Transcript P142_Lect2

•CONTENT
-know facts
•CONCEPTS
-understand principals
•PROBLEM SOLVE -figure things out
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An electron and a proton are set free, near
each other, deep in outer space. The
electron moves towards the proton with
1) constant velocity ( v = constant )
2) increasing velocity but constant
acceleration ( a = constant )
3) increasing velocity and increasing
acceleration
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An electron and a proton are set free, near
each other, deep in outer space. The
electron moves towards the proton with?
There is a net force on the electron due to the proton’s
charge, so the electron accelerates (1 is out). As the electron
moves closer to the proton, the force it experiences grows
stronger (Coulomb’s Law holds F  1/r2). If the force
becomes stronger, and the mass does not change, then
Newton’s Second Law (F = ma or a = F/m) says that the
acceleration increases.
The answer must be (3).
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An electron and a proton are set free,
near each other, deep in outer space.
What about the proton?
mproton = 0.000 000 000 000 000 000 000 000 001 6748 kg
melectron=0.000 000 000 000 000 000 000 000 000 0009 kg
mproton = 1836melectron
1e  qe
-19
=1.6021010
Coulomb
How many electrons are there in
1 Coulomb of negative charge?
How many protons are there in
1 Coulomb of positive charge?
Three pith balls are suspended from thin threads.
Various objects are charged (rubbing glass against
polyester, nylon against silk, rubber against fur, etc)
and each pith ball is charged by touching them with
one of these objects.
It is found that pith balls A and B repel each other
and that B and C repel each other.
1)
2)
3)
4)
A and C carry charge of opposite sign.
A and C carry charge of the same sign.
All three carry charge of the same sign.
Additional experiments are necessary to
determine the sign of all charges.
Three pith balls are suspended from thin threads.
Various objects are charged (rubbing glass against
polyester, nylon against silk, rubber against fur, etc)
and each pith ball is charged by touching them with
one of these objects.
It is found that pith balls A and B attract each other
and that B and C repel each other.
1)
2)
3)
4)
A and C carry charge of opposite sign.
A and C carry charge of the same sign.
All three carry charge of the same sign.
Additional experiments are necessary to
determine the sign of all charges.
Two uniformly charged spheres are firmly fastened by insulated
stands to frictionless pucks which are set on an air table. The
charge on sphere 2 is 3 the charge on sphere 1. Which force
diagram correctly shows the magnitude and direction of the
electrostatic forces?
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Charles Coulomb (1738-1806)
What force exists between two concentrations of charge,
each 1 coulomb separated center-to-center by 1 meter?
What force exists between two concentrations of charge,
each 1 coulomb separated center-to-center by 1 meter?
q1q2
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2
2 ( 1 C )( 1 C )
F = k 2 = (8.9875  10 Nm / C )
R
(1 m2)
= 8.9875109 N
What force exists between two concentrations of charge,
each 1 coulomb separated center-to-center by 1 meter?
q1q2
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2
2 ( 1 C )( 1 C )
F = k 2 = (8.9875  10 Nm / C )
R
(1 m2)
= 8.9875109 N 0.2248 lb/N
= 2,020,390,000 lbs
= 1,010,195 tons
assume 2 equally charged
balloons (with uniformly
distributed charge)
 = 15o
0.75 meters
0.8 grams
each
W  0.008 N
We can calculate
the distance, x, by

1) 0.75sin
2) 0.75cos
3) 0.75/sin
4) 0.75/cos
5) sin/0.75
6) sin/0.75
0.75 m
x
x
sin  =
0.75meter

0.75 m
x = (0.75meter) sin 
= (0.75meter) sin 15
= 0.194meter
o
x

STRING’S
TENSION
ELECTROSTATIC
REPULSION
BALLOON’S
WEIGHT

T
F
W

F
T
T
W
F
W
Felectric
We can calculate
the tan by
W
T

1) T/F
2) T/W
3) F/T
4) F/W
5) W/F
6) W/T
Felectric
With
tan15o=
Felectric
W
W
T
Felectric=(0.008 N)tan15o

= 2.14410-3 N
(8.9875109 Nm2/C2) q1q2
0.002144 N =
(0.388 m)2
R = 0.388 meters
W  0.008 N
(8.9875109 Nm2/C2) q1q2
0.002144 N =
(0.388 m)2
q1q2 = (0.388 m)20.002144 N / (8.9875109 Nm2/C)
= 3.5910-14 C2
R = 0.388 meters
W  0.008 N
Assuming q1  q2, that means each q  1.8910-7 Coulomb
How many electrons would that be?
A hydrogen atom is composed of a nucleus containing a
single proton, about which a single electron orbits. The
electrical force between the two particles is 2.31039
greater than the gravitational force!
If we adjust the distance separating the two particles,
can we find a separation at which the electrical and
gravitational forces are equal?
1) Yes, by moving the particles farther apart.
2) Yes, by moving the particles closer together.
3) No, at any distance.