Welcome to Phys 208! - UW-Madison Department of Physics
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Transcript Welcome to Phys 208! - UW-Madison Department of Physics
From Last Time…
Forces between
charges
Electric dipole
1
Exam 1
Wed. Feb. 20, 5:30-7 pm
Covers Chap. 21.5-7, 22-23,25-26
+ lecture, lab, discussion, HW
8 1/2 x 11 handwritten note sheet (both sides) allowed
Students with schedule class conflicts:
stay after lecture today to arrange time
Review Group/Quiz (solutions on website).
Review lab question sheets.
Review sample exams on website.
HW4 (due Fri. next week)
covers exam material.
2
From Last Time…
Forces between
charges
Electric dipole
3
Unusual dipoles:
Electrogenic fish
• Dipole +
nearby conducting object
Some fish generate charge
separation - electric dipole.
Dipole is induced in nearby
(conducting) fish
Small changes detected by
fish.
4
The idea of electric fields
• EM wave made up of
oscillating electric and magnetic fields.
• But what is an electric field?
• Electric field is a way to describe the force on
a charged particle due to other charges
around it.
• Force = charge electric field
• The direction of the force is the direction of
the electric field.
5
Electric field of a point charge
Force on this charge…
Q2
+
+
+
Q1Q2
F k 2 rˆ
r
Q1
…due to this charge
E F r /Q2
+
= Force/unit charge
Units? N/C
6
+
Question
Which vector best represents the electric
field at the red dot?
A
B
E
C
-
D
7
Electric field
• Electric field vector defined at every point in
space.
• Gives magnitude and direction of force on
test particle
e.g. wind velocity
(speed and
direction) in
different parts of
the country.
8
Relationship Between F and E
• Fe = qE
This is valid for a test charge small
enough that it does not disturb the
source charge distribution
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• If q is positive, F and E are in the same direction
Qp=1.6x10-19 C
+
Electric field 1Å away
from proton
E
r=
9
2
19
9
10
N
M
/C
1.6
10
C
10
10
m
2
1x10-10
m
E
2.9 1011 N /C
(to the right)
9
Electric Field Direction
• a) q is positive, F is
•
•
•
directed away from q
b) The direction of E is
also away from the
positive source charge
c) q is negative, F is
directed toward q
d) E is also toward the
negative source charge
10
Superposition with Electric Fields
• At any point P, the total electric field due to a group of
source charges equals the vector sum of electric fields of
all the charges
Find the electric field due to q1, E1
Find the electric field due to q2, E2
E = E1 + E2
Remember, the fields add as vectors
11
Quick Quiz
Which is the direction of the electric field at dot?
A. Left
B. Right
C. Up
Away from positive charge (right)
D. Down
E. Zero
y
+
-
x
Net E field is to right.
12
Quick Quiz
In this electric dipole, what is the direction of the
electric field at point A?
A) Up
A
B) Down
C) Left
D) Right
x=-a
+Q
x=+a
-Q
E) Zero
13
Electric field: summary
• Electric field ->
will be a force on a charged particle.
• This force ( and electric field) can arise from
electric charges (via Coulomb’s law)
• But once electric field is known, don’t need
to know the charges that produce it.
14
Calculating dipole electric field
y
On the y-axis
E E y yˆ
E y E E
kq
kq
1
1
kq
2
2
y s /2
y s /2
+ s/2
2ys
y s /2 y s /2
1
k 2qs 3
y
2
2
For y s
x
-
s/2
Since p points from - charge to + charge
Ek
2p
r3
on y-axis of dipole only
15
Question: electric dipole
A and B are the same (large) distance from dipole.
How do the magnitude of the electric fields at A
and B compare?
A) E A E B
A
B) E A E B
C) E A E B
B
D) E B 0
16
Pictorial representation of E: Electric Field Lines
17
Electric field lines
• Local electric field tangent to
•
•
•
•
field line
Density of lines proportional to
electric field strength
Fields lines can only start on +
charge
Can only end on - charge.
Electric field lines can never
cross
18
Point particles
19
Electric field of a dipole
+
-
20
Electric field of two + charges
21
Question
How are the charges A and B related?
A) A+, B-, A B
B) A-, B+, A B
C) A+, B-, A B
D) A-, B+, A B
E) A+, B-, A B
A
B
22
Hydrogen Atom Example
• The magnitude of the electric force between the electron and
•
proton:
Fe = ke|e| |-e| / r2 = 8.2 x 10-8 N, r ~ 0.53 x 10-10 m
The gravitational force between the electron and the proton
Fg = Gmemp / r2 = 3.6 x 10-47 N
me = 9.11 x 10-31 kg, mp = 1.67 x 10-27 kg
G = 6.67 x 10-11 N m2/kg2
Fe/Fg ~ 2 x 1039
Mp=1.67x10-27 kg
Me = 9.11 x10-31 kg
+
r = 0.53x10-10 m
23