Chapter 9 Electrostatics
Summary Assignment Key Points
Section 9.2 The Electrical Nature of Matter
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All atoms in matter always contain electric charges, but you may not notice them
until you make them move from their normal position.
Static Electricity is a charge on a substance that stays in one place.
The study of static electric charge is called Electrostatics.
Positively charged and negatively charged objects attract most neutral objects.
The Law of Electric Charges states that like charges repel one another and
unlike charges attract one another.
A Model for the Electrical Nature of Matter
o All matter is made of atoms
o The nucleus contains positively charged protons and neutral neutrons.
o A cloud of electrons surrounds the nucleus
o Like charges repel; unlike charges attract
o Some elements have weak attractions for electrons, allowing electrons to
move from atom to atom
o A single atom is always electrically neutral (same number of electrons and
protons)
o Atoms that gain electrons become Negative Ions
o Atoms that lose electrons become Positive Ions
Section 9.3 Charging by Friction
• The transfer of electric charges between objects by a rubbing action.
• The Electrostatic Series shows the kind of electric charge substances will have.
Section 9.5 Transferring Charge by Contact
• Transferring a charge by friction is difficult to avoid.
• When charging by contact occurs, one object is already electrically charged. The
other object may or may not be charged as well.
• The important factor is that there must be a difference in the amount of charge
already on the two objects.
Section 9.6 Insulators and Conductors
• An electrical insulator is a substance in which electrons cannot move freely form
atom to atom (ex. wood)
• An electrical conductor is a substance in which electrons can move freely form
one atom to another. (ex. metal)
Section 9.7 Discharging Electrically Charged Objects
• If a charged object has all the excess electric charges removed it is said to be
discharged or neutralized.
• Ways to discharge objects: Grounding, discharge at a point, exposure to air
• Grounding: connect an object to earth by a conductor such as a wire to a metal
rod buried in the ground
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Discharge at a point: grounding wires don’t work for aircraft or cars; removing an
electric charge by repelling electrons off the tip of a conductor that is pointed at
the end
Section 9.8 Induction
• Third way to charge an object
• Induction is transferring an electric charge from one substance to another
without direct contact
• Induced Charge Separation: A slight shift in position of electrons that produces
opposite charges on the two side of a particle (dust on a TV)
• Induction is the principle used in the design of many devices that remove
pollutants and dust from the air or as a way of coating surfaces.
Section 9.10 Lightning
• Lightning is part of a natural process of exchanging electric charges between the
atmosphere and Earth
• Thunderclouds have huge numbers of negative charges concentrated near the
bottom of the cloud. When they move close to tall objects, it is sometimes close
enough to return to the ground in a huge spark
• The jagged nature of lightning is caused by the path of least resistance.
• Lightning rods: a metal rod attached to the highest part of the building with a
thick conductor connected to a metal plate buried in the ground.
• Lightning rods provide 2 types of protection: they help prevent lighting from
striking and if it does strike, they will direct the charge into the ground