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COULOMB’S LAW
Let’s Get Charged Up!!!!!!!!!!
What is a Coulomb?
• The unit of charge is the
coulomb (C)
• The charge on a proton
is +1.602 X 10-19
coulomb.
• The charge on a electron
is -1.602 X 10-19
coulomb.
Coulomb’s Law
• Explains the relationship between the
amount of each charge , the distance
between their centers and the electrical
force.
• The force between 2 charged objects
depends on the amount of charge and the
distance between charges.
Coulomb’s Law cont’
• Force is directly proportional to the charge of
each object. (greater the charge = stronger the
force)
• Force between 2 charges varies inversely with
the square of the distance between the 2
charges. (Inverse Square Law)
Coulomb’s Law
Coulomb’s Law
FE = electric force (N = Newtons)
k = “Coulomb’s constant” = 9 X 109 N·m2/C2
q1 = charge #1 (C)
q2 = charge #2 (C)
r = distance between charges (m)
Practice Problem #1
1. Two steel marbles are each given a net charge of
0.001 C. Calculate the size of the force on the
marbles if they are held 2 meters apart.
Practice Problem #2
2. Calculate the size of the force if the marbles are
held 4 m apart?
2a. Is the force half the value of when the marbles
were 2 meters apart? Why or why not?
Practice Problem #3
3. Two charged boxes are 4 meters apart from each
other. The blue box has a charge of 0.000337 C and
is attracting the red box with a force of 626 N.
Determine charge of the red box.
Practice Problem #4
• 4. The force between two charged objects is 4350
N. One object has a charge of 0.028 C and the
other has a charge of 0.033 C. What is the distance
between the 2 objects?
What is Electricity?
• The movement of
electrons from one
place to another.
• e- movement:
negatively charged
area  positively
charged area
Electricity that moves…
• Current: The flow
of electrons from
one place to
another.
• Measured in
amperes (A)
• Kinetic energy –
the energy of
motion
What is Voltage?
• Electrical Voltage measures the
difference in electric potential energy
between 2 places in a circuit.
• Differences in voltage are what cause
electric currents to flow.
• Circuit = the path through which
electricity travels
What is Resistance?
• The opposition to the flow of
an electric current, producing
heat.
• The greater the resistance, the
less current gets through.
• Good conductors have low
resistance.
• Measured in ohms.
Ohm’s Law
• Electric current is dependent on voltage and
resistance.
Examples of Problems
1. A toaster oven has a resistance of 12
ohms and is plugged into a 120-volt outlet.
How much current does it draw?
Examples of Problems
2. The motor in a toy car has a resistance of
3.2 ohms and needs 1.5 amperes of
current to run properly.
What battery voltage is needed?