Transcript Electric Field

Electric Charge and Electric
Field
Ch 16
Static Electicity
• Electricity comes
from the Greek work
elektron which means
“amber”.
• Static Electricity =
amber effect
• Ex: a glass rod or
plastic ruler rubbed
with a cloth attracts
tiny pieces of paper
Other Examples
• Clothes out of clothes dryer
• Combing your hair
• Shock when you touch a metal doorknob
Electric Charge
• An object becomes “charged” due to a
rubbing process and it is said to possess
a net electric charge.
• There are two types of electric charge,
positive and negative.
• These denominations are to be taken
algebraically- plus and minus sign
Unlike Charges attract; Like
Charges repel
Law of Conservation of Charge
“The net amount of electric charge
produced in any process is zero”
• Firmly established as those for energy and
momentum
• Benjamin Franklin (1706-1790)
• Whenever a certain amount of charge is
produced on one body in a process, an
equal amount of the opposite type of
charge is produced on another body
Electric Charge in an atom
• Electricity starts inside
the atom itself
• Positively charged
nucleus surrounded by
negatively charged
electrons
• Ion: atom with a positive
or negative charge
• Charge can leak off onto
water molecules in the air
Water molecule is polar
• Why is it hard to
perform any static
electricity demos or
experiments on humid
or rainy days?
• On dry days, the air
contains fewer water
molecules to allow
leakage. On humid, it
is difficult to make any
object hold its charge
for long.
Insulators and Conductors
• Conductors: materials where some of the
electrons are bound very loosely and can
move about freely within the material (free
electrons)
Ex: metals
• Insulators: materials where the electrons
are bound very tightly to the nuclei
Ex: wood, rubber
Semiconductors
• Semiconductors: materials that fall into
an intermediate category
Ex: silicon, germanium, carbon
• Conductors: a lot of free electrons
• Semiconductors: very few free elctrons
• Insulators: almost no free electrons
Methods of Charging
• By Conduction: or by
contact. The two
objects end up with
the same sign of
charge
• By Induction: a
charged object is
brought close to a
neutral object but
does not touch.
Charging by Induction
• Inducing a charge on an object connected
to the ground by a metal wire
(“grounded”), the object will acquire a
charge opposite to the charged object.
• Earth is so large and can conduct, can
easily accept or give up electrons. It acts
like a reservoir for charge.
Electroscope
• Electroscope: is a
device that can be
used for detecting
charge.
• The greater the
amount of charge, the
greater the separation
of the leaves
Electrostatic Conduction
• Electroscope charged by conduction
Electrostatic Induction
• Electroscope charged by induction:
Sign of charged object
• How can you use an electroscope to
determine the sign of a given charge?
• Bring a charged object of known charge
close to a charged electroscope with an
unknown charge and observe the
separation of the leaves
Coulomb’s Law
• An electric charge exerts a force on other
electric charges. What factors affect the
magnitude of this force?
• charges
• distance between charges
• F = k Q1 Q2
r2
Direction of the electric force
The direction of the electric force is always along
the line joining the two objects and will depend
on whether the charges have the same sign or
opposite signs.
Coulomb’s Law
• It gives the force between two points charges,
Q1 and Q2, a distance r apart
• k : the proportionality constant
• k = 8.988 x 109 N. m2/C2
• Unit for charge: the coulomb
• 1 C is a very large charge. Charges produced by
rubbing ordinary objects are typically around a
microcoulomb ( 1 µC = 10-6 C)
Elementary Charge
• Charge of the electron, the elementary
charge, is the smallest charge
• 1 e = 1.602 x 10-19 C
• Electron : -e
• Proton: +e
• Electric charge is quantized, existing only
in discrete amounts: 1e, 2e, 3e, etc
Coulomb’s Law in terms of ε0
• The constant k is often written in terms of
another constant,ε0, called permittivity of
free space.
• k=1
4Лε0
• Other fundamental equations are simpler
in terms of ε0 rather than k
Coulomb’s Law
• Apply to objects whose size is much smaller
than the distance between them
• Point charges: spatial size negligible compared
to other distances
• Charges are at rest (electrostatic)
• It gives the force on a charge due to only one
other charge
Solving Problems involving
Coulomb’s Law
• Ignore the signs of the charges
• Determine direction based on whether the
force is attractive or repulsive
• If several charges are present, the net
force on any of them will be the vector
sum of the forces due to each of the
others
Con’t
• When dealing with several charges, it is often
helpful to use subscripts on each of the forces
involved
• The first subscript refers to the particle on which
the force acts; the second refers to the particle
that exerts the force
• Ex: F31 means the force exerted on particle 3 by
particle 1.
