Unit 3 - Part 2: Electric Fields
7.2 Coulomb’s Law
Matter is made of particles which are ____________________ or __________________ charged.
Law of electric charges: Electric force between two like charges (charges with the same sign) is
repulsive, whereas electric force between two unlike charges (charges with opposite signs) is
attractive.
A neutral object and a charged object will always _______________________ each other
In the SI system, the unit of charge is the _______________________________ ( ).
Charges are conserved, meaning that they cannot be _________________________.
Law of Conservation of Charge: Charge can be transferred from one object to another, but the
total charge of a closed system remains constant.
Also, it is thought that the total charge of the entire universe is constant and neutral.
Charges are also ________________________, meaning that they occur in finite packages.
The smallest unit of charge is called the ____________________________________________
which is equal to the charge on one proton (+e) or one electron (-e).
Charge of a single electron is: −1.60 × 10 C.
Charge of a single proton is: _________________________________.
Conductor: Any substance in which electrons are able to move easily from one atom to another.
Insulator: Any substance in which electrons are not free to move easily from one atom to another.
Coulomb’s Law
Coulomb determined that the force between two charged objects is proportional to their charges and
inversely proportional to the square of their distances.
where: 𝐹 = magnitude of electric/electrostatic force
𝒌 𝒒𝟏 𝒒𝟐
𝑭𝐄 = q1 = first charge
𝒓𝟐
q2 = second charge
𝑟 = distance between charges
𝑘 = Coulomb’s constant
The proportionality constant 𝑘 is called Coulomb’s constant. Its value is 8.99 × 10 N.m2/C2 .
Do not confuse this constant 𝑘 with the spring constant in Hooke’s law.
There are four important things to notice from this equation.
Electric force can be ________________________ or __________________________.
Electric force depends on ____________________________, not mass.
Electric force gets significantly smaller as 𝑟 _____________________________.
The Coulomb’s constant 𝑘 is much _____________________ than the gravitational constant 𝐺.
There is a very important difference between gravitational forces and electrostatic forces:
Gravity ALWAYS _________________________.
Electrostatic force can ___________________ or ___________________.
When solving for electrostatic forces we will NOT use ± signs.
Instead, we will determine the direction of the force based on attraction or repulsion.
Example: Find the magnitude of the electric force between two electrons separated by a distance of 0.80 nm.
Example: Two point charges of 1.8×10 -6 C and −2.4×10-6 C produce a force of 2.2×10-3 N on each other.
How far apart are these two charges?
Example:
A charge of 𝑞 = 1.7×10-6 C is placed 0.02 m from a charge of 𝑞 = 2.5×10-6 C and 0.035 m from a charge of
𝑞 = −2.0×10-6 C as shown below. Find the net electric force on charge 𝑞 = 1.7×10-6
𝑞 = +1.7×10-6 C 𝑞 = +2.5×10-6 C 𝑞 = −2.0×10-6 C
Example:
Find the magnitude and direction of the net electric force on charge 𝑞 .
