Electricity and Magnetism
• Explore the second of the four fundamental
forces in nature
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Gravity
Electricity and Magnetism
Weak Nuclear Force
Strong Force
• Huge effect on everyday life
Electricity and Magnetism
• The electromagnetic force is what binds
electrons to the nuclei of atoms
• Responsible for all of chemistry
• When we liberate electrons from individual
atoms, we can make them do work for us
• So, the topic is extremely important if we
are to understand the world around us
Electricity and Magnetism
• Many of the fundamental particles have
something we call electric charge
• We don’t know what this is, we can only
describe the results when particles have the
property
• There are two kinds of charges which we
call Positive and Negative
Electricity and Magnetism
• We observe that like charges repel
• We observe that unlike charges attract
• The forces are inverse square law forces,
just like gravity, but very much more
powerful
• Atoms have a positive nucleus with
surrounding negatively charged electrons
Electricity and Magnetism
• Each electron in an atom is identical to
every other electron so they all have the
same mass and the same negative charge
• The nucleus is composed of positively
charged protons and uncharged neutrons
• All protons are identical and the charge of
the proton is exactly the same size as the
charge of the electron, but it is opposite
Electricity and Magnetism
• Normal atoms have identical numbers of
protons and electrons
• Atoms with a missing electron are called
positive ions
• Atoms with an extra electron are called
negative ions
Conservation of Charge
• Another important conservation law is
Conservation of Charge
• Add to our set of tools along with
Conservation of Energy and Conservation
of Momentum
• We don’t know why, but we observe it to
always be true
• Another deep meaning that isn’t simple
Coulomb’s Law
• Relationship describing the force between
two charged particles
q1 q 2
Fk 2
r
• Charge is measured in coulombs
• Force in newtons
• Distance in meters
Coulomb’s Law
• One Coulomb is a HUGE charge
• Takes 6.25 x 1018 electrons to amount to
this much charge
• The constant k is 9 x 109 Nm2/C2
• Shows how huge the electric force really is
Coulomb’s Law
Gravitational Force
M pme
FG
r2
F  6.7  10
11
1.67  10 27  9.1  10 31

5 10 11

2
F  4  10 47 N
Coulomb Force
Fk
q p qe
r
2
F  9  10
9
1.6 10 19  1.6  10 19
F  9.2  10 8 N
5  10 
11 2
Charge Polarization
• Make a tiny difference in the average
positions of electrons in an atom
Distorts electrons like
tides on the earth and
pulls the nucleus to the
right
Electric Field
• How do things interact with each other
when they don’t physically touch?
• Problem has occupied many great minds
• When we talked about gravity, we said that
Einstein demonstrated that mass distorts
spacetime and the bending of spacetime
made objects appear to experience a force
• Can talk about a gravitational field
Electric Field
• Place a charged particle in spacetime and it
creates an electric field
• Using Coulomb’s Law, we can calculate the
force on a test charge at every point in space
• The force/unit charge is the definition!
E  F/ q
Electric Field
Electric Potential
• Lift an object into the air and we give it
gravitational potential energy
PE  Work
PE  Force  distance
PE  mg  h
PE  mgh
Electric Potential
• Separate a positive charge from a negative
charge and give it electric potential energy
PE  work
PE  force  distance
PE  Eq  h
PE  Eqh
Electric Potential
• Define electric potential as the electric
potential energy per unit charge
electric potential energy
Electric Potential =
amount of charge
1 Joule
1 Volt =
Coulomb