12.1
Electrostatics
Electrostatics is the area of physics that deals with objects that have an electric
charge. Electric charge is a property of matter that is responsible for all electric
and magnetic forces and interactions. Electric charge is arbitrarily described as
negative or positive. Static electricity, which is a buildup of stationary electric
charge on a substance, is one of the most difficult problems encountered in a
high-tech clean room, where electronic equipment is designed and assembled
(Figure 1). Often the electronic equipment is so sensitive that even small electric
discharges can cause permanent damage. Anyone working in such an environment needs a complete understanding of the properties of static electricity.
The Electrical Structure of Matter
The Bohr-Rutherford model of the atom (Figure 2) can help us understand electrical phenomena. The principal concepts in this atomic model are as follows:
1. Matter is composed of sub-microscopic particles called atoms.
Figure 1
In a high-tech clean room, many precautions
are taken to protect sensitive devices from
electrostatic discharge.
electric charge: a basic property of
matter described as negative or positive
static electricity: a buildup of stationary
electric charge on a substance
atom: sub-microscopic particle of which all
matter is made
electron: negatively charged particle which
moves around the nucleus of an atom
2. Electric charges are carried by particles within the atom that are
called electrons and protons.
3. Protons are found in a small central region of the atom called the
nucleus. They are small, heavy particles, and each one carries a positive electric charge of a specific magnitude, called the elementary
charge (e).
4. Electrons move in the space around the nucleus. They are small, very
1
light particles (each with only slightly more than 2000 the mass of a
proton), yet each of them carries a negative electric charge equal in
magnitude to that of the proton.
5. Atoms are normally electrically neutral, because the number of
(positive) protons in the nucleus is equal to the number of (negative)
electrons in the space around the nucleus.
6. Neutrons are small, heavy particles (each slightly heavier than a
proton) found in the nucleus. They carry no electric charge.
proton: positively charged particle found in
the nucleus of an atom
7. If an atom gains an extra electron, it is no longer neutral but has an
excess of electrons and a net negative charge. Such an atom is called
a negative ion.
nucleus: the central region of an atom,
where protons and neutrons are found
8. If an atom loses an electron, it will have a deficit of electrons and a
net positive charge. Such an atom is called a positive ion.
elementary charge: (e) electric charge
of magnitude equal to the charge on a proton
and an electron
neutron: a neutral particle found in the
nucleus
negative ion: an atom that has at least
one extra electron and is negatively charged
positive ion: an atom that has lost at
least one electron and is positively charged
fundamental laws of electric
charges: Opposite charges attract each
other. Similar electric charges repel each
other. Charged objects attract some neutral
objects.
424 Chapter 12
The atoms of a solid are held tightly in place; their nuclei vibrate but are not
free to move about within the solid. Since the nuclei contain all of the protons,
the positive charges in a solid remain fixed. However, it is possible for negative
charges within a solid to move because some of the outermost electrons can
move from atom to atom.
The Basics of Electrostatics
When charged objects are brought close to each other they interact at a distance,
causing them to attract or repel. Even some neutral objects are attracted to
charged objects. Through experimentation, the American statesman, inventor,
and scientist Benjamin Franklin (1706–1790) discovered that there are two types
of charges; he gave them the names positive and negative. The fundamental laws
of electric charges are as follows:
12.1
Fundamental Laws of Electric Charges
• Opposite electric charges attract each other.
• Similar electric charges repel each other.
• Charged objects attract some neutral objects.
electron
regions
10-10 m
Investigation 12.1.1
protons
Electric Charges and Forces
In this investigation, you will explore the interactions between objects possessing
different types of electric charges and the effects that they have on each other.
10-14 m
nucleus (enlarged)
neutrons
Question
Can the Bohr-Rutherford model of the atom be used to predict the interaction
of charged objects?
Figure 2
The Bohr-Rutherford model of the atom
Prediction
(a) Using the Bohr-Rutherford model of the atom and the laws of electric
charges, predict the type of electrical interaction that will occur between
two ebonite (plastic) rods rubbed with fur and brought close to one
another.
INQUIRY SKILLS
Questioning
Hypothesizing
Predicting
Planning
Conducting
Recording
Analyzing
Evaluating
Communicating
Materials
ebonite rods or polyethylene strips and fur
insulated rod hanger
glass rods or acetate strips and silk
pith ball suspended on thread
small bits of paper
sawdust
iron filings
Do not perform this investigation if you are allergic to
animal fur.
