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Coulomb’s Law and Electric Fields. Q p =1.6x10 -19 C. Q e = -1.6x10 -19 C. +. -. r = 1x10 -10 m. Recall Coulomb’s Law. Magnitude of the force between charges q 1 and q 2 separated a distance r: F = k q 1 q 2 /r 2 k = 9x10 9 Nm 2 /C 2 Force on nucleus of Hydrogen from e -. - PowerPoint PPT Presentation
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Coulomb’s Lawand Electric Fields
Recall Coulomb’s Law
• Magnitude of the force between charges q1 and q2 separated a distance r:
F = k q1q2/r2 k = 9x109 Nm2/C2
• Force on nucleus of Hydrogen from e-
F = (9x109)(1.6x10-19)(1.6x10-19)/(10-10)2 N
= 2.3x10-8 N (to the right)
+ -
r = 1x10-10 m
Qp=1.6x10-19 C Qe = -1.6x10-19 CF
5
Electric Field• Charged particles create electric fields.
– Direction is the same as for the force that a + charge would feel at that location.
– Magnitude given by: E F/q = kq/r2
+
r = 1x10-10 m
Qp=1.6x10-19 C
E
E = (9109)(1.610-19)/(10-10)2 N = 1.41011 N/C (to the right)
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What is the direction of the electric field at point B?
1) Left
2) Right
3) Zero
x
yA
B
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Since charges have equal magnitude, and point B is closer to the negative charge net electric field is to the left
E FieldWhat is the direction of the electric field at point C?
1) Left
2) Right
3) Zero
x
y
C
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Away from positive charge (right)
Towards negative charge (right)
Net E field is to right.
Red is negative
Blue is positive
Comparison:Electric Force vs. Electric Field
• Electric Force (F) - the actual force felt by a charge at some location.
• Electric Field (E) - found for a location only – tells what the electric force would be if a charge were located there:
F = Eq• Both are vectors, with magnitude and direction.
Add x & y components. Direction determines sign.
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What is the direction of the electric field at point A?
1) Up
2) Down
3) Left
4) Right
5) Zerox
yA
B
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Red is negative
Blue is positive
E Field IIWhat is the direction of the electric field at point A, if
the two positive charges have equal magnitude?
1) Up
2) Down
3) Right
4) Left
5) Zero
x
yA
B
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Red is negative
Blue is positive
Electric Field Lines• Closeness of lines shows
field strength- lines never cross
• # lines at surface Q
• Arrow gives direction of E- Start on +, end on -
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A B
X
Y
Charge A is
1) positive 2) negative 3) unknown
Field lines start on positive charge, end on negative.
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A B
X
Y
Compare the ratio of charges QA/ QB
1) QA= 0.5QB 2) QA= QB 3) QA= 2 QB
# lines proportional to Q
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A B
X
Y
The magnitude of the electric field at point X is greater than at point Y
1) True 2) False Density of field lines gives E
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The electric field is stronger when the lines are located closer to one another.
E Field Lines
Compare the magnitude of the electric field at point A and B
1) EA>EB 2) EA=EB 3) EA<EB
A
B
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E inside of conductor• Conductor electrons free to move
– Electrons feels electric force - will move until they feel no more force (F=0)
– F=Eq: if F=0 then E=0
• E=0 inside a conductor (Always!)
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A B
X
Y
"Charge A" is actually a small, charged metal ball (a conductor). The magnitude of the electric field inside the ball is:
(1) Negative (2) Zero (3) Positive
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Recap• E Field has magnitude and direction:
– EF/q
– Calculate just like Coulomb’s law
– Careful when adding vectors
• Electric Field Lines– Density gives strength (# proportional to charge.)
– Arrow gives direction (Start + end on -)
• Conductors– Electrons free to move E=0
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