Download File - Mr. Ahearn`s Science

Electricity
Mr. Ahearn
2009
Electricity
• Protons and
Electrons have
positive and
negative charge
• Atom becomes +
if it loses
electrons, and – if
it gains electrons
Electrons Move in Solids
e- can move from atom to atom by
rubbing
– Ex. Rubbing a balloon on hair
» Hair holds e- more loosely, so
they go to the balloon, hair
becomes +, balloon becomes –
» Causes static charge- imbalance
of electric charge
Ions Move in Solution
• Movement of charge
can be caused by
movement of ions
• NaCl dissolved in
water Na+ and Clmove freely
• Play important role
in human body
(Nerve cell)
Electric Force
• Attractive or repulsive force that all charged
objects exert on each other
• “opposites attract and like repel”
• depends on distance and amount of charge
• Electric Fields- surrounds every electric
charge and exerts forces on other electric
charges
Insulators and Conductors
•
Insulator- e- can’t move freely
•
•
Ex. Plastic, wood, glass, rubber
Conductor- e- can move easily
•
•
Ex. Copper, gold, aluminum
Metals are best because e- are loosely
attached
Induced Charge
• Rubbing feet across
floor collects e- from
carpet, spread over
body as you go to
touch metal (door
knob), e- in your hand
move to the metal,
you feel an electric
discharge
Induced Charge
Grounding
• Electrical discharge can cause damage and
injury.
i. Lightening
ii. Even small amt. can damage circuits
• One way to avoid this is to direct that current
into the ground called grounding.
Examples include: lightening rods, 3rd prong on a plug
Question?
1. Why when objects become charged it is
electrons that are transferred from one
object to another rather than protons?
2. Why are metals good conductors?
3. Why does an electric discharge occur?
http://virlab.virginia.edu/VL/VDG.htm
Virtual Lab
Electric Current
Vocabulary:
Electric current
Circuit
Voltage
Resistance
Flow of Charge
• Electric Current- flow of electric charge
– Liquids  ions
– Solids  electrons
Measured in amperes (A)
Model for Simple Circuit
• Flowing water is good model for electric
current.
• Circuit- closed conducting loop that
electric charge flows.
Electric Circuits
• simplest electric circuit:
source of electrical
energy (battery), and an
electric conductor (wire),
connected to the battery.
• As long as there is a
closed path for electrons
to follow, electrons can
flow in a circuit.
Voltage
• voltage- of a battery is a measure of how
much electrical potential energy each
electron can gain.
• Measured in volts (V)
• voltage increases, more electrical potential
energy is available to be transformed into
other forms of energy.
How a Current Flows
• When the ends of a wire are connected to
a battery, the battery produces an electric
field in the wire.
• Electric field forces electrons to move 
(+) battery terminal.
• electron collides with other electric
charges and continues  (+) terminal
Batteries
• battery supplies
energy to an electric
circuit.
• alkaline battery two
terminals are
separated by a moist
paste.
Why do batteries die?
Battery Life
• Battery contains limited amt. of chemical.
• Reaction change chemicals into new
compounds.
• When chemicals used up battery is dead
Resistance
• measure of how difficult it is for electrons
to flow through a material.
• unit of resistance is the ohm (Ω).
• Insulators generally have much higher
resistance than conductors
Buildings Use Copper Wires
• Copper has low resistance and is one of
the best electric conductors.
• Less heat is produced as electric current
flows in copper wires, compared other
materials.
Resistance of Wires
• Depends on length, thickness, material.
• Increases – longer, thinner
Resistance at work
Light bulb Filaments
• filament is made of
wire so narrow that it
has a high resistance.
• electric current flows
in the filament, it
becomes hot enough
to emit light.
• made of tungsten
metal (high melting
pt)
Question
1. Compare and contrast and electric discharge
with electric current?
2. Describe how a battery causes electrons to
move in a circuit?
3. Describe how the electric resistance of a wire
changes as the wire becomes longer? And
thicker?
4. Explain why the electric wires in a house are
made of copper?
5. In an electric circuit, where do the electrons
come from that flow in the circuit?
Answers
1. Both involve movement of e-. Discharge is rapid
and discontinuous. Current is continuous.
2. A battery has a positive terminal that attracts e- and
a negative terminal that repels e-. These forces
cause e- to flow.
