a charged object touches the
ball of the electroscope, the leaves become permanently charged and remain separated. If a like charged
object is brought near the plate or ball, the leaves move farther apart, and if an oppositely charged
object is brought near the plate or ball, the leaves move closer together.
⫹
⫹
⫹
⫹
⫹
⫹
rod with excess
positive charge
⫺⫺
⫺
⫺ ⫺
⫺
⫺ ⫹
⫹ ⫹
⫹
⫹
⫹
⫹
rod with excess
positive charge
⫹
⫹
⫹
⫹
neutral to start
⫺
⫺
⫺
rod with excess
negative charge
⫺
⫹⫹ ⫺⫺
⫹ ⫺
⫺
⫺
⫺
⫺
⫺⫺
⫺
⫺
negatively charged
to start
4.
⫹
⫹
⫹
rod with excess ⫹ ⫺ ⫺
positive charge
⫺
⫹
⫺
⫺
⫹
⫺
⫺
negatively charged
to start
The atoms of one material more readily give up electrons, whereas the atoms of the other material more
readily accept electrons. When the materials touch, electrons move toward the atoms that more readily
accept electrons, giving that material a negative charge and leaving the other material with a positive charge.
(b)
5. (a)
charged
⫹
⫹
pith ball ⫹ ⫹
792
⫹
⫹
neutral to start
⫹
⫺
⫹
⫹ ⫺⫺⫺
⫺
⫺ ⫹
⫺
⫺
⫹ ⫹
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oppositely
⫺
⫺ charged
⫺⫺
pith ball
⫹
⫹
⫹⫹
⫹
⫹
⫹⫹
like charged
pith balls
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(c)
charged
neutral
⫹
⫹ ⫺
⫹
pith ball ⫹ ⫹
⫺ ⫹ pith ball
6. (a)
The charge will be positive.
(b)
The charge will be negative.
7.
Typical answers are electrostatic paint sprayers, dusters, precipitators, air cleaners, and footprint lifters.
8.
In a laser printer, a laser shines on a drum, “writing” symbols and “drawing” objects in the form of
negatively charged areas on the drum. The drum rolls over positively charged toner particles, which are
attracted to the negatively charged areas on the drum. The drum then rolls over paper with a higher
negative charge, causing the toner particles to move toward the paper where they are melted into
permanent print.
9.
Definition
Facts/Characteristics
The buildup of electric charge on the surface of
an object.
• caused by electrons moving from one material onto
another
• more protons than electrons means a positive charge;
less protons than electrons means a negative charge
• like charges repel, opposite charges attract
• any charged object will attract a neutral object
• static charges attract particles like dust
Static Electricity
Examples
• rubbing a balloon on your hair
• an electrostatic buildup on clothes in the dryer
• shuffling your feet on a carpet on a cold, dry day
• sliding down a plastic slide
Non-Examples
• electricity in a circuit
• a conductor connected to ground
• objects coated with a layer of water in a humid room
WHAT DO YOU UNDERSTAND?
10.
No. Static electricity involves the movement of electrons (negative charges) from one object onto another.
11.
Answers will vary. The diagram should show that protons are in the centre of atoms and are “protected” by
surrounding electrons and are held to each other and to neutrons by very strong forces. Electrons, on the
other hand, are on the outside of atoms and can be removed with much less difficulty.
12.
The decals stick to the window because of static electricity. As the backing of the decal is peeled off, it is
charged by friction. The charge allows the decal to stick to a surface.
13.
No. Liquid paint, for example, can be electrostatically charged and then sprayed onto an oppositely
charged object. The paint is attracted to the object and little paint is wasted.
14.
If you are holding a conductor, you will ground it and it will not be able to build up an electrostatic
charge.
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15. (a)
two conducting spheres
connected by a conductor
⫹ ⫺
⫹
⫺ ⫺
⫹ ⫺⫹
rod with a positive charge
⫹
⫺⫹ ⫹⫺ ⫹ ⫹⫺ ⫹
⫹
⫺⫹
⫹⫺ ⫹
⫹⫺
⫹
⫹ ⫺
⫺ ⫹⫺
⫹ ⫺⫹
⫹⫺ ⫺
⫺⫹
⫺ ⫺⫺
⫹ ⫺⫹⫺
⫹
⫹
⫹ ⫹
(b) conducting sphere
⫺ ⫹⫺
⫹
⫺⫹
⫺
⫺⫹
⫹
⫺ ⫹⫺
⫹
⫺⫹
⫺
⫺⫹
⫹
conducting rod
⫹⫺ ⫺ ⫹ ⫺ ⫺ ⫹ ⫺
⫹ ⫺ ⫺ ⫺⫹ ⫺ ⫺
⫺
⫺⫹
electrons
⫺ ⫹
⫹⫺ ⫺ ⫹
⫹ ⫺ ⫺ ⫺⫹ ⫺ ⫺
⫺
⫺⫹
⫺ ⫹⫺
⫹
⫺
⫺ ⫺⫹
⫹
⫹ ⫺ ⫺⫺
⫺
(c)
⫺⫹ ⫺ ⫺ ⫹ ⫺
⫹⫺
⫹
⫹ ⫺
⫺
⫹
⫺
⫺ ⫹⫺
⫹
⫺
⫺ ⫺⫹ ⫺
⫹
⫺
⫺
⫹
⫺
⫺ ⫹⫺
⫹
⫺⫹
⫺
⫺⫹
⫹
16. (a)
17.
