Coulom
b’s Law
• Charles Coulomb (1736-1806: French)–Studied relationship between
force and charge Unit of charge = 1 coulomb (C)
The charge on 1 electron is (1.6 x 10-19 C)
Coulom
b’s Law
• The LAW!– Force directly related to magnitude of
the charges– Force inversely related to the square of
the distance between the two charges
F = r2
kqQF α q Q
r2
1F α
r2
q QF αk is a constant that corrects the proportion into an equality.
F = r2
k qQ
• This is similar to a formula • we talked about in Lesson 5.
Only Attractiv
e
Attractive or
Repulsive
F = r2
G mM
Newton’sLawOf
Universal Gravitation
Coulomb’sLaw
Coulom
b’s Law
Product of
masses
Product of
charges
F = r2
G mM
F = r2
k qQ
• Keep comparing:
Coulom
b’s Law
• Keep Comparing:
Both areinverse-squarelaws
F = r2
G mM
F = r2
k qQ
• Keep Comparing
F = r2
G mM
9 x 109 N·m2/C2
Large Magnitude
6.67 x 10-11 N·m2/kg2
Small Magnitude
F = r2
k qQ
Exponent Keys on Calculators: Borrow one from your instructor. Ipods and cell telephones don’t work well with exponent keys.
TI 30: 2nd EETI 83: 2nd EE
Others: Might see exp, or x 10x These keys are used when decimal places are too many for the calculator to display or use.
A number like .0000000000000000000000000003 so it needs to be typed by using the exponent key. TI 30: 3EE-28, others: 3exp-28
More Practice:
56900000000000000000000000
5.69EE25
.000000000000000000000256
2.56EE-22
Coulom
b’s Law
F= r2kq Q
• Sample problem 1! YEAH!–What is the magnitude of the
electrostatic force between 2 protons in the nucleus? (assume the distance to be 3.0x10-13 cm)
(9 E109)=
(1.6 E10-
19)(3.0 E 10-15
)2
F=25.6 N
(1.6 E 10-
19)
Coulom
b’s Law
F= r2
G mM
• Sample problem 2! – What is the magnitude of the gravitational
force between 2 protons in the nucleus? (assume the distance to be 3.0x10-13 cm)
(6.67 x 10-11)
=(1.67 x 10-
27 )(3.0 x 10-15 )2
F= 2.07x10-35 N
(1.67 x 10-27)
Electric Fields: An area around a charged object that affects all other charged objectsUnit: N/C
The more the lines the stronger the field
Two More Rules for Drawing Electric Field Lines:
Lines are always drawn out of the positive and into the negative.
The more the lines the stronger the field.
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