Download Document

Charge
• Comes in + and –
• Is quantized
• elementary charge, e, is charge on
1 electron or 1 proton
• e = 1.602  10-19 Coulombs
• Is conserved
• total charge remains constant
Coulomb’s Law
• F = kq1q2
2
/r
• k = 8.99  109 N m2 / C2
• q1, q2 are charges (C)
• r2 is distance between the
charges (m)
• F is force (N)
• Applies directly to spherically
symmetric charges
Spherical Electric Fields
F =kqq0
2
r
E = F = kq
2
q0
r
Why use fields?
Forces exist only when two
or more particles are
present.
Fields exist even if no
force is present.
The field of one particle
only can be calculated.
Field around + charge
Positive
charges
accelerate
in
direction
of lines of
force
Negative
charges
accelerate in
opposite
direction
Field around - charge
Positive charges
follow lines of
force
Negative charges
go in opposite
direction
For any electric field
F = Eq
F: Force in N
E: Field in N/C
q: Charge in C
Principle of
Superposition
When more than one charge
contributes to the electric
field, the resultant electric
field is the vector sum of the
electric fields produced by
the various charges.
Field around dipole
Caution…
Electric field lines are NOT
VECTORS, but may be used to
derive the direction of electric
field vectors at given points.
The resulting vector gives the
direction of the electric force
on a positive charge placed in
the field.
Field
Vectors
Electric Potential
U = kqq0
r
V = U = kq
q0
r
(for spherically symmetric charges)
Electrical Potential
V = -Ed
V: change in electrical
potential (V)
E: Constant electric field
strength (N/m or V/m)
d: distance moved (m)
Electrical Potential Energy
U = qV
U: change in electrical
potential energy (J)
q: charge moved (C)
V: potential difference (V)
Electrical Potential
and Potential Energy
Are scalars!
Potential Difference
Positive charges like to
DECREASE their potential.
(V < 0)
Negative charges like to
INCREASE their potential.
(V > 0)
Potential surfaces
positive
highest
high
medium
low
negative
lowest
Equipotential surfaces
low
high
Definition: Capacitor
Consists of two “plates” in close
proximity.
When “charged”, there is a
voltage across the plates, and they
bear equal and opposite charges.
Stores electrical energy.
Capacitance
C = q / V
C: capacitance in Farads (F)
q: charge (on positive plate) in
Coulombs (C)
V: potential difference between
plates in Volts (V)
Energy in a Capacitor
UE = ½ C
2
(V)
U: electrical potential energy (J)
C: capacitance in (F)
V: potential difference
between plates (V)
Capacitance of parallel
plate capacitor
C = ke0A/d
C: capacitance (F)
ke: dielectric constant of filling
0 : permittivity (8.85 x 10-12 F/m)
A: plate area (m2)
d: distance between plates(m)
Parallel Plate Capacitor
+Q
V1
V2
V3
V4
V5
E
dielectric
-Q
Cylindrical Capacitor
-Q
E
+Q
Problem #1
+4 C
60o
+1 C
60o
+1 C
Calculate the
force on the
4.0 C
charge due to
the other two
charges.
Problem #2
Calculate the mass of ball B,
which is suspended in midair.
A
q = 1.50 nC
R = 1.3 m
B
q = -0.50 nC
Problem #3
Two 5.0 C positive point
charges are 1.0 m apart.
What is the magnitude and
direction of the electric field
at a point halfway between
them?
Problem #4
Calculate the magnitude of the charge on each
ball, presuming they are equally charged.
40o
1.0 m
A
0.10 kg
1.0 m
B
0.10 kg