• Very important to draw the free body diagram for
each body showing all the forces acting on that
body
Gravitational x Electric Force
• Both inverse square laws (F ˜ 1/r2)
• Mass for gravity, charge for electricity
• Gravity always attractive
• Electric force can be either attractive or
repulsive
Electric Field
• The idea of forces acting at a distance was
a difficult one for early thinkers
• The idea of field was introduced by
Michael Faraday (1791-1867)
• Electric Field: extends outward from
every charge and permeates all of space
Electric Field
• We can investigate the
electric field surrounding
a charge or group of
charges by measuring the
force on a small positive
test charge
• Test charge: a charge so
small that the force it
exerts does not
significantly alter the
distribution of the charges
that create the field being
measured
Definition of Electric Field
• The electric field is defined in terms of the
force on such positive test charge
• E=F
q
“The electric field at any point in space is a
vector whose direction is the direction of
the force on a positive test charge at that
point, and whose magnitude is the force
per unit charge.”
Direction of Electric Field
• The electric field due to a positive charge
points away from the charge
• The electric field due to a negative charge
points toward that charge
• If q is positive F and E will point in the
same direction. If q is negative, F and E
point in opposite directions
Electric Field due to one point
charge
• E = k q Q/ r2
q
=kQ
r2
• In terms of ε0
• E= 1 Q
4Лε0 r2
• Notice that E is
independent of q, that
is, it depends only on
the charge Q which
produces the field,
and not on the value
of the test charge q
Superposition principle for electric
fields
• If the field is due to more than one charge,
the individual fields, E1, E2,etc, due to
each charge are added vectorially to get
the total field at any point
• E = E1 + E2 + …..
Problem Solving Electrostatic
1. Draw a careful diagram
2. Apply Coulomb’s law to get the magnitude of
the forces or the electric field
3. Determine the direction of each force or
electric field physically (like charges repel,
unlike attract)
4. Add vectorially forces or fields to get resultant
5. Use symmetry (geometry) whenever possible
Field Lines
• In order to visualize the electric field, we
draw a series of lines to indicate the
direction of the electric field at various
points in space. These electric field lines
(sometimes called lines of force) are
drawn so that they indicate the direction of
the force due to the given field on a
positive test charge.
Fild Lines
• Number of lines starting on a positive charge, or
ending on a negative charge, is proportional to
the magnitude of the charge
• The closer the lines are together, the stronger
the electric field in that region
Field Lines
• The direction of the
field at any point is
directed tangentially
Field Lines
• For unequal charges, for example +2Q and -Q,
twice as many lines leave + 2Q as there are
lines entering -Q
Field Lines
• Field lines between
two opposite charged
parallel plates
• Field lines are parallel
and equally spaced
except near the
edges
• Electric field has the
same magnitude,
constant, in the
central region
Properties of Field Lines
1. They indicate the direction of the electric field
(tangent to the field line at any point)
2. The magnitude of the electric field is
proportional to the number of lines crossing
unit are perpendicular to the lines (closer the
lines, stronger the field)
3. They start on + charges and end on – charges
(the # is proportional to the magnitude of the
charge)
Electric Fields and Conductors
• The electric field inside a
good conductor is zero in
the static situation
• If there were an electric
field within the conductor,
there would be a force on
its free electrons, the
electrons would move
(not static situation)
• Main consequence: Any
net charge on a good
conductor distributes
itself on the surface.
• The electric field is
always perpendicular
to the surface outside
of a conductor
• If there were a
component parallel to
the surface, electrons
would move along the
surface in response to
that force (not static
situation)
Why are you safer inside your car
during a thunderstorm?
• The electric field inside is zero
Shielding and safety in a storm
• Conducting metal box is often used for
shielding delicate instruments and
electronics circuits from unwanted external
electric fields
• A relatively safe place to be during a
lightning storm is inside a car, surrounded
by metal