Transcript 15-1. Did you complete the reading assignment?

Charging a rod
• Charge by conduction
• Charge by induction—
• induced charge
• A Hydrogen atom is composed of a nucleus
containing a single proton about which a single
electron orbits. The electric force between the
two particles is 2.3 e 39 (that's 2.3 times 10 to
the power 39) greater than the gravitational
force. If we could adjust the distance between
the two particles, can we find a separation at
which the electric and gravitational forces are
equal? explain your answer
A Hydrogen atom is composed of a nucleus containing a single proton
about which a single electron orbits. The electric force between the two
particles is 2.3 e 39 (thats 2.3 times 10 to the power 39) greater than the
gravitational force. If we could adjust the distance between the two
particles, can we find a separation at which the electric and gravitational
forces are equal? explain your answer
●Yes, because Fe=keqq0/r2, If we increase the distance, the electric force
decreases, then eventually it equals to the gravitational force.
●Yes, as long as the gravitational force does not decrease at exactly the
same rate as the electrical force, there should be a point at which they are
equal.
●No. The gravitational and electric forces will never be equal because they
are both inversely proportional to the square of the distance between the
proton and election. Therefore, if you change the distance, both the
gravitational and electric forces will change in equal proportion, and the
electric force will still be 2.3 e 39 times greater than the gravitational force.
• Charges q1 and q2 are placed in space.
• They are free to move.
+q1
+q2
• Charge q1 will feel a force due to charge q2 and
q2 will feel a force due to q1 .
• Charge q1 will move left and charge q2 will
move right.
+q1
+q2
 kq1q2
F  2 rˆ
r
Which force is larger?
The force on q1 due to q2 or the force on q2 due
to q1
The two forces are the same—Equal and
opposite forces like gravity.
F
m
+q
F
M
F
-q
+Q
The direction of Fe is along a line
connecting the two charges —
but can attract or repel
depending on the sign of the charges.
Electric Charge
• Like charges repel.
• Unlike charges attract.
Three balls are suspended from thin insulating
threads. Onto each ball is transferred some charge. It is
found that balls 1 and 2 repel each other and that balls 2
and 3 also repel each other. What can we conclude about
the charges on the balls? Explain your answer.
●If the balls repel each other they must have the same
charge. 1 and 2 are the same, 2 and 3 are also the
same, so all three balls must have the same charge.
However, we can't say whether that charge is positive or
negative just from this information.
Discussion question: Three rods are suspended from
thin insulating threads. It is found that rods 1 and 2 attract
each other and that rods 2 and 3 repel each other. What
can we conclude about the charges on the rods?
1.
2.
3.
4.
5.
1 and 3 carry charges of
opposite sign.
1 and 3 carry charges of
the same sign.
All three carry charges of
the same sign
One of the objects carries
no charge.
We need to do more
experiments to answer the
question above.
Discussion question: Three charges are placed
as shown. Charges A and B are held fixed in
place. What direction is the force on C?
A
C
B
+q
+q
-q
A.
B.
C.
D.
E. Zero magnitude – no direction
Two charges are placed as shown. What
direction is the electric force on a positive
charge at point A?
A
+q
A.
B.
C.
D.
E. Zero magnitude – no direction
+q
Which vector has a greater magnitude?
What direction is the net force?
What if I make q1 negative. Then what direction is the net force?
What is the z component of a unit vector in the
direction of vector A = (3, -2, 2) m ?
(magnitude only)
A.
B.
C.
D.
E.
3.000 m
2.000 m
0.7276 m
0.4851 m
1.000 m
Electric Field
• The direction of the Electric field is the direction of the
force on a positive point charge at a given point.
• Force is a vector
• Electric field is a vector
q1 feels a force due to an electric field produced by q2.
 kq1q2

F  2 rˆ  q1 E2
r
Three charges are placed as shown. Charges A
and B are held fixed in place. What direction
is the electric field at C?
A
C
B
+q
+q
-q
A.
B.
C.
D.
E. Zero magnitude – no direction
Math review
• In packet
– Study formulas and chapter summaries
– Study “What does it mean to integrate?”
Next three class periods we will do a lot of math
review.
The math review test starts next Wednesday
Answers
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10-5
11-b
12-b
14-d
16-b