Procedure
1. Place an ebonite rod on a hanger suspended by a thread. Bring another
ebonite rod close to the suspended rod and note whether the suspended
rod moves in response to the other rod’s proximity.
2. Rub one of the ebonite rods vigorously with the fur and place it in the
hanger. Then rub the other ebonite rod with fur and bring it near the suspended rod. Note the effect that the second rod has on the first.
3. Repeat steps 1 and 2 with the glass rods, rubbing them with the silk.
4. Suspend a glass rod rubbed with silk from the hanger and bring an ebonite
rod rubbed with fur near it. Note the effect that the ebonite rod has on the
glass rod.
5. Suspend a pith ball from a thread and touch it with your finger. Slowly bring a
charged ebonite rod near to, but not touching, the pith ball, and note its effect.
Note what happens after the pith ball and the ebonite rod have touched.
6. Repeat the procedure using a charged glass rod.
7. Using first a charged glass rod and then an ebonite rod, approach each of
the following in turn and note the results: small bits of paper, sawdust, iron
filings, and a thin, continuous stream of water from a tap (Figure 3).
Figure 3
A charged rod is held near a water stream.
Will other liquids such as cooking oil show
the same behaviour?
Electricity 425
Analysis
(b)
(c)
(d)
(e)
How many different types of electric charge were you able to identify?
What was the purpose of steps 1 and 3?
Why did you touch the pith ball with your finger in step 5?
Give simple descriptions of the interaction between similarly charged
objects and between oppositely charged objects, and explain your observations using the Bohr-Rutherford model of the atom.
(f) What happened when a charged rod was brought near some neutral
objects? Will the same thing occur with all neutral objects? Explain your
observations using the Bohr-Rutherford model of the atom.
(g) What must be true about water molecules for them to behave as they do in
the presence of a charged rod?
Evaluation
conductor: solid in which charge flows
freely
insulator: solid that hinders the flow of
charge
+
+ +
+
+
+
+
+
+
+
+ + +
conductor
(b)
+
+
+
+ +
+
+
Charging an Object
Charging an object involves the addition or removal of electrons. If electrons are
removed from an object, it will be charged positively; if electrons are added, it will
be charged negatively. In both cases the number of protons in each object remains
constant. The change in charge on the object is due to the loss or gain of electrons
(Figure 4). The net charge in solids is due to an excess or deficit of electrons.
(a)
+
(h) Evaluate your predictions.
(i) Identify sources of error in the procedure and suggest a few changes that
will help reduce error.
++
+++ +
+ + ++
++
+ ++
+
–+
–
– + –
–+
–+
– –+ –
neutral
object
negatively
charged object
–+
–+
–+
+ –
+
– +
+
insulator
+
+
+ ++
++ +++
+ + ++++++
+ +++++
+
+
Figure 5
(a) On a spherical conductor, the charge
spreads out evenly. On an insulator, the
charge remains in the spot where it was
introduced.
(b) On other shapes of conductors, the
charges tend to repel one another toward
the more pointed surfaces.
426 Chapter 12
positively
charged object
Figure 4
Neutral and charged objects can be represented by sketches with positive and negative signs
marked on the objects.
Most solids fall into one of two broad categories based on their electrical
properties. They are either conductors, solids in which charge flows freely, or
insulators, solids that hinder the flow of charge. Most metals are excellent conductors, the best being silver, gold, copper, and aluminum. Some of the electrons
in these conductors are called “conduction electrons” because of their ability to
move about. On a spherical conductor, charge spreads out evenly (Figure 5(a)).
On other shapes, charges tend to repel one another toward the more pointed surfaces (Figure 5(b)). Plastic, cork, glass, wood, and rubber are excellent insulators.
If you rub wool on one end of a plastic comb so it develops a charge, the other end
will remain neutral due to the limited mobility of charges in insulators.
12.1
Charging by Friction
ebonite rod
An atom holds onto its negative electrons by the force of electrical attraction of
its positive nucleus. Some atoms exert stronger forces of attraction on their electrons than others (Figure 6).
When charging by friction, the type of charge that develops can be determined using a chart called the electrostatic series (Table 1). When rubbed
together, substances at the top of the series will lose electrons to substances lower
in the series, meaning that the substance higher in the table becomes positively
charged while the substance lower in the table becomes negatively charged.