Polyester becomes negatively charged and nylon becomes positively charged.
(b)
Wool becomes negatively charged and acetate becomes positively charged.
(c)
Silk becomes negatively charged and glass becomes positively charged.
(d)
No charge develops.
⫺⫹⫺ ⫹ ⫺
A ⫺⫹⫹⫹⫹⫺⫹⫹ ⫹
⫹
⫺ ⫹ ⫺⫹
⫹ ⫹ ⫺ ⫹ ⫺⫹⫺ ⫹ ⫺ B
⫺ ⫹
⫹ ⫺ ⫹ ⫺⫹
electrons
⫺⫹⫺ ⫹ ⫺ ⫹ ⫹ ⫺ ⫹ ⫺⫹⫺ ⫹ ⫺ B
A ⫺⫹⫹⫹⫹⫺⫹⫹ ⫹
⫹
⫹ ⫺ ⫹ ⫺⫹
⫺ ⫹ ⫺⫹ ⫺ ⫹
⫺⫹⫺ ⫹ ⫺
A ⫺⫹⫹⫹⫹⫺⫹⫹ ⫹
⫹
⫺ ⫹ ⫺⫺ ⫹
794
⫹ ⫹ ⫺ ⫹ ⫺⫹ ⫹
⫹
⫹ ⫺ ⫹ ⫺⫹ B
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18.
When a charged object is brought near a neutral object, electrons move either toward or away from the
charged object, depending on its charge. This results in attraction between both objects. Only electrons
(negative charges) move, not protons (positive charges).
19.
The neutral object could develop an induced charge, with negative charges moving toward the side farther
from the negatively charged object. This would cause the neutral object to be attracted to the negatively
charged object.
20.
If originally neutral, the leaves of an electroscope separate when a charged object is brought near the plate
or ball of the electroscope. A neutral pith ball, on the other hand, is attracted to a charged object and
moves towards it.
21.
If the scientist had charged an electroscope and the leaves were apart, he or she could touch a rod made of
Element X to the ball or plate of the electroscope. If the leaves fall together, Element X is a conductor.
The cat fur would likely give the rubber boot a negative charge, and the boot would give the pith ball a
negative charge as well. If the ebonite repels the pith ball, it must also have a negative charge.
Ebonite rubbed with fur will have a negative charge. Because the ebonite repels C, C is negative. Because
A attracts C, A is positive. Because A repels B, B is also positive.
A humidifier in the room could reduce the electrostatic buildup that occurs between socks and carpet. The
socks could also be rubbed with a dryer sheet to coat them with a conductive chemical.
Cotton clothes would be much better than synthetic fibres or wool garments. Cotton lies more toward
the middle of the electrostatic series and is less likely to develop static charges that could cause sparks.
However, cotton is more flammable than wool, so wool would ultimately be safer.
In some cases, laws are passed that force factory owners to install electrostatic precipitators. Responsible
factory owners, however, should realize that keeping the environment clean saves a great deal in health
care, agriculture and other areas, savings that would benefit the owners in both direct and indirect ways.
Answers will vary. Key features should be a conductive material, and a clear conductive path around the
outside of the structure to the earth (ground). Whether or not the lightning rod should be pointed is still
a matter of scientific debate. Sample diagram:
22.
23.
24.
25.
26.
27.
rod
wire
ground
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28.
Sample answer: Smoke passes through negatively charged plate and acquires a negative charge. The
negatively charged particles then stick to positively charged plates farther down the tube, due to the law of
opposite charges. The particles then fall to a collection plate for removal. Sample diagram:
exhaust without ash and dust particles
positively charged
plate
negatively charged
particles
collection plate
negatively charged
plates
ash and dust particles
emissions containing ash and dust particles
29.
Answers will vary. One strategy is to follow the electrostatic series to deposit a known charge on an object.
A test material that is repelled by the charged object must have a like charge, and a test material that is
attracted to the charged object must have an opposite charge.
30.
Student answers will depend entirely on the products they test. Eco-friendly dusting means staying away
from aerosol products and using traditional methods such as lambswool or feather dusters.
31. (a)
Temporary charging does not involve touching. When the objects are moved apart, the charges arrange
themselves as they were originally. Permanent charging does involve touching and leads to a new
distribution of the negative charges on the objects after they are moved apart.
(b)
Answers will vary.
(c)
Answers will vary. Students should comment that it would take a huge number of words to describe
with accuracy the information that a diagram easily conveys.
32.
Sample answer. The rover that went to Mars had to deal with dust devil storms, which caused static
electricity problems with sensors and other equipment. The scientists learned that in the dry Martian
atmosphere, static electricity could build up easily. They are taking special precautions against static in
future spacecraft.
33.
Answers will vary depending on the brands examined, quality standards, speed, and so on. Current pricing
for colour laser printers seems to be reasonably close to black-and-white printers, but toner costs for a
colour printer can be high.
34.
A photocopier does not use a laser; instead, white light reflecting from a page to be copied strikes a
“platen,” removing charge. Dark areas on the page do not reflect light and the corresponding areas on
the platen stay positively charged (similar to negatively charged areas on the drum of a laser printer).
Negatively charged toner (which is positively charged in a laser printer) is attracted to the positively
charged areas on the platen, and then a charged piece of paper pulls the toner from the platen, after which
the toner is fused by heat to the paper.
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