Transcript File

Forces
• By the early 19th century, physicists had
classified the apparent myriad of forces in
nature to just 3 kinds:
– Gravitational force
– Electric force
– Magnetic force
Forces
• By the end of the 19th century, they had
narrowed the list to just 2 forces:
– Gravitational force
– Electromagnetic force
Forces
• The 20th century first added two new forces
to this list that are observed only inside the
atomic nucleus:
– Gravitational force
– Electromagnetic force
– Weak force
– Strong force
Forces
• And then found theoretical links that
narrowed the list back to 2 kinds of forces:
– Gravitational force
– Strong/Electroweak force
21.1 Electric forces
• Electric forces are created between all electric charges.
• Because there are two kinds of charge (positive and negative)
the electrical force between charges can attract or repel.
Elementary Charges
• Protons carry the smallest positive charge.
• Protons and uncharged neutrons generally
reside in an atom’s nucleus.
• Protons are held in the nucleus by the strong
force.
21.1 Electric Charge
• All ordinary matter contains
both positive and negative
charge.
• You do not usually notice the
charge because most matter
contains the exact same
number of positive and
negative charges.
• An object is electrically neutral
when it has equal amounts of
both types of charge.
21.1 Electric Charge
• Objects can lose or gain electric
charges.
• The net charge is also sometimes
called excess charge because a
charged object has an excess of either
positive or negative charges.
• A tiny imbalance in either positive or
negative charge on an object is the
cause of static electricity.
21.1 Electric Charge
• Electric charge is a property
of tiny particles in atoms.
• The unit of electric charge is
the coulomb (C).
• A quantity of charge should
always be identified with a
positive or a negative sign.
Electrical Charge
• An object with an excess of electrons is
negatively charged.
• An object with too few electrons (too many
protons) is positively charged.
• An object with the same number of electrons
and protons is neutral.
Units of Charge
• The SI unit of charge is the Coulomb.
1 Coulomb = the charge of 6.24 x 1018
electrons
21.1 Electric current
 The direction of current was historically defined as the
direction that positive charges move.
 Both positive and negative charges can carry current.
• In conductive liquids (salt
water) both positive and
negative charges carry current.
• In solid metal conductors, only
the electrons can move, so
current is carried by the flow
of negative electrons.
21.1 Electric current
• Current is the movement of electric charge
through a substance.
Current
(amps)
I=q
t
Charge that flows
(coulombs)
Time (sec)
Two basics type of materials
• Conductors
– Materials, such as metals, that allow the free
movement of charges
• Insulators
– Materials, such as rubber and glass, that don’t
allow the free movement of charges
21.1 Conductors and insulators
• All materials contain
electrons.
• The electrons are what carry
the current in a conductor.
• The electrons in insulators
are not free to move—they
are tightly bound inside
atoms.
21.1 Conductors and insulators
A semiconductor has a few free electrons and atoms
with bound electrons that act as insulators.
21.1 Conductors and insulators
• When two neutral objects
are rubbed together, charge
is transferred from one to
the other and the objects
become oppositely charged.
• This is called charging by
friction.
• Objects charged by this
method will attract each
other.
Superconductors
• Superconductors are materials that lose all
resistance to charge movement at
temperatures near absolute zero (0 K or
about -273oC).
• Recently, “high temperature” (above 100 K)
superconductors have been discovered.
Polarization
• Bringing a charged object near (but not
touching) a neutral object polarizes
(temporarily separates) the charge of the
neutral object.
– Like charges in the neutral object are repelled
by the charged object.
– Unlike charges in the neutral object are
attracted by the neutral object.
• The neutral object returns to normal when
the charged object is removed.
21.2 Coulomb's Law
• The force between two
charges gets stronger
as the charges move
closer together.
• The force also gets
stronger if the amount
of charge becomes
larger.
21.2 Coulomb's Law
• The force between two
charges is directed along
the line connecting their
centers.
• Electric forces always
occur in pairs according
to Newton’s third law,
like all forces.
21.1 Coulomb's Law
• The force between charges
is directly proportional to
the magnitude, or amount,
of each charge.
• Doubling one charge
doubles the force.
• Doubling both charges
quadruples the force.
21.1 Coulomb's Law
• The force between charges is
inversely proportional to the
square of the distance between
them.
• Doubling the distance reduces the
force by a factor of 22 = (4),
decreasing the force to one-fourth
its original value (1/4).
• This relationship is called an
inverse square law because force
and distance follow an inverse
square relationship.
21.2 Fields and forces
• Mass creates a gravitational field that exerts
forces on other masses.
21.2 Fields and forces
• Gravitational forces are far weaker than electric forces.
21.2 Drawing the electric field
21.2 Fields and forces
• The concept of a field is used to describe any quantity that
has a value for all points in space.
• You can think of the field as the way forces are transmitted
between objects.
• Charge creates an electric field that creates forces on other
charges.
Electric Field
• The Electric Force is like the Gravitational
Force
• Action at a Distance
• The electric force can be thought of as being
mediated by an electric field.
Electric Field
• We say that when a charged object is put at
a point in space,
•
The charged object sets up an Electric
Field throughout the space surrounding the
charged object
• It is this field that then exerts a force on
another charged object
Electric Field Lines
• Possible to map out the electric field in a
region of space
•
An imaginary line that at any given point
has its tangent being in the direction of the
electric field at that point
• The spacing, density, of lines is related to the
magnitude of the electric field at that point
Electric Field Lines
• At any given point, there can be only one field
line
• The electric field has a unique direction at any
given point
• Electric Field Lines
•
Begin on Positive Charges
•
End on Negative Charges
21.2 Electric shielding
• Electric fields are created all around us
by electric appliances, lightning, and
even static electricity.
• These stray electric fields can interfere
with the operation of computers and
other sensitive electronics.
• Many electrical devices and wires that
connect them are enclosed in
conducting metal shells to take
advantage of the shielding effect.
21.3 Capacitors
• A capacitor is a storage device for electric charge.
 Capacitors can be connected in series or parallel
in circuits, just like resistors.
21.3 Capacitors
• A capacitor can be charged by connecting it to a battery or
any other source of current.
• A capacitor can be discharged by connecting it to any closed
circuit that allows current to flow.
21.3 Capacitors
The current flowing into or out of
a particular capacitor depends
on four things:
1. The amount of charge already in
the capacitor.
2. The voltage applied to the
capacitor by the circuit.
3. Any circuit resistance that limits
the current flowing in the circuit.
4. The capacitance of the capacitor.