–
–
+
–
+
– +
+ –
–
+
–
+
+
–
–
–
+
+
+
–
+
fur
Table 1 The Electrostatic Series
acetate
Figure 6
When ebonite and fur are rubbed together,
some of the electrons from the atoms in the
fur are “captured” by the atoms in the
ebonite, which exert stronger forces of attraction on those electrons than do the atoms in
the fur. After rubbing, the ebonite has an
excess of electrons and the fur has a deficit.
weak hold on electrons
glass
wool
cat fur, human hair
calcium, magnesium, lead
silk
aluminum, zinc
increasing tendency
cotton
to gain electrons
electrostatic series: chart that shows a
substance’s tendency to gain or lose electrons
paraffin wax
ebonite
polyethylene (plastic)
carbon, copper, nickel
rubber
sulphur
platinum, gold
strong hold on electrons
Electroscopes
An electroscope is a device that is used to detect the presence of an electric charge
and to determine the charge’s “sign” (that is, whether it is positive or negative).
Pith-Ball Electroscope
electroscope: device that is used to
detect the presence of an electric charge and
to determine the charge’s “sign” (that is,
whether it is positive or negative)
A pith-ball electroscope is a light, metal-coated ball suspended on an insulating
thread (Figure 7(a)). If the ball is charged, it can be used to detect the presence
of a charge on nearby objects. It will be repelled by a similarly charged object and
attracted to an oppositely charged object or a neutral object.
(a)
(b)
–
+
+
negative
– charge
–
+
–
–
–
–
–
+
–
–
–
+
–
+
–
+
–
–
+
–
+
–
+
–
–
+
–
–
+
positive
charge
–
+
+
Figure 7
(a) Repulsion of a negative pith ball by a
charged ebonite rod
(b) Similar net charges on a pair of metal
leaves cause the leaves to move apart,
indicating the presence of a charge on
the electroscope.
Electricity 427
(a)
Metal-Leaf Electroscope
pith-ball electroscope
–
+
–
––
+
–
––
–+
– – +
– + – – –+
–
– +––
flow of electrons
(b)
A metal-leaf electroscope consists of two thin metal leaves suspended from a
metal rod in a glass container. A metal knob or plate is attached to the top of the
metal rod. Since the centre of the electroscope is made of a conducting material,
any charge on it spreads out over the entire knob, rod, and leaves. Since the leaves
are then charged similarly, they repel each other and move apart, indicating the
presence of a charge (Figure 7(b)). The farther apart the leaves move, the greater
the charges they are carrying.
Charging by Contact
When a charged ebonite rod is touched to a neutral pith-ball electroscope, some
of the excess electrons on the ebonite rod are repelled by their neighbours and
move onto the pith ball (Figure 8(a)). The pith ball and the ebonite rod share the
excess of electrons that the rod previously had, and both end up with net negative charges. A similar sharing of electrons occurs when a charged ebonite rod
touches the knob of a metal-leaf electroscope (Figure 8(b)).
When a positively charged glass rod is used (Figure 9), some of the electrons
on the neutral pith-ball or metal-leaf electroscope are attracted to the glass rod
until the electroscope shares the deficit of electrons that the rod previously had.
Both end up with net positive charges. These examples show that an object that
is charged by contact has the same charge as the charging object.
flow of electrons
–
+
Figure 8
Charging by contact with a charged ebonite rod
+
+
– + + –
+
–+
+ +
–
+
+–
+
–+ +–
+
–
+
–
flow of electrons
+
Figure 9
Charging by contact with a charged glass rod
+
+
Try This
Activity
Charging by Contact
How is an object charged by contact?
• Rub an ebonite rod with fur and touch it to the neutral pith ball or
leaf electroscope. Bring the rod close to the pith ball again and
observe what happens.
• Repeat the procedure for a glass rod charged with silk.
(a) When the charged ebonite rod touched the pith ball, what type
of charge did the pith ball acquire? Explain by discussing the
way the electrons moved. Use sketches showing before, during,
and after contact.
(b) When the charged glass rod touched the pith ball, what type of
charge did the pith ball acquire? Explain using diagrams.
428 Chapter 12
12.1
Charging by Induction
When a charged ebonite rod is brought close to the knob of a neutral metal-leaf
electroscope, electrons on the electroscope will move as far away as possible from
the negative rod. This results in an induced charge separation on the electroscope since the positive charges cannot move. (If the charged rod is removed the
electrons will return to their original distribution.) If you ground the electroscope by touching it, some electrons are induced to flow through your finger,
leaving the electroscope. When you remove your finger from the electroscope,
the device is left with a deficit of electrons, resulting in a net positive charge
(Figure 10(a)).
