Download - Electrostatic Force
Electrostatic Force
Coulomb’s Law
ChargesTwo charges of the same type repel one another
+ +
The two charges will experience a FORCE pushing them apartor pulling them together
Two charges of the opposite type attract one another
+-
How much force?The amount of force that two charged objects experience
depends on three factors
q1 q2r
1. The charges on the objects (q1 and q2)
2. The distance between them (r)
3. The electrostatic constant (k) = 8.99 x 109 N·m2/C2
We use Coulomb’s Law to calculate this force
221
r
qkqFe
Coulomb vs. Newton
221
r
qkqFe
Look carefully at the Coulomb’s Law equation
221
r
mGmFg
This equation looks very much like Newton’s Law of Universal Gravitation!
Both have an INVERSE SQUARE relationshipbetween FORCE and DISTANCE
F
r
Gravitational force concerns MASSCoulomb force is about CHARGE
Which is stronger?
• Consider two electrons 1μm apart
– What is the gravitational force between them?
• Depends on mass
– What is the electrostatic force between them?
• Depends on charge 26
1919229
)101(
)1060.1)(1060.1)(/1099.8(
mx
CxCxCmNx
26
31312211
)101(
)1011.9)(1011.9)(/1067.6(
mx
kgxkgxkgmNx
221
r
mGmFg
5.54 x 10-59 N
221
r
qkqFe
2.30 x 10-16 N
Electrostatic force is MUCH stronger thangravitational force for small, chargedobjects like electrons and protons!!!
Example
221
r
qkqFe 2
229
)2(
)2)(2)(/1099.8(
m
CCCmNxFe
What is the electrostatic force between these two objects?
r = 2.0 m
q1 = -2C q2 = +2C
NxFe91099.8
A negative answer shows that the force is ATTRACTIVE
Example
221
r
qkqFe 29
66229
)105(
)102)(105)(/1099.8(
mx
CxCxCmNxFe
What is the electrostatic force between these two objects?
r = 5 nm
q1 = -5μC q2 = -2μC
NxFe151060.3
A positive answer shows that the force is REPULSIVE