chapter 21
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Chapter 21Electric Fields
An electric force of4.5 x 10-5 N is measured
between two particles. One particle has a charge of
2.0 x 10-6 C & the other has a charge of 3.0 x 10-8 C. Calculate the distance
between them.
Electric force like gravitational force is
inversely proportioned to the square of the
distance between the two points of concern
Electric Field (E)•A vector quantity that relates the force exerted
on a charge to the amount of the charge
Electric Field (E)
E =Fon q’
q’
Electric Field (E)
Fon q’ = q’E
Calculate the electric field strength when a 25 N force at 37o NoE is exerted on a charge
of + 5.0 x 10-6 C
Typical Field Strengths
Field Value (N/C)TV tube 1 x 105
Spark req 3 x 106
H orbital 5 x 1011
Electric Field Lines
•Lines representing the force vectors in
an electric field
Electric Field Lines
+
Electric Field Lines
-
+ -
Electric Field Lines
Electric Field Lines
•Always point from positive to
negative
Electric Field Lines
•Do not exist , but provide a model
of a field
The electric field between two
parallel plates is uniform
+ -
Electric Potential•The electric
potential difference of charges
measured in volts
Electric Potential•As with heat, we can only measure
potential difference (V)
Electric PotentialDifference (V)•The change in potential energy per unit charge
Electric PotentialDifference (V)•The work done moving a charge
thru a field charge
Electric PotentialDifference (V)
•Measured in J/C
•J/C = volt (V)
Electric PotentialDifference (V)
W on q’
q’V =
Electric PotentialDifference (V)
U = W
Electric PotentialDifference (V)
Uq’
q’V =
Electric PotentialDifference (V)
W on q’
q’V =
Electric PotentialDifference (V)
W = Fd
Electric PotentialDifference (V)
Fd on q’
q’V =
Electric PotentialDifference (V)
F
q’V = x d
Electric PotentialDifference (V)
F
q’E =
Electric PotentialDifference (V)
V = Ed
Basic Equations•V = Ed•W = qV•F = qE
Equipotential
•When the electric potential
difference is 0
Equipotential
•Charge rearranges itself to reach equipotential
Equipotential•When two spheres have
the same charge, the larger one has lower
electric potential
Equipotential•When two spheres have
the same electric potential, the larger one has the greater charge
Equipotential•When a charged object comes in contact with a
neutral one, the charge in equally distributed
Equipotential•Because of the size of
Earth, when objects touch Earth, their charge
is passed to the Earth
Grounding•When a charged object
touches Earth, all its charge flows to Earth creating equipotential
Electric Fields
•All charges are on the outside of a conductor
Electric Fields
•In pointed object, the field strength is
greatest at the point
Capacitor
•A device designed to store a charge
Capacitance
•The ratio of charge to electric potential
difference
Capacitance (C)
C =
qV
Farad (F)
•Unit for capacitance measured in coulombs
per volt: F = C/V
Basic Equations•V = Ed•W = qV•F = qE•q = CV
A charge of 1.6 x 10-6 C is stored to create a
capacitance of 4.0 x 10-3 F acting over
2.0 m. Calculate: V, E, F, & W
A charge of 1.5 x 10-6 C is stored to create a
capacitance of 4.0 x 10-3 F acting over
2.0 mm. Calculate: V, E, F, & W
A charge of 3.2 x 10-4 C is stored to create a
capacitance of 8.0 mF acting over 4.0
m. Calculate: V, E, F, & W
Charge =1.6 x 10-6 CForce = 3.2 x 10-3 NDistance = 64 nm.
Calculate: V, E, C, & W
Calculate: 3.2 x 10-144
x 1.5 x 10162
8.0 x 10-256 7.5 x 10175
x 4.0 x 10122 =
Calculate: 3.2 x 10144
x 1.5 x 10162
8.0 x 10-254 7.5 x 10-175
x 2.0 x 10125 =
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