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CHAPTER 17
SECTION 1:
Electric Charge and Static Electricity
 All matter is made up of very small particles called atoms.
 Atoms are made up of even smaller particles – protons,
neutrons, and electrons.
 The charged particles of atoms are protons and electrons –
protons are positively charged and electrons are negatively
charged.
Charges Exert Forces
1. Charge is a physical property – an object can have a positive
charge, a negative, or no charge.
2. Charged objects exert a force – a push or a pull – on other
charged objects.
3. The Law of Electric Charge – states that like charges repel, or
push away and opposite charges attract.
Like charges repel
Unlike charges attract
 Because protons are positive and electrons have opposite
charges, they are attracted to each other.
 Without this attraction, electrons could not be held in atoms.
4. The electric force is the force between charged objects.
5. There are two factors that determine the size of the force are:
1) the amount of the charge on the object – the greater the
charge – the greater the electric force and
2) the distance between the charges – the closer together the
charges are – the greater the electric force.
6. An electric field - the region around a charged object in which
an electric force is exerted on another charged object.
Three ways to charge an object
1. There are three ways to charge an object: friction, conduction,
and induction. Atoms become charge when they gain or lose
electrons.
2. Friction – charging by friction happens when electrons are
“wiped” from one object onto another.
 Ex: Using a cloth and rubbing a plastic ruler - the electrons
move from the cloth to the ruler.
 Also, rubbing a balloon on your hair causes electrons to
move from your hair to the balloon.
3. Conduction – when electrons move from one object to another
by direct contact. Electrons can be transferred from a positively
charged object to an uncharged object.
 Ex: A negatively charged plastic ruler touches an uncharged
metal rod and transfers electrons to the uncharged metal rod.
 The rod becomes negatively charged by conduction.
4. Induction – happens when charges in an uncharged metal object
are rearranged without direct contact with a charged object.
 Ex:If you hold a metal object near a positively charged
object, the electrons in the metal are attracted to and move
toward the positively charged object.
CONSERVATION OF CHARGE - When an object is charged
by any method, NO charges are created or destroyed. Because
charges are not created or destroyed, charge is said to be
conserved.
5. Electroscope – used to detect whether an object is charged.
Moving Charges
1. Electrical conductor – is a metal material in which charges can
move easily. Most metal are good conductors because some of
their electrons are free to move.
 Ex: copper, aluminum, and mercury
2. Electrical insulators – materials in which charges to do not
move easily. They do not conduct charges very well because
electrons cannot flow freely.
 Ex. plastic, rubber, glass, wood, and air
Static Electricity
1. Static electricity – is the electric charge at rest on an object;
generally produced by friction or induction.
 The term static means “not moving” so the charges do not
move away from the object they are in.
2. Electric Discharge – is the release of electricity stored in a
source or the loss of static electricity as charges move off an
object.
 Ex: clothes stuck together by static electricity – this happens
slowly.
3. Sometimes electric discharge happens quickly. One of the most
dramatic examples of electric discharge is lightning.
 See figure 6 How lightning Forms on page 479 in the text.
4. A lightning rod is a pointed rod connected to the ground by a
wire.
 They are mounted so that they are the tallest point on a
building.
 When lightning strikes a lightning rod, the electric charges
are carried safely to Earth through the rod’s wire which
prevents lightning from damaging buildings.
SECTION 2: Electric Current and Electrical Energy
Electric Current
1. Electric current - is the rate at which charges pass a given point.
2. The higher the current is, the greater the number of charges that
pass the point each second.
3. Electric current is expressed in units called Amperes – which are
often shortened to amps.
 The symbol for unit ampere is A.
 The symbol for current is the letter I.
Making Charges Move
1. When you flip a switch, an electric field is set up in a wire at
the speed of light.
2. The electric field causes the free electrons in the wire to move.
The energy of each electron is transferred instantly to the next
electron.
3. The electric field is created so quickly that all electrons start
moving through the wire at the same instant.
AC AND DC
1. There are two kinds of electric current
 direct current (DC) and
 alternating current (AC).
2. In direct current the charges always flow in the same direction.
 The electric current from the batteries used in a camera is DC
3. In alternating current (AC), the charges continually shift from
flowing in one direction to flowing in the reverse direction.
 The electric current from outlets in your home is AC
Voltage
1. Voltage – is the potential difference between two points in a
circuit. (the push behind the current)
2. Voltage is expressed in the unit volts (V).
 In equations, the symbol for voltage is the letter V.
3. Voltage is a measure of how much work is needed to move a
charge between two points.
4. Think of voltage as the amount of energy released as a charge
moves between two points in the path of a current.
 The higher the voltage is, the more energy is released per
charge.
5. As long as there is a voltage between two points on a wire,
charges will flow in the wire.
Resistance
1. Resistance – is the opposition to the flow of electric charge.
2. Resistance is expressed in the unit ohms (Ω, the Greek letter
omega).
 In equations, the symbol for resistance is the letter R.
3. Resistance can be thought of as “electrical friction.”
 The higher the resistance of a material – the lower the current
in the material is.
4. If voltage doesn’t change, as resistance goes up, current goes
down.
5. An objects resistance varies depending on the object’s material,
thickness, length, and temperature.
 As resistance increases, current decreases.
Generating Electrical Energy
1. Energy cannot be created or destroyed. It can only be
transformed or changed into other kinds of energy.
2. Many things change different kinds of energy into electrical
energy.
 Generators convert mechanical (kinetic) energy to electrical.
 Cells and batteries convert chemical or radiant energy into
electrical energy.
o The two kinds of cells are wet cells and dry cells.
 Wet cells have liquid electrolytes.
 A car battery is made of several wet cells
that use sulfuric acids as electrolytes.
 Dry cells work similar to wet cells, but the
electrolytes are solid or paste-like.
 Batteries used in small radios & flashlights
are dry cells.
7. Thermocouples – a device that converts thermal energy into
electrical energy.
8. Photocells – a device that converts light energy in electrical
energy.