A positively charged rod held near the knob of an electroscope will also
result in an induced charge separation (Figure 10(b)). If grounded, electrons will
move through your finger onto the electroscope. When you remove your finger
from the electroscope, the device is left with an excess of electrons, resulting in a
net negative charge. From this example we can conclude that an object that is
charged by induction has the opposite charge to that of the charging rod.
induced charge separation: distribution of charge that results from a change in
the position of electrons in an object
ground: a connection of an object to Earth
through a conductor
(a)
–
+
–
–
+ –
– – – –
+
–
electrons repelled
to ground by
negative rod
– –
+
–
–
+
Activity 12.1.1
Charging by Induction
+
–
–
+
+
In this activity, you will observe how an object is charged by induction.
Procedure
grounded electroscope
in the presence of
a negatively charged rod
1. Place two short metal rods on top of two glass beakers so they are touching
each other, as shown in Figure 11.
(b)
metal rods
+
charged
object
glass beakers
Move this beaker
away once charged
object is close.
–+
+–
+
–
+
–
+
electrons attracted
from ground by
positive rod
– –
–
–
Figure 11
–
2. Bring a charged object close to one of the metal rods and then pull the
other metal rod away by touching the glass beaker only. Put down the
charged object and test each object for a charge with an electroscope.
3. Put the two metal rods back together by touching only the beakers and test
them again for a charge.
– +
+
–
+
–
Analysis
(a) What happens to the electroscope when it is brought close to the metal
rods after separating them? Explain why this occurs by discussing the
movement of electrons.
(b) What happens to the electroscope after the metal rods are put back
together? Explain.
(c) Draw a diagram of the charged rod near the two metal rods before they are
separated, showing the positions of the positive and negative charges.
grounded electroscope
in the presence of
a positively charged rod
Figure 10
Notice that the leaves of the electroscope
fall, indicating a neutral condition of the
leaves while the finger is in position.
Electricity 429
A summary of the methods of charging objects is shown in Table 2.
Table 2 Summary of Methods of Charging Objects
Method of
charging
Friction
Contact
Induction and
grounding
Initial charges
on objects
both objects
neutral
one neutral and
the other charged
one neutral and
the other charged
Steps
rub two objects
together
touch charged
object to
neutral object
bring charged object
close to neutral
grounded object
remove ground and
then charged object
Final charges
on objects
oppositely
charged
similarly
charged
oppositely
charged
Practice
Understanding Concepts
1. If an ebonite rod is rubbed with fur, the rod becomes negatively
charged. What is the source of the charge?
2. If the knob of a positively charged electroscope is approached slowly
by a negatively charged rod, what happens to the leaves of the electroscope? Why?
3. Explain fully what happens when a positively charged rod touches
the knob of a neutral metal-leaf electroscope.
4. Why are metallic fibres used in the pile of some carpets?
5. (a) Define electrostatic induction.
(b) Given a glass rod, silk, and two metal spheres mounted on insulating stands, describe how to charge the spheres oppositely by
electrostatic induction.
6. A negatively charged rod is brought near a neutral sphere supported
by an insulating stand. What type of charge would you expect to find
on the side of the sphere nearest the rod and on the other side if the
sphere is (a) a conductor and (b) an insulator? Explain.
Controlling Static Charges in a Clean Room
How does a worker in a clean room use knowledge about the electrical nature of
matter to prevent electric discharges from damaging sensitive devices? Keep in
mind that people are prime generators of static charge and even just walking
around can develop a significant charge. When the worker touches a sensitive
piece of equipment, the charge will pass into it, causing damage. To prevent this
from happening many precautions are taken.
Workers often wear wrist straps that are connected to a ground cord. Special
clothing and shoes are worn to inhibit the buildup of charge and to help dissipate any charge that might develop. Special floors are installed, floor mats are
used, and just about everything is grounded, including chairs, desks, and even
the devices being constructed.
For those items that cannot be grounded, mostly insulators like plastic, the
charge is dissipated by the air. Devices are installed that create charged particles
in the air. These charged particles are attracted to the opposite charges that
develop on objects and help to neutralize them.
430 Chapter 12
12.1
SUMMARY
Electrostatics
• The fundamental laws of electric charges state that opposite electric
charges attract each other; similar electric charges repel each other; and
charged objects attract some neutral objects.
• The net charge in solids is due to an excess or deficit of electrons.
