Download 5.1 Electrostatics

Coulomb’s Law
IB Physics 5.1
Electric Force and the Electric
Field
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.
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Coulomb’s Law
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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
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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.
So what is a Coulomb?
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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 1.602176487 × 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?)
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.
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Alternative Definition
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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: