Transcript Physics 17

Physics
Chapter 17:
Electric Forces and Fields
Electric Forces and Fields
Electrical Charge
Positive Charge
Caused by the Removal of Electrons
Negative Charge
Caused by an Excess of Electrons
Electrically Neutral
Balance of Nucleus and Electron Cloud
Charges
Electric Forces and Fields
Electrical Charge
Like Charges
Repel Each Other
Opposite Charges
Attract Each Other
Electric Forces and Fields
Electrical Charge
Charge is Conserved
Some Atoms May Gain Electrons
Anions
Some Atoms May Lose Electrons
Cations
Charge is Transferred
Electric Forces and Fields
Electrical Charge
Quantity of Charge
Charges are Dependant on
Addition or Removal of Electrons
Charges can be +1, +2, +3 …
or -1, -2, -3 …
One Unit of Charge is 1.6x10-19C
Electric Forces and Fields
Electrical Charge
Quantity of Charge
Charge of an Electron
- 1.6x10-19C
Charge of a Proton
+ 1.6x10-19C
Electric Forces and Fields
Electrical Charge
Quantity of Charge
Units
Coulomb (C)
Charles Augustin de Coulomb
1736 - 1806
Electric Forces and Fields
Transfer of Electrical Charge
Conductors
Materials that Allow Charge
(electrons) to Move
Most Metals
Most Salts
Water (with electrolytes)
Electric Forces and Fields
Transfer of Electrical Charge
Insulators
Materials that Inhibit the Flow of
Charge
Glass
Wood
Plastic
Water (pure)
Electric Forces and Fields
Transfer of Electrical Charge
Semiconductors
Insulator, but Dynamically
Conductive by Temperature and
Addition of Impurities
Silicon
Electric Forces and Fields
Transfer of Electrical Charge
Charging by Contact
Static Charge
Insulators
Conductors
Ground
Reservoir
of Electrons
Rub-ee
Rub-er
Charge
Glass Rod
silk
+
Acrylic tube
fur
−
Electric Forces and Fields
Transfer of Electrical Charge
Charging by Induction
Distributing Charge
Electric Forces and Fields
Transfer of Electrical Charge
Conduction
Charging a Neutral Object Through the
Contact of a Charged Object
Induction
Charging a Neutral Object Through a
Charged Object without Contact
Electric Forces and Fields
Electrical Force
Force can Act Over a Distance
Force is Directly Proportional to
Distance
Electric Forces and Fields
Electrical Force
Coulomb’s Law
Forces from Charge
The Greater the Charge, the Greater the
Force
The Closer the Distance, the Greater the
Force
q1q2
F k 2
r
Electric Forces and Fields
Electrical Force
Coulomb’s Law
F = Electrostatic Force
q = Magnitude of Point Charge (Not -)
r = Distance Separating Point Charges
k = 8.99x109 Nm2/C2
q1q2
F k 2
r
Electric Forces and Fields
Electrical Force
Coulomb’s Law
Units
Coulomb (C)
1 C = 6.25x1018 electrons
The Charge of One Electron = -1.60x10-19C
q1q2
F k 2
r
Electric Forces and Fields
Electrical Force
Field Force (No Contact)
Can be Attractive or Repulsive
Gravity Only Attractive
Strong Force
Gravitational Force is Relatively Much Weaker
q1q2
F k 2
r
Electric Forces and Fields
Electrical Force
Superposition
More than Two Charged Particles
Vector Sum of the Forces
-q2
F1-2
+q1
F1-3
-q3
q1q2
F k 2
r
Electric Forces and Fields
Problem
Two electrons in an atom are separated by 1.5x10–10 m,
the typical size of an atom. What is the electrical force
between them?
Electric Forces and Fields
Solution
Two electrons in an atom are separated by 1.5x10–10 m,
the typical size of an atom. What is the electrical force
between them?
19 2
q1q2
(
1
.
6
x
10
)
9
2
2
8
F  k 2  9 x10 Nm / C

1
.
1
x
10
N
10
2
r
(1.5 x10 m)
Electric Forces and Fields
Problem
Two negatively charged bodies each with charge of
-5.0x10–5 C are 0.20 m from each other. What force acts
on each particle?
Electric Forces and Fields
Solution
Two negatively charged bodies each with charge of
-5.0x10–5 C are 0.20 m from each other. What force acts
on each particle?
5
2
q1q2
9
2
2 (5 x10 C )
2
F  k 2  9 x10 Nm / C
 5.6 x10 N
2
r
(0.20m)
Electric Forces and Fields
Problem
Two charges, q1 and q2, are separated by a
distance, d, and exert a force, F, on each other.
What new force will exist if q1 is doubled?
q1q2
F k 2
r
Electric Forces and Fields
Solution
If q1 is doubled it becomes 2q1 which doubles F
(2q1 )( q2 )
2F  k
2
r
q1q2
F k 2
r
Electric Forces and Fields
Problem
Two charges, q1 and q2, are separated by a
distance, d, and exert a force, F, on each other.
What new force will exist if q1 and q2 are cut in
half?
q1q2
F k 2
r
Electric Forces and Fields
Solution
If q1 and q2 are cut in half they become ½ q1 and
½ q2 which reduce F, 4 fold
 1  1 
 q1  q2 
1
2  2 

