Transcript ps ch 21 Electricity

Chapter 21
Electricity
21-1 Electric Charge
Static Electricity is the accumulation of electric
charges on an object: One object loses
electrons while another object gains electrons.
• Objects with “opposite” charges will be
“attracted”
• Objects with “like” charges will “repel” each
other.
21-1 Electric Charge
• Charged objects can cause electrons to
rearrange their position on a neutral object by a
process called induction. (this can cause your
hair to “stand up” from static electricity)
• The electrons create an electric field that
exerts a force on objects that have an electric
charge.
•It is strongest near the electron and becomes
weaker as distance from the e- increases.
21-1 Electric Charge
Conductors are materials that allow electrons to move
easily through them. (carry electricity)
• Metals make good conductors because they only
have one or two electrons in their outer shells so they
don’t hold on to them very well.
• Copper is used in wiring.
• Silver is a good conductor.
• Gold is also a very good conductor but it is very
expensive.
• Metals are also used as cooking utensils and pans
since they conduct heat very well. Stainless steel is
an alloy of metals often used.
21-1 Electric Charge
Insulators are materials that DO NOT allow e- to move
through it easily; EX: plastics, wood, rubber, and glass.
• Earth contains a large supply of e- therefore it
functions as a conductor.
• An object connected to Earth is grounded and will
carry a current, such as lightning to Earth.
21-1 Electric Charge
The Electroscope is a device that is able to detect
electric charges; made of two thin metal leaves/strips
attached to a metal rod with a knob at the top.
(p.549) When a charged object touches the knob
the electrons travel down the rod to the leaves. Both
leaves become negatively charged as they gain
electrons. Since they have similar charges, they repel
each other.
Rubbing a glass rod with silk will cause e- to
leave the glass rod and build up on the silk, leaving the
glass rod positively charged. This will positively charge
the electroscope leaves and cause them to repel each
other.
Electroscope
21- 2 Lightning – Causes & Effects
Lightning is a very large discharge of static electricity.
Air currents in a cloud build up areas of positive and
negative charges.
If the bottom portion of a cloud has a negative charge it
can induce a positive charge on Earth’s surface.
As the difference in charge increases, electrons are
attracted toward the positively charged ground.
A lightning bolt occurs when many electrons are
transferred at the same time. Each lightning bolt that
strikes Earth may contain several billion billion
electrons.
21- 2 Lightning – Causes & Effects
• Most lightning doesn’t strike Earth, just moves from
the negative area of one cloud to the positive area of
another cloud.
•The lightning ionizes atoms and produces great
amounts of heat which causes the air to expand
rapidly, producing thunder.
• Lightning strikes can cause power outages, fires,
injury, and death.
• Many buildings have lightning rods to attract the
lightning bolt and carry it harmless to the ground.
21- 2 Lightning – Causes & Effects
• Lightning plays an important role in the nitrogen
cycle of the Earth’s forests.
• Many, if not most, forest fires are started by
lightning.
• Most often the flames travel along the ground and
don’t harm healthy trees.
•This clears the dead debris from the forest floor and
reduces the risk of rapidly spreading fires that can
become out of control.
21- 3 Electric Current
• Electrons flow from one object to another because of
a potential difference. (One object has more e- than
the other object.)
• Static electricity isn’t a constant flow of electrons
and cannot keep a bulb lit or an appliance working.
• This potential difference between two places is
measured in volts (V) and is often called voltage. It is
measured by a voltmeter.
21- 3 Electric Current
• The closed pathway through which the
electrons flow is called a circuit.
•If a switch is turned on, closing the pathway, a
bulb is lit, or an appliance starts working
because electrons have a “path to flow”.
• The “flow” of e- through a wire or conductor is
called current.
21- 3 Electric Current
• The amount of current depends on the number
of e- passing a point in a given time. Current is
measured in amperes (A).
• One ampere = one coulomb flowing past a
point in one second. One coulomb = the charge
of 6.24 x 10 18 electrons.
• Current is measured with an instrument called
an ammeter.
21- 3 Electric Current
• In order to keep the current moving through a circuit, a
device must maintain the potential difference. One
device that can maintain this potential difference
(measured in volts) is a battery.