3. Longer wire  more resistance. Thicker wire 
less reistance.
4. Copper is a good conductor and has low
resistance. Less heat produced when current flows
in copper wire.
5. Electrons are in the atoms of the elements that
make up the wire.
Electric Circuits
Vocabulary
Ohm’s Law
Parallel Circuit
Series Circuit
Electric Power
Controlling the Current
Amount of current depends on:
– voltage supplied by the battery
– resistance of the conductor
Bucket height is like voltage 
higher up = more volts
Hose is like resistance 
longer and thinner = more resistance
Ohm’s Law
Voltage (v)= current (amps) x resistance (omh’s)
If voltage in a circuit increases current increases.
voltage in the circuit doesn’t change  the current in the circuit decreases
when the resistance is increased.
Ohm’s Law example
A lightbulb is plugged into a wall outlet. If the
lightbulb has a resistance of 220Ω and the
current is 0.5 A, what is the voltage of the
outlet?
Current: I =0.5 A
Resistance: R= 220Ω
V = IR = (0.5 A) (220Ω) = 110V
Practice
1.
An electric iron plugged into a wall has a resistance of
24Ω. If the current in the iron is 5.0 A, what is the voltage
provided by the wall socket?
120 V
2.
What is the current in a flashlight bulb with a resistance
of 30 Ω if the voltage of the batteries is 3.0 V?
0.1 A
3.
What is the resistance of a bulb connected to a 110 V
wall outlet if the current in the bulb is 1.0 A?
110 Ω
Series Circuits
• Series Circuit- has
only one path for
electric current to
follow.
• electrical devices are
connected along the
same current path.
Series Circuit
Each new device that is added to the circuit
decreases the current because each
device has electrical resistance. The total
resistance to the flow of electrons
increases as each additional device is
added to the circuit. By Ohm’s law, if the
voltage doesn’t change, the current
decreases as the resistance increases.
Parallel Circuit
• A circuit that has more than 1
path for electric current to
flow.
• If one path is broken,
electrons continue to flow
through the other paths.
• resistance in each branch
can be different, depending
on the devices in the branch.
Protecting Electric Circuits
• In a parallel circuit, current increases as
more devices are added  wire heats up.
• To keep the wire from becoming hot
enough to cause a fire, the circuits in
houses and other buildings have fuses or
circuit breakers.
Protecting Electric Circuits
Electric Power
Rate at which electrical energy is converted
into other forms of energy
• SI unit of power is the watt.
Power (watts) = Current (amps) x Voltage (volts)
Practice
A lightbulb is plugged into a 110 V wall outlet.
How much electric power does the lightbulb
use if the current in the bulb is 0.55 A?
Voltage: V = 110 V
Current: I = 0.55 A
P = IV = (0.55 A) (110 V) = 60 W
Practice
1.
The batteries in a portable CD player provide 6.0 V. If
the current in the CD player is 0.5 A, how much power
does the CD player use?
3.0 W
2.
What is the current in a toaster uses 1,100 W of power
when plugged into a 110 V wall outlet?
10 A
3.
An electric clothes dryer uses 4,400 W of electric
power. If the current in the dryer is 20.0 A, what is the
voltage?
220 V
Cost of Electric Power
• sell it in units of kilowatt-hours, amount of
electrical energy equal to using 1 kW of
power continuously for 1 h.
Electrical Safety
• 1997- electric shocks killed an estimated
490 people (USA)
Electrical Safety
• You experience an electric shock when an
electric current enters your body
• In some ways your body is like a piece of
insulated wire.
• The fluids inside your body are good conductors
of current.
• The electrical resistance of dry skin is much
higher.
• Skin insulates the body like the plastic insulation
around a copper wire.
Lightening Safety
• On average, more people are killed every year
by lightning in the United States than by
hurricanes or tornadoes.
• If you are outside and can see lightning or hear
thunder, take shelter indoors immediately.
• If you cannot go indoors, you should take these
precautions:
– Avoid high places and open fields
– Stay away from tall objects such as trees, flag poles,
or light towers.
Lightening Safety
• Avoid object that
conduct current such
as bodies of water,
metal fences, picnic
shelters, and
bleachers
Study for your upcoming test!!