F k
2
4
r
q1q2
F k 2
r
Electric Forces and Fields
Problem
Two charges, q1 and q2, are separated by a
distance, d, and exert a force, F, on each other.
What new force will exist if d is tripled?
q1q2
F k 2
r
Electric Forces and Fields
Solution
If d is tripled, F is reduced by 32
1
q1q2
F k
2
9
(3r )
q1q2
F k 2
r
Electric Forces and Fields
Problem
Two charges, q1 and q2, are separated by a
distance, d, and exert a force, F, on each other.
What new force will exist if d is cut in half?
q1q2
F k 2
r
Electric Forces and Fields
Solution
If d (r) is halved, F is increased by 22
q1q2
4F  k
1 2
( r)
2
q1q2
F k 2
r
Electric Forces and Fields
Problem
Two charges, q1 and q2, are separated by a
distance, d, and exert a force, F, on each other.
What new force will exist if q1 is tripled and d is
doubled?
q1q2
F k 2
r
Electric Forces and Fields
Solution
If q1 is tripled it becomes 3q1 and if d is doubled
the denominator becomes 3r2, so F is reduced
to ¾
3
(3q1 )( q2 )
F k
2
4
2r
q1q2
F k 2
r
Electric Forces and Fields
Problem
A force of 4.4x103 N exists between a positive charge of
8.0x10–4 C and a negative charge of –3.0x10–4 C. What
distance separates the charges?
Electric Forces and Fields
Solution
A force of 4.4x103 N exists between a positive charge of
8.0x10–4 C and a negative charge of –3.0x10–4 C. What
distance separates the charges?
kq1q2
(9.0 x109 Nm2 / C 2 )(8.0 x104 C )(3.0 x104 C )
r

 0.70m
3
F
4.4 x10 N
Electric Forces and Fields
Problem
A positive charge of 3.0 µC is pulled on by two negative
charges. One, –2.0 µC, is 0.050 m to the north and the
other, –4.0 µC, is 0.030 m to the south. What total force
is exerted on the positive charge?
Electric Forces and Fields
Solution
A positive charge of 3.0 µC is pulled on by two negative
charges. One, –2.0 µC, is 0.050 m to the north and the
other, –4.0 µC, is 0.030 m to the south. What total force
is exerted on the positive charge?
6
6
(
3
.
0
x
10
C
)(
2
.
0
x
10
C)
9
2
2
F1  (9.0 x10 Nm / C )
 22 N , north
2
(0.050m)
Electric Forces and Fields
Solution
A positive charge of 3.0 µC is pulled on by two negative
charges. One, –2.0 µC, is 0.050 m to the north and the
other, –4.0 µC, is 0.030 m to the south. What total force
is exerted on the positive charge?
6
6
(
3
.
0
x
10
C
)(
4
.
0
x
10
C)
9
2
2
F2  (9.0 x10 Nm / C )
 120 N , south
2
(0.030m)
Electric Forces and Fields
Solution
A positive charge of 3.0 µC is pulled on by two negative
charges. One, –2.0 µC, is 0.050 m to the north and the
other, –4.0 µC, is 0.030 m to the south. What total force
is exerted on the positive charge?
F  F2  F1  120N  22N  98N , south
Electric Forces and Fields
Homework
Page 655
Problems
18 (3.5x103 N)
19 (45N attractive)
21 (1x10-7N, 12o below +x axis)
23 ?
Electric Forces and Fields
Electric Fields
Electric Fields Exist Around Any Object
that Carries a Charge
The Potential Difference Between Two
Charged Objects Can Transfer Energy in
the Form of Electricity
Electric Forces and Fields
Electric Fields
Electrical Force is Measured in
C/m, Units are Newtons
Electric Field (E) is Measured in
F/q0, Units are N/C
Electric Forces and Fields
A Charge Creates an
Electric Field in All
Directions
A Second Charge (test
charge), Within the
Electric Field of the
First Charge, Will be
Affected by the Force of
the Electric Field
-q2
q0
+q1
-q3
Electric Forces and Fields
Electric Fields (E)
F - Electric Force
2
kq
2
F
q
r
E

k 2
q0
q0
r
F
E
q0
q
Ek 2
r
Electric Forces and Fields
Units
q`= Coulomb
F = Newton
E = Newton/Coulomb (N/C)
q
Ek 2
r
F
E
q
Electric Forces and Fields
The Magnitude of an Electric Field is
Proportional to the Distance From the
Charge Creating the Field
The Closer to the Charge, the Greater the Field
The Further from the Charge, the Weaker the
Field
Electric Forces and Fields
Electric Fields Have Direction
Fields Extend Away from Positive Charges
Fields Extend Toward Negative Charges
Electric Forces and Fields
Electric Field Lines
Electric Forces and Fields
Electric Field Lines
Electric Forces and Fields
Conductors in
Electrostatic
Equilibrium
St. Elmo’s Fire
Charge Accumulation
at Points of
Conductive Surfaces
Corona
St. Elmo’s Fire Video
Electric Forces and Fields
Conductors in Electrostatic Equilibrium
Van de Graff Generator
Rubber Belt Pulls e- from Felt
Electrons Transfer to Screen
Excess e- at Sphere
Seeking Equilibrium
No Points = No Corona
Electric Forces and Fields
Homework
Page 656-657
Problems
38 (12x103 N/C toward the 30nC Charge)
43 (a, 2x107N/C +x b, 40N)
56 ?