• Small batteries are dry cell batteries. Dry cells are
electron pumps because they create a potential
difference between the (+) and (-) terminals/end of the
batteries.
• When the two ends are connected in a circuit, e- are
released from the carbon rod, making the C rod (+), and
e- accumulate on the zinc, making it negative (-). The
potential difference between these two ends causes
current to flow through the circuit as long as the
chemical reaction occurs between the carbon and the
zinc.
21- 3 Electric Current
• Connecting dry cells (batteries) can produce higher
voltage. Most radios require six or more 1.5V batteries
to operate.
• Appliances can also operate from a wall socket. The
potential difference between two holes in a wall socket
is usually 120 V. The electricity from the wall socket is
provided by an electric generator.
• Batteries can also be wet cells which contain two
connected plates of different metals in an electrolyte
solution. EX: car batteries; most car batteries use lead
plates in a sulfuric acid solution.
21- 3 Electric Current
• Resistance is the tendency for a material to “oppose”
the flow of electrons. Most all conductors have some
resistance. It is measured in Ohms (W).
• As a current flows through a light bulb part of the
electrical energy is converted by the Tungsten
filament that glows white-hot as current passes
through. It has a high resistance and the current loses
electrical energy as it moves through the filament.
• Copper is an excellent conductor because it has low
resistance. It is used in household wiring because very
little electrical energy is converted to thermal energy as
current passes through the wires.
21- 3 Electric Current
• The size (diameter and length) of the wire affects
the resistance.
• Thick wires allow more electrons to travel so they
have less resistance.
• Thin wires have greater resistance to electron
flow because fewer electrons can travel.
• The longer the wire, the greater the resistance
because it takes longer for the electrons to travel
through it.
• In most conductors, resistance increases as
temperature increases.
21- 3 Electric Current
• George Simon Ohm found experimentally that the
current in a metal conductor is directly proportional to
the potential difference across its ends and inversely
proportional to the resistance.
•This statement is expressed as Ohm’s Law:
• Potential difference = current X resistance
V (volts) = I (amperes) x R (ohms)
21- 3 Electric Current
• A light bulb with a resistance of 160 W is plugged into
a 120V outlet. What is the current flowing through the
bulb?
• Given:
resistance: R = 160 W
voltage: V = 120 V
Unknown: Current: (I)
Equation: I = V/R = 120V / 160 W = 0.75 A
21- 3 Electric Current
1) (a.) Find the current flowing through a 20 W wire
connected to a 9 V battery. (b.) What if it were
connected to two 1.5 V batteries?
2) The current flowing through a lamp is 4.5 A. It is
plugged into a 120 V outlet. What is the resistance
of the lamp?
21- 4 Electrical Circuits
 Series circuits have only one path for the current to
travel.
When any part of the circuit is turned off or goes out, or
“disconnected”, no current can flow through the circuit,
creating an “open” circuit.
The electrons will have no complete path to flow.
21- 4 Electrical Circuits
 Parallel circuits contain separate branches (paths)
for current to move through.
More current flows through the paths of lowest
resistance.
Because all branches connect the same two points of
the circuit, the potential difference is the same in each
branch.
When one branch is open (light switch off, appliance
off), the others remain working because electrons can
still flow.
A fuse or a circuit breaker is wired between every
parallel circuit and the main switch box as a safety
device.
21- 5 Electrical Power and Energy
 Power is the rate at which electrical energy is
converted to another form of energy such as
mechanical energy, light, or heat energy.
 Power is expressed in watts (W) or kilowatts (kW).
The amount of power used by an appliance can be
calculated by multiplying the potential difference (V)
by the current (I).
P=IxV
units: watts = (amperes) (volts)
 Appliances use different amounts of energy
depending upon their electrical needs.
 Solve practice problems 1 & 2 page 566
21- 5 Electrical Power and Energy
 Electrical energy in your home is measured by an
electrical meter.
 The amount of electrical energy you use depends on
the power required by the appliances in your home
and how long they are used.
E = P x t energy = (power) (time)
kWh = (kW) (t)
 The unit of electrical energy is the kilowatt-hour
(kWh).
 One kilowatt-hour is 1000 watts of power used for one
hour. The power company charges for each kWh you
use.
 Solve practice problems 1 & 2 on page 568.