Electric Charge and Fields. AP Physics B Chapter 16 Notes. Static Electricity. Charge is a property of subatomic particles Charges can be positive or negative Opposites attract likes repel. Static Electricity. Charge can be transferred by rubbing two objects together
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Electric Charge and Fields
AP Physics B Chapter 16 Notes
Requires contact
No contact is required—you bring a charge object near a conductor and polarize it (charge separation) and then ground it.
FG∝ mm’
FG∝ 1/r2
FG =(Gmm’)/r2
FE∝ qq’
FE∝ 1/r2
FE =(kqq’)/r2
Where k=8.988 x 109Nm2/C2
(constant of proportionality)
Example: An electron is released above the surface of the Earth. A second electron
directly below it exerts an electrostatic force on the first electron just great enough to cancel out the gravitational force on it. How far below the first electron is the second?
Electric Forces and Fields obey Newton’s Laws.
FNet= F E– F G= 0
F E= F G
Fe
e
mg
5.1 m
r = ?
e
Consider three point charges, q1 = 6.00 x109 C (located at the origin), q2 = 2.00x109 C and q3= 5.00x109 C, and, located at the corners of a RIGHT triangle. q2 is located at y= 3 m while q3 is located 4m to the right of q2. Find the resultant force on q3.
+
Which way does q2 push q3?
Which way does q1 push q3?

4m
q2
q3
3m
Fon 3 due to 1
5m
q
q1
+
Fon 3 due to 2
q = 37
q3
q= tan1(3/4)
4m
q2
q3
3m
Fon 3 due to 1
5m
q
q1
F3,1sin37
Fon 3 due to 2
q = 37
q= tan1(3/4)
q3
F3,1cos37
5.6 x109 N
7.34x109 N
1.1x108 N
64.3 degrees above the +x
E = F/q [N/C]
E is a vector quantity
c
a
+Q
b
E= (kQ)/r2
Electric Fields
Gravity Fields
P. 36 Pg. 466 Two point charges are separated by 12 cm. The electric field at P is zero. How far from Q1 is P?
Electric Dipole
(E is constant)
P. 41 Pg. 467 An electron is accelerated in an E field (1.45 x 104 N/C2) between two plates 1.1 cm apart. The electron is accelerated from rest and passes through a hole in the positive plate. How fast is it going at the hole?
E field ⊥ to surface
Another situation
There must be a negative charge on this side OR this side was induced positive due to the other side being negative.
There must be a positive charge on this side
Units are Nm2/C