Electric Charge
Chapter 17.1
Examples of Electric Charge
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Rubbing a plastic comb through your hair.
Rubbing a balloon on your hairs.
Rubbing your shoes across a wool rug.
Sliding your clothing across a car seat.
• Experiments like these work best when the air
is dry, moisture creates a pathway for the
charge to leak off the materials.
Two Kinds of Charge
• There is a Positive Charge (+) and a negative
charge (-).
• Benjamin Franklin (1706-1790) coined the
term positive and negative.
• Opposites attract and likes repel.
Positive & Negative
• When rubbing a balloon across your hair, the
balloon becomes negatively charged and the
hair becomes positively charged.
• Positive and negative are said to be opposite
because an object with an equal amount of
positive and negative charge has no net
charge.
Atoms
• An atom is composed of three parts:
– Protons: Positively Charged (+)
– Neutrons: Neutral Charge
– Electrons: Negatively Charged (-)
• The protons and neutrons are relativity fixed
in the atom, but the electrons located on the
outside of the atom can be transferred from
one atom to another quite easily.
Charged Ions
• When electrons are balanced with the
protons, the atom has no net charge.
• If an electron is transferred from one neutral
atom to another, one of the atoms gains a
negative charge and the other loses a negative
charge, therefor becoming positive.
• Ions – Atoms with a net positive or negative
charge.
Balloon and Hair
• Rub the balloon against hair and a charge
starts to build.
• The balloon pulls electrons away from the hair
– Hair loses electrons and becomes positively
charged.
– Balloon gains electrons and becomes negatively
charged.
• By increasing the surface area of the objects,
the charges can be increased.
• The net positive charge will be equal to the
net negative charge.
Millikan Oil Drop Experiment
• Robert Millikan (1886-1953) performed an
experiment by levitating a charged oil droplet
in an electric field.
• From the experiment he learned that electric
charge is quantized.
– Multiples of a fundamental unit of charge.
• Electrons have a charge of –e
• Protons have a charge of +e
Charges
• The value of electric charge:
e = 1.60219x10-19 C
• The SI Unit for charge is a Coulomb (C)
-1C = 6.2x1018 Electrons
Charge and Mass of Atomic Particles
Particle
Charge (C)
Mass (kg)
Electron
Proton
Neutron
-1.60x10-19 C
1.60x10-19 C
0
9.109x10-31 kg
1.673x10-27 kg
1.675x10-27 kg
Transfer of Electric Charge
• Depending on the ability for electrons to flow
in a material determines if the material is a
conductor or an insulator.
• A rubber balloon can receive a net charge, but
the charge can not flow freely around the
balloon, therefore it’s an insulator.
• Copper can receive a net charge, but the
charge can flow through the material making
it a good conductor.
Conductors
• Conductor – Material that transfers charge easily.
• Outer electrons of the atoms in a metal are not
anchored to the nuclei of particular atoms, but
are free to roam in the material.
• Metals are good conductors for the motion of
electric charges because their electrons are
“loose.”
• Ex:
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Copper
Gold
Aluminum
Silver
Insulators
• Insulator – Material that does not transfer charge
easily.
• Electrons insulators are tightly bound and remain
with their particular atoms.
• They are not free to wander about to other
atoms in the material.
• Ex:
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–
–
–
Rubber
Silk
Glass
plastic
Semiconductors & Superconductors
• Semiconductors are a material characterized by
electrical properties between insulators and
conductors.
• Atoms in a semiconductor hold their electrons
until given small energy boosts.
• This occurs in photovoltaic cells that convert solar
energy into electrical energy.
– Germanium and silicon
• Superconductors are materials that have no
resistance to the follow of electrons.
– Super cooling metals
Charge by Contact
• Electrons can also be transferred from one
material to another by simply touching.
• When a charged rod is placed in contact with a
neutral object, some charge will transfer to the
neutral object.
• This method of charging is called charging by
contact.
• If the object is a good conductor, the charge will
spread to all parts of its surface because the like
charges repel each other.
Induction
• Induction – The process of charging a
conductor by bringing it near another charged
object and grounding the conductor.