Transcript Electric field strength (E)

ELECTRIC FIELDS,
POTENTIAL DIFFERENCE
& CAPACITANCE
The fundamental
rule at the base of
all electrical
phenomena is that
like charges repel
and opposite
charges attract.
ELECTRIC FIELD
A charge creates an electric
field around it in all
directions. When another
charged object enters this
electric field, it experiences
an electric force (magnitude
and direction).
ELECTRIC FIELD
Michael Faraday developed
the concept of an electric
field.
MR. CHARGE
BABY CHARGE
Here is a positive charge
(+Q). What is the
strength of the electric
field (E) at a point that is
distance d from the
charge?
E=?
d
Electric field strength (E) is
defined as the force
experienced by a small positive
test charge (+q) when it is
placed at a point some distance
(d) from the charge.
F
E 
q
+
d
Here is a negative charge
(-Q). What is the
strength of the electric
field (E) at a point that is
distance d from the
charge?
E=?
d
Electric field strength (E) is
defined as the force
experienced by a small positive
charge +q when it is placed at
that point.
F
E 
q
+
d
ELECTRIC FIELD
INTENSITY
F
E
q
E  Electric Field Intensity
in Newtons/Co ulomb (N/C)
F  Force in Newtons (N)
q  charge in Coulombs (C)
Overview Problems
• Problem 1
• Problem 2
• Problem 3
ELECTRIC FIELDS
The direction of the electric
field at any point is the
direction that a positive
charge (+q) would move when
placed at that point.
ELECTRIC FIELDS
Electric Field Lines are
imaginary lines drawn so that
their direction at any point is
the same as the direction of the
electric field at that point.
ELECTRIC FIELDS
The strength of the electric
field is indicated by the spacing
between the lines. The closer
the electric field lines the
stronger the electric field.
POSITIVE CHARGE
The electric field lines around
a positive charge will point
away from the charge.
NEGATIVE CHARGE
The electric field lines around
a negative charge will point
towards the charge.
TWO POSITIVE
CHARGES
POSITIVE &
NEGATIVE CHARGE
WHICH CHARGE IS
STRONGER A OR B?
1.
2.
A
B
A
B
http://dev.physicslab.org/asp/applets/pointcharges/default.asp
ELECTROSTATIC
EQUILIBRIUM
A good conductor contains charges
that are not bound to any atom
and are free to move within the
material. When no net motion of
charges occurs within a
conductor, the conductor is said
to be in electrostatic equilibrium.
PROPERTIES OF AN
ISOLATED CONDUCTOR
1. The electric field is zero
everywhere inside the conductor.
2. Any excess charge on an
isolated conductor resides entirely
on its surface.
PROPERTIES OF AN
ISOLATED CONDUCTOR
3. The electric field outside a
charged conductor is perpendicular
to the conductor’s surface.
4. On an irregularly shaped
conductor, the charge tends to
accumulate at sharp points.
ROBERT VAN DE
GRAAFF
VAN DE GRAAFF
At Museum of Science
ENERGY AND
ELECTRICAL POTENTIAL
Consider a fixed negative charge
placed at a point B and a fixed
positive charge at point A.
There is a force of attraction
between the two charges.
Point A
Point B
ENERGY AND
ELECRICAL POTENTIAL
Work has to be done against
the force of attraction to move
the negative charge from point
B to point C. Therefore
negative charge will have a
change in its potential energy.
Point A
Point B
Point C
ELECTRICAL POTENTIAL
DIFFERENCE
measured in volts
the change in potential energy
(work) per unit charge.
Use this formula when
the work on a charge is
given.
W
V
Derived
q
Equation:
F d

q
F
 d
q
 Ed
Use this formula when
working with parallel plates
(uniform electric field)
Overview Problems
•
•
•
•
•
•
•
Example
Problem 4
Problem 5
Problem 6
Problem 7
Problem 8
Problem 9
Parallel Plates
When two parallel plates are
connected across a battery, the
plates will become charged and an
electric field is established between
them.
PARALLEL PLATES
Since the field lines are parallel and
the electric field is uniform between
two parallel plates, a test charge would
experience the same force of
attraction or repulsion no matter where
it was located.
F=qxE
The direction of the electric field is defined as
the direction that a positive test charge would
move if placed in the field. So in this case, the
electric field would point from the positive plate
to the negative plate. The field lines are parallel
to each other and so the electric field is
uniform.
V  Ed
V measured in Volts (V)
E measured in Newtons/Coulomb (N/C)
d measured in meters (m)
Therefore 1 N/C = 1 V/m
Since the field lines are parallel and
the electric field is uniform between
two parallel plates, a test charge would
experience the same force of
attraction or repulsion no matter
where it was located. That force is
calculated with the equation:
F = q E
• In the diagram above, the distance between the
plates is 0.14 meters and the voltage across the
plates is 28V.
V
28V
E 

 200V / m
d
.14m
If a positive 2 nC charge were inserted anywhere
between the plates, it would experience a force in
the direction of the negative, bottom plate, no
MATTER where it is placed in the region between
the plates.
9
F  Eq  (200V / m)( 2  10 )
7
 4  10 N
WHAT HAPPENS TO E
AS d INCREASES &
DECREASES
E and d have an inverse relationship (mathematically)
Millikan Oil Drop
Experiment
Robert Millikan discovered the charge of
an electron.
Millikan Oil Drop
Experiment
Fine oil drops were sprayed from an
atomizer in the air. Gravity acting on the
drops caused them to fall. A potential
difference was placed across the plates.
The resulting electric field between the
plates exerted a force on the charged
drops. The resulting electric field between
the plates was adjusted to suspend a
charged drop between the plates.
REMEMBER
 An electron always carries the same charge.
 Charges are quantized.
 Changes in charge are caused by one or
more electrons being added or removed.
qE  mg
q g

m E
V
where : E 
d
Capacitor
A device designed to store electric charge.
A typical design of a capacitor consists of two
parallel metal plates separated by a distance.
The plates are connected to a battery.
Electrons leave one plate giving it a positive
charge, transferred through the battery and to
the other plate giving it a negative charge. This
charge transfer stops when the voltage across
the plates equals the voltage of the battery.
Thus the charged capacitor acts as a storehouse
of charge and energy that can be reclaimed
when needed for a specific application.
The capacitance (C) of a capacitor
is defined as the ratio of the
magnitude of the charge on either
conductor to the magnitude of the
potential difference (voltage)
between the conductors.
Q
C
V
C = Capacitance
(Farad) (F)
Q = Charge
(Coulomb) (C)
V = Potential Difference
(Volts) (V)
Overview Problems
• Example
• Problem 10
• Problem 11