1. IB Physics 5.1 Electric Force and the Electric Field
Coulomb’s Law
IB Physics 5.1
Electric Force and the Electric Field

2. Like charges repel, unlike charge attract.
What we already know!
Like charges repel, unlike charge attract.
So ……….
There must be force/s involved for this to happen.
And…….
These force/s would be electric in nature.

3. Coulomb’s Law
The electric force acting on a point charge q1 as a result of the presence of a second point charge q2 is given by Coulomb's Law:
where ε0 = permittivity of free space

4. Note that this satisfies Newton's third law because it implies that exactly the same magnitude of force acts on q2 .
Coulomb's law is a vector equation and includes the fact that the force acts along the line joining the charges.
Like charges repel and unlike charges attract.
Coulomb's law describes a force of infinite range which obeys the inverse square law, and is of the same form as the gravity force.

5. So what is a Coulomb?
The coulomb (symbol: C) is the SI unit of electric charge.
It is named after Charles-Augustin de Coulomb.
1 coulomb is the amount of electric charge transported by a current of 1 ampere in 1 second.
The elementary charge is  × 10-19 C
if 1 amp of current equals the flow of 1C of charge per second then….
Quick Quiz: How many electrons is this? (Hint what is the charge on an electron?)

6. A little History!
The ampere was historically a derived unit—being defined as 1 coulomb per second. Therefore the coulomb, rather than the ampere, was the SI base electrical unit.
In 1960 the SI system made the ampere the base unit.

7. Alternative Definition
It can also be defined in terms of capacitance and voltage, where one coulomb is defined as one farad of capacitance times one volt of electric potential difference: