Electrostatics
Electric Charge
• The source of negative charge is the electron
• The source of positive charge is the proton
• The smallest possible amount of charge, the
elementary charge, is 1.6 x 10-19 Coulombs
• Electron charge = - 1.6 x 10-19 C
• Proton charge = + 1.6 x 10-19 C
– All other amounts of charge are multiples of these
• Electric Charge is a conserved quantity
What is an Electric Field?
• One charged object can influence another
charged object without any direct contact.
• We say a charged object is surrounded by an
electric field, a region of influence. Any other
charged object in that space will interact with
that field and experience an electrical force.
Electric Field Strength
• The electric field strength at a given location is
defined as the amount of force per unit of
charge at that location.
E=F/q
(units of E: N/C)
This is analogous to the way we described gravitational field
strength earlier in the year: g = Fg/m
Field around a positive charge
• A positive charge placed in this field will feel a force
directed away from the source of the field…repulsion
• A negative charge placed in this field will feel a force
directed towards the source of the field…attraction
Field around a negative charge
• A positive charge placed in this field will feel a force
directed toward the source of the field…attraction
• A negative charge placed in this field will feel a force
directed away from the source of the field…repulsion
Field around two opposite charges
(same amount)
Any charge placed in the field will feel a force
in a direction tangent to the field lines
Field around two like charges (same
amount)
Electric Field Lines
• When drawing, or interpreting, electric field lines
keep in mind the following…
– Filed lines originate at + charge or infinity
– Field lines terminate at – charge or infinity
– Filed lines are always perpendicular to the surface of a
charged object
– Line density is an indicator of field strength
– The number of lines leaving a + and terminating at a –
is proportional to the magnitude of charge
– Field lines NEVER cross each other (Why?)
Electric Force – how strong is it?
• The magnitude of force between two charges is
described by Coulomb’s Law:
𝑘𝑄1 𝑄2
𝐹=
𝑟2
𝑘 – constant = 9.0 x109 Nm2/C2
𝑄 – amount of charge
𝑟– distance between charged objects
Determine the electrostatic force on the
electron in a hydrogen atom from the proton in
the nucleus
• Q1 =
• Q2 =
• r=
For comparison purposes, determine the
gravitational force between them as well…