Transcript Chapter 15: Electric Charge, Forces, and Fields

Chapter 15: Electric
Charge, Forces, and Fields
Static Electricity – Electrical
charge that stays in one place
Electric Charge: a fundamental property of
matter associated with the particles that make
up the atom.
• Electricity is the study of the interaction between
electrically charged objects.
• Structure of matter
Neutral vs Charged Objects
• Atoms with equal numbers of protons and
electrons are described as being electrically
neutral.
• Atoms with an unequal number of protons and
electrons are electrically charged (and in fact, is
then referred to as an ion rather than an atom).
• Any particle which contains less electrons than
protons is said to be positively-charged.
• Any particle which contains more electrons than
protons is said to be negatively charged.
Quantifying Charge, Q – The Coulomb
• The magnitude of charge on an electron is the
fundamental unit of charge, or the smallest observable
charge in nature, abbreviated e
• To determine the total charge of an object with an excess
of protons, subtract the total number of electrons from
the total number of protons.
• Example Problem 1: Identify the following particles as
being charged or uncharged. If charged, indicate whether
they are charged positively or negatively.
Measurement of Charge
• SI Unit for charge is the coulomb, C
• The charge on a single electron is qe= -1.602 x 10 -19 C.
• The charge on a single proton is qp=+1.602 x 10 -19 C.
• In calculations, always convert charges to coulombs
Example Problem 2 (rewrite false answers to make them
true)
A. TRUE or FALSE: An object which is positively charged
contains all protons and no electrons.
B. TRUE or FALSE: An object which is negatively charged
could contain only electrons with no protons.
C. TRUE or FALSE: An object which is electrically neutral
contains only neutrons.
Example Problem 3: Determine the quantity and
type of charge on an object which has 3.62 x 1012
more protons than electrons.
Example Problem 4: Complete the following
statements… After some rather exhausting counting
a physics student determines that a very small
sample of an object contains ...
A. 8.25749 x 1017 protons and 5.26 x 1014 electrons; the
charge on this object is ____ Coulombs.
B. 3.12 x 1014 protons and 4.5488 x 1016 electrons; the
charge on this object is ____ Coulombs.
C. 2.40277 x 1019 protons and 9.88 x 1016 electrons; the
charge on this object is ____ Coulombs.
D. 2.6325 x 1015 protons and 2.6325 x 1015 electrons;
the charge on this object is ____ Coulombs.
Example Problem 5: The amount of charge carried
by a lightning bolt is estimated at -10.0 Coulombs.
What quantity of excess electrons is carried by the
lightning bolt?
Interactions of Charges
• Electrical force keeps electrons in orbit around
the nucleus and holds matter together.
• Law of Charges (aka The Charge-Force Law) : Like
charges repel, and unlike charges attract.
• Charge on an electron and proton are equal in
magnitude, but opposite in sign, thus acting on
separate objects, the forces follow Newton’s
Third Law
Interaction Between Charged
and Neutral Objects
• What type of interaction is observed between
a charged object and a neutral object?
• Any charged object - whether positively
charged or negatively charged - will have an
attractive interaction with a neutral object.
• Positively charged objects and neutral objects
attract each other; and negatively charged
objects and neutral objects attract each other.
Example Problem 6: On two occasions, the following
charge interactions between balloons A, B and C are
observed. In each case, it is known that balloon B is
charged negatively. Based on these observations, what can
you conclusively confirm about the charge on balloon A
and C for each situation.
Example Problems 7 and 8
7. Two objects are charged as shown at the right.
Object X will ____ object Y.
A. Attract
B. Repel
C. Not affect
8. Two objects are shown at the right. One is
neutral and the other is negative. Object X will
____ object Y.
A. Attract
B. Repel
C. Not affect
Conductors and Insulators
• Conductors are materials which permit electrons to flow
freely from atom to atom and molecule to molecule.
• Conductors allow for charge transfer through the free
movement of electrons.
• A conductor will permit charge to be transferred across
the entire surface of the object.
• Insulators are materials
which 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 and charge is
seldom distributed
evenly across the surface
of an insulator.
Semiconductors have an intermediate ability to
conduct charge.
• Movement of electrons is more complex than
the valence electron theory can describe and
is only understood by the aid of quantum
mechanics
• Conductivity of semiconductors can be
adjusted by adding atomic impurities to the
substance, making them useful in technology
Superconductors are elements, inter-metallic alloys, or
compounds that will conduct electricity without
resistance below a certain temperature.
Distribution of Charge via Electron Movement
• To reduce the overall repulsive affects within the object,
there is a mass migration of excess electrons throughout
the entire surface of the object.
• Excess electrons migrate to distance themselves from their
repulsive neighbors so excess negative charge distributes
itself throughout the surface of the conductor.
• What if electrons are removed from a conductor at a given
location, giving the object an overall positive charge? If
protons cannot move, then how can the excess of positive
charge distribute itself across the surface of the material?
• Electrons are loosely bound within atoms and are free to
move within a conductor. Electron migration happens
across the entire surface of the object, until the overall
sum of repulsive affects between electrons across the
whole surface of the object are minimized.
Example Problem 9: One of these isolated
charged spheres is copper and the other is
rubber. The diagram below depicts the
distribution of excess negative charge over the
surface of two spheres. Label which is which and
support your answer with an explanation.
Electrostatic Charging
• Law of Conservation of Charge:
The net charge of an isolated
system remains constant
• When one material becomes
negatively charged, another
must become positively
charged.
• Charging by Friction: transfer of
charge due to contact between
materials
• Amount of charge depends on
the nature of the materials
• Charging by Conduction:
transfer of charge due to
the flow of electrons by
the contact of a charged
object to a neutral object
• for electrons to move from
the atoms of one material
to the atoms of another
material, there must be an
energy source (a motive)
and a low-resistance
pathway.
Electrostatic Charging, cont.
• Charging by Induction: separation of charges in a
two-body system due to repulsion between like
charges.
• Induction charging is a method used to charge an
object without actually touching the object to any
other charged object.
Electrostatic Charging, cont.
• Polarization: a separation of charge within an
object where the net charge is zero
• Electric dipoles can be induced or permanent
• The density of the electron cloud can be distorted by
nearby atoms
• Electrons shared in covalent bonds may establish a
dipole when the atoms involved have different
electronegativity
Example Problem 10: You shuffle across a carpeted
floor on a dry day and the carpet acquires a net
positive charge. Will you have a deficiency or an
excess of electrons? If the charge the carpet
acquired has a magnitude of 2.15 nC, how many
electrons were transferred?
Since the carpet has a net positive charge, it must
have lost electrons and you must have gained
them.
Given: qc = +2.15 nC = 2.15 x 10-9 C
qe = -1.602 x 10-19 C
net charge on you, q = -qc = -2.15x10-9 C
Grounding
• Grounding is the process of removing the
excess charge on an object by means of the
transfer of electrons between it and another
object of substantial size.
• A ground is simply an object which serves as a
seemingly infinite reservoir of electrons, often
called an “electron sink”
• **Why would grounding a charge be
important?**
Homework
• p. 530 – 531; 1, 6, 9, 10, 12, 13, 18
• Egg Car Plan Sheet Due 02/10/11