• A conductor is a solid in which charge flows freely; an insulator is a solid
that hinders the flow of charge.
• Solids can be charged by friction, contact, induction, or grounding.
Section 12.1 Questions
Understanding Concepts
1. Two rods, one brass and the other plastic, have been charged by
contact at one end by an ebonite rod rubbed with fur while being
supported by an insulator. Compare the distribution of electric
charge on the two rods.
2. Sometimes when you are taking off a wool sweater by pulling it
over your head, you find that your hair is attracted to the sweater.
Explain why this is so by referring to the movement of electrons.
3. State the type of charge that would develop on each material
when each pair of materials is rubbed together.
(a) wool and ebonite
(b) glass and paraffin wax
(c) polyethylene and fur
4. A suspended polyethylene (plastic) strip is given a negative
charge. What can you conclude for certain if you bring another
object close to the polyethylene strip and the two objects
(a) repel each other?
(b) attract each other?
5. Figure 12 shows an arrangement for demonstrating one of
Newton’s laws of motion. The three-armed device is called a
pinwheel.
(a) Decide the direction in which the pinwheel will rotate when
the generator is turned on, and explain why.
(b) Which one of Newton’s laws does this action illustrate?
– –
–
connected to
the generator
Figure 12
For question 5
6. In the 16th century, William Gilbert (1540–1603), an English physicist, discovered several insulators that would accept an electric
charge, but no conductors. Give possible reasons why he did not
discover that conductors will also accept charges.
7. The following observations are made of the interactions between
various combinations of four pith balls, A, B, C, and D. The force
between C and D is a repulsion. A attracts both B and D. If A is
repelled by an ebonite rod that has been rubbed with fur, what
are the possible charges on each pith ball?
Applying Inquiry Skills
8. Several dangerous explosions have occurred when powerful jets
of water were used to wash ocean-going oil tankers. What was
the likely cause of these explosions? What could be done to prevent them?
(continued)
Electricity 431
9. A pith-ball electroscope may be in a charged or a neutral state.
Describe the steps you would take to determine whether it is
charged or neutral. If the electroscope is charged, how would you
determine what charge it possesses?
Making Connections
electric field: the space around a charged
object where forces of attraction or repulsion
act on other objects
10. While metal car bodies are being spray-painted at the factory,
they are grounded. Explain the advantages of this. (Hint: As the
spray leaves the nozzle at high speeds, it becomes charged.)
11. When clothes made of different materials tumble in a warm
clothes dryer they have a tendency to cling to one another.
Coated strips or spray may be purchased to prevent or remove
the “static cling.” Research how “anti-static cling” formulations
work. Follow the links for Nelson Physics 11, 12.1.
GO TO
www.science.nelson.com
12. Electrostatic printing, commonly known as xerography or photocopying, uses the principles of static electricity to print on paper
or other materials. Research and explain the steps in the process
where static electricity plays a role. Follow the links for Nelson
Physics 11, 12.1.
+
GO TO
www.science.nelson.com
Reflecting
13. Would you feel comfortable working in a high-tech clean room
knowing that you would have to be extremely careful about static
charge? List some advantages and disadvantages of such a job.
Figure 1
Electric field around a single positive charge
12.2
(a)
+
+
Electric Fields and
Electric Charge
What causes electric force? How is electric charge measured? The study of
charges will be far more productive if we address these questions.
Electric Fields
(b)
–
+
Figure 2
Some typical electric fields. Notice that
these field lines are directed away from the
positive spheres and toward the negative
spheres.
(a) Like charges
(b) Opposite charges
432 Chapter 12
Electric charges exert forces that can attract and repel each other even when they
are not in direct contact. What causes the force? We don’t see anything between
the charges that could be responsible for it. Yet this kind of force is already
familiar to you. The force of gravity was explained in terms of a gravitational
field of force—when a mass is placed in the gravitational field of another mass,
the first mass experiences a force of attraction toward the second mass.
It is reasonable to assume that the forces between charged objects may
also be due to a field of force. If this is true, then every charged object creates
an electric field of force in the space around it (Figure 1) and any other charged
object in that field will experience a force of electrical attraction or repulsion.
The electric field is represented by drawing a series of field lines around the
charged object. Field lines show the direction of the electric force on a small positive test charge placed at each and every point in the field. It is customary to use
a positive charge as a test charge. The relative distance between adjacent field
lines at a given point is an indication of the strength of the electric field at that
point. Some typical electric fields are shown in Figure 2.