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COULOMB’S LAW

Unit 7: Electrostatics

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ELECTRIC FORCE

The force between two electrically charged objects is an electric force
Like gravity, this is a non-contact force that works over a distance
It is a vector, so it has both magnitude and direction

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ELECTRIC FORCE

The size of the electric force depends on 2 things:

The amount of charge (the greater the charge, the greater the force)
The distance between charges (the further the distance, the less the force)

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COULOMB'S LAW

Coulomb’s law relates the force between two single charges separated by a distance

Force

(N)

Constant

9x10⁹ N^.m²/C²

Distance (m)

F = K q₁ q₂

r²

Charges (C)

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COULOMB'S LAW

The force between two charges gets stronger as the charges move closer together
The force also gets stronger if the amount of charge becomes larger

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COULOMB'S LAW

The force between charges is directly proportional to the magnitude, or amount, of each charge

Doubling one charge doubles the force
Doubling both charges quadruples the force

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COULOMB'S LAW

The force between charges is inversely proportional to the square of the distance between them

Doubling the distance reduces the force by a factor of 2²= 4, decreasing the force to ¼ its original value
Reducing the distance by ¼ increases the force by 4² = 16

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DIRECTION ON A POINT CHARGE

If q1 is positive and q2 is negative, determine the direction of the force on each

q₁

+

q₂

-

If both q1 and q2 are positive, determine the direction of the force on each

Force of q1 on q2: left

Force of q2 on q1: right

q₁

+

q₂

+

Force of q1 on q2: right

Force of q2 on q1: left

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EXAMPLE 1

Two balls are each given a static electric charge of one ten-thousandth (0.0001) of a coulomb. Calculate the force between the charges when they are separated by one-tenth (0.1) of a meter.

F = Kq₁q₂

r²

F = (9x10⁹⁾(0.0001)(0.0001)

0.1²

k = 9x10⁹N•m²/C²

q₁ = 0.0001C

q₂ = 0.0001C

r = 0.1 m

F = 9,000 N

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EXAMPLE 2

A charge of -2.0x10^-4 and a charge of 8.0x10^-4 are separated by 0.3 m.

What is the magnitude and direction of the force the negative charge exerts on the positive charge?

F = Kq₁q₂

r²

F = (9x10⁹⁾(-2.0x10^-4)(8.0x10^-4)

0.3²

K = 9x10⁹N•m²/C²

q₁ = -2.0x10^-4 C

q₂ = 8.0x10^-4 C

r = 0.3 m

F = 16,000 N

F(-) on (+) 🡪 attractive force

F(-) on (+) = 16,000 N left

q₁

-

q₂

+

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EXAMPLE 2

A charge of -2.0x10^-4 and a charge of 8.0x10^-4 are separated by 0.3 m.

What is the magnitude and direction of the force the positive charge exerts on the negative charge?

K = 9x10⁹N•m²/C²

q₁ = -2.0x10^-4 C

q₂ = 8.0x10^-4 C

r = 0.3 m

F = 16,000 N

F(+) on (-) 🡪 attractive force

F(+) on (-) = 16,000 N right

q₁

-

q₂

+

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FIELDS AND FORCES

An electric field is the region around a charged object where electric forces can be exerted on another charged object (repelled or attracted)

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FIELDS AND FORCES

Mass creates a gravitational field that exerts forces on other masses

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FIELDS AND FORCES

Gravitational forces are far weaker than electric forces

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+

–

+

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DRAWING THE ELECTRIC FIELD

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DRAWING THE ELECTRIC FIELD

1. Field lines point away from positive and toward negative.	2. Electric Field Lines cannot cross.� � �No! � Yes!�	3. The Electric Field is stronger in places where the field lines are closer together. �At point A the field is stronger than point B

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EXAMPLE 3

Identify if the set of electric field lines represents a positive or negative charge.

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LESSON CHECK 7.3

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0.01

0.02

0.03

0.01

0.02

0.03

0.04

0.05

Force (N)

Effect of Radius on Force