Foundations of
Physics
Four Fundamental Forces
Cosmic and Macroscale
Macroscale and
Microscale
Subatomic
Nuclear
Electromagnetic Force
• Force of attraction or repulsion due to an
imbalance of electric charge
Opposite
Charges
Attract
Like Charges
Repel
Electric Charge
• Atoms are
composed of
positive protons,
neutral neutrons,
and negative
electrons
• Neutral atoms
have equal
numbers of
positive protons
and negative
electrons
electron
neutron
proton
Electric Charge
• Protons and neutrons are locked in the
nucleus and can not move freely
• Electrons are found outside the nucleus in
energy levels and are free to move
• A positive or negative electric charge
results from objects gaining or losing
electrons
Conservation of Charge
• When electrons are transferred from one
object to another, no electrons are created
or destroyed.
• The electrons removed from one object
equals the electrons added to another
object.
Charging By Friction
• Results from rubbing objects together
• Called triboelectric charging
• Friction strips electrons from atoms of one
object and deposits them on another
object
Friction transfers the electrons
from the sweater to the balloon
The balloon and sweater have
a balance of electric charge
initially
Resulting in a slightly more
positive sweater and a slightly
more negative balloon
Electron Affinity
• In some materials, the electromagnetic
force of attraction between the positive
nucleus and the negative electrons is very
strong. This attraction is called electron
affinity.
• Electrons move from materials with low
electron affinity to materials with high
electron affinity.
Conductors and Insulators
• Conductors are materials which allow
electric charge to move through them
easily.
• Insulators are materials which do not
allow electric charge to move through
them easily.
Iron Nail
Plastic Rod
Electrons strongly bound to
positive nucleus
Electrons are loosely
bound to positive nucleus
Conduction
• Charging by contact
• Charging a neutral object by making
contact with a charged object
• The resulting charge is always the same
as the original charge.
• Requires a conductor
Conduction
• When a negatively charged object touches a
neutral object, the repulsive force between the
electrons pushes the mobile electrons off of the
negative object and onto the neutral object.
• Thus, the original negative charge is spread
Objects are separated after initial touch
over both objects.
- Charged object touches neutral object
Both objects have a negative charge
Some electrons move onto sphere
Notice the negative charge on the original object is reduced because
some electrons moved to the sphere
Conduction
• When a positively charged object touches a
neutral object, the force of attraction pulls
mobile electrons off of the neutral object onto
the positively charged object
• Results in both objects obtaining a positive
Objects are separated after initial touch
charge
+ Charged object touches neutral object
Some electrons move to positive object
Both objects have a positive charge
Notice the positive charge on the original object is reduced because
some electrons moved onto the object from the sphere
Polarization
• A negatively charged balloon is brought near a
neutral sphere
• The negatively charges repel the electrons of
the sphere pushing them to the opposite side of
the sphere.
• Results in an object with two oppositely
charged ends
Van de Graaff Generator
• A motor at the base of the generator turns
a large rubber belt which is stretched
between two rollers located at each end of
a long clear tube.
Metal Sphere
Rubber Belt
Motor in Base
Van de Graaff Generator
• As the rubber belt moves over the rollers,
electrons are removed from the rollers and
begin to build up on the belt.
• Wire brushes at the top of the clear tube
remove the excess electrons from the belt
and conduct the negative charge to the
metal sphere at the top of the generator.
Van de Graaff Generator
• The negative charge on metal
sphere repels each other
causing the charge to spread
out evenly around the sphere.
• When the electromagnetic
force of repulsion becomes
too large, the negative charge
“jumps” off the sphere to
another object or the air.
Induction
• Charging of a neutral object by bringing a
charged object near (but not touching) it.
• Includes two processes: Polarization and
Grounding
• Results in two oppositely charged
objects
Induction
• The negative charged electrons on the sphere
are removed by touching it with a finger (or
other object)
• Grounding – connecting a charged object to
Earth to allow electrons to travel to or from the
object depending on the situation
Induction
• The finger (or grounding object) is
removed and the balloon and sphere are
separated
Notice the positive charge is
equal to the negative charge
on the original balloon (there
Is no reduction of charge)