Electric Charge
What is it?
How do we move it?
Comparison of Subatomic Particles
Relative
Particle Symbol Charge
Mass
Electron
e-
-1
Proton
p+
+1
1
Neutron
n0
0
1
Charge is Quantized
• There is a minimum amount of charge that
can exist (1 electron)
• All charges are an integer multiple of this
charge
Millikan’s Oil Drop Experiment
• Electron charge = – 1.60 10-19 C
• Proton charge = + 1.60 10-19 C
Charge is Conserved
• Due to its dependence on matter (electrons &
protons), charge must be conserved
• The total amount will be the same at the end
as it was at the start
• When electrons are transferred, the value of
charge created is equal
Charged Objects
• An object becomes charged when it has an
imbalance of subatomic particles.
• More e- than p+ = negative charge
• More p+ than e- = positive charge
Effects of Unbalanced Charges
• Objects that have an imbalance of protons and
electrons will experience a change in motion
– Like charges repel
– Opposite charges attract
Properties of objects determines the
behaviors of excess charge
• Conductors
• Electric charges move
freely within a
framework of protons
• Most metals are
conductors
• When any part of the
conductor gains an
excess charge, that
charge is distributed
throughout the entire
object
Properties of objects determines the
behaviors of excess charge
• Insulators
• Electrons do not travel
freely between atoms
• Atoms that acquire an
excess charge will
maintain that charge; it
will not be distributed
throughout the entire
object
Charging
• Interaction between objects will result in an
imbalance of protons and neutrons
• Electrons are transferred between objects
– The object that gains electrons will have a
negative charge
– The object that loses electrons will have a positive
charge
• The direction of electron transfer depends on
the materials involved
Charging by Friction
- +
- +
- +
- +
- +
- +
Charging by Conduction
• Electrons are transferred by contact
• When objects that have different affinity for
electrons come into contact, electrons are
transferred
• Conductors and insulators can both be
charged by conduction, but what happens to
the excess charge depends on whether the
object is a conductor or insultor
Grounding
• Electrons are transferred to or from the Earth
by contact
• The Earth can accept or donate an infinite
number of electrons without gaining a charge,
due to the incredible number of atoms in the
Earth
Induction
• In certain cases, conductors can allow charges
to move within the conductor without actually
gaining an imbalance of positive or negative
charge
• The nature of conductors is responsible for
this ability
Induction
• The charged object will repel the like charges
in the conductors
• Conductors have the ability to transfer charge
easily
Induction
Forces Between Charges
• Electric charge can cause changes in motion,
which means that they exert force
– Like charges repel
– Opposite charges attract
Coulomb’s Law
•
• q1 & q2 = value of two charges
• d = distance between charges
• kC = 8.99 109
(Coulomb Constant)
Coulomb’s Law
•
• The force is directly related to the sizes of the
charges
• The force is inversely related to the distance
between the charges squared
Field Forces & Field Theory
• Certain forces (gravity, electric charge,
magnetism) have the ability to affect objects in
the space around them even though they do not
actually touch these objects
• Field theory was proposed by Michael Faraday to
explain “action at a distance” (field forces)
– He actually proposed it for electric fields, but it
applies to other types of field forces as well
• The area around the object where it can affect
other objects is called the “force field”
Electric Field
• What happens to a positive charge around the
charge(s) in question?
Electric Field
• The lines move away from positive charges
and towards negative charges