Download Electricity - Effingham County Schools

Electricity
Positive and Negative Charge
• 2 Electric Charges:
• Proton = Positive charge
• Electron = Negative Charge
• The amount of positive charge on a
proton equals the amount of negative
charge on an electron.
Transferring Charge
• Some materials hold onto their electrons
more tightly or loosely.
• When you walk
on the carpet,
electrons are
transferred from
the carpet to the
soles of your
shoes.
Transferring Charge
• The soles of your shoes have an excess of
electrons and become negatively charged.
• The carpet has lost electrons and has an
excess of positive charge.
• The accumulation of
excess electric
charge on an object
is called static
electricity.
Conservation of Charge
• According to the law of conservation of
charge, charge can be transferred from
object to object, but it cannot be created or
destroyed.
• Whenever an object becomes
charged, electric charges have moved
from one place to another.
Behavior of Charges
Opposite charges attract
Like charges repel
• Unlike charges
attract each
other, and like
charges repel
each other.
• The force between
any two objects that
are electrically
charged decreases
as the objects get
farther apart.
Electric Fields
• An electric field surrounds every electric
charge and exerts the force that causes
other electric charges to be attracted or
repelled.
• Any charge that is placed in an electric
field will be pushed or pulled by the field.
Conductors and Insulators
• If you reach for a metal doorknob after
walking across a carpet, you might see a
spark.
• The spark is
caused by
electrons
moving from
your hand to
the doorknob.
Conductors and Insulators
• A material in
which
electrons are
able to move
easily is a
conductor.
*Metals
•Electrolytes
•Human body
•Earth’s Crust
• A material in
which
electrons are
not able to
move easily is
an insulator.
•Plastics
•Styrofoam
•Wood
•Rubber
•Paper
Charging Objects
• Rubbing two materials together can result
in a transfer of electrons.
• Then one material is left with a positive
charge and the other with an equal
amount of negative charge.
• The process of transferring charge
by touching or rubbing is called
charging by contact.
Charging Objects
• Because electrical forces act at a distance,
charged objects brought near a neutral
object will cause electrons to rearrange
their positions on the neutral object.
• The rearrangement
of electrons on a
neutral object
caused by a nearby
charged object is
called charging by
induction.
Induction of an Electric Field
(Balloon and Board Drawing)
Series Circuit
• The current only has 1 loop to flow
through.
• Used in holiday lights and flashlights
• Disadvantage: one break in the circuit
disrupts entire loop.
Parallel Circuits
• Contain 2 or more
pathways for current to
move through.
• Ex: Homes, cars and
airplanes are wired with
parallel circuits.
• Advantages: Some
branches can be turned off
without affecting the others.
Fuses
• 1 useful device that prevents electric
circuits from overheating.
• Fuses are small pieces of metal that melt if
the current becomes too high. The
melting causes a break in the circuit and
stops the flow of current. Fuses must be
replaced once “blown”
Circuit Breaker
• Device used to prevent electric
circuits from overheating.
• Contains a piece of metal that
heats up and expands when the
current is too large.
• They can be reset by switching
back to “on” position.
Electric Power
• Depends on the current and voltage used
in an appliance.
• It is measured in Watts or Kilowatts.
• P = IV
• Power = Current (amps) x voltage (volts)
Current and Voltage Difference
• Ohm’s Law: The relationship between
voltage, resistance, and current.
• Electrons move through conductors from an
area of high voltage to an area of low
voltage.
• EX: from a battery to a machine.
• Batteries usually provide the voltage
difference, which causes electrons to move.
Voltage Difference
• In some ways, the electric force that
causes charges to flow is similar to the
force acting on the water in a pipe.
• Water
flows
from
higher
pressure
to lower
pressure.
Voltage Difference
• A voltage difference is related to the
force that causes electric charges to flow.
Voltage difference is measured in volts.
3 Variables in a Circuit
1. Voltage: the measure of volts or the
potential difference.
• **Electric charges have potential energy
that depends on the position in electric
fields.
• Just as a ball rolls down a hill, a negative
charge will move away from another
negative charge. (Repulsive forces)
This is known as Potential Difference.
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3 Variables in a Circuit
2. Electrical Current: is the flow of charges
through a wire. (I)
3 Variables in a Circuit
3. Resistance: The tendency for a material
to oppose/resist the flow of electrons.
• Resistance is
measured in
ohms ().
2 Ways to change resistance
1. Materials: can have hi or low
resistance
• Ex: copper has low resistance and can
carry electrons quickly. This wire doesn’t
lose much energy or heat.
• Ex: Tungsten (W) has a high resistance
and doesn’t let electrons flow fast.
Therefore it heats up quickly. It loses
heat and glows. Used in light bulbs.
2 Ways to change resistance
2. Thickness of wire:
– Thick wire allows electrons to move fast and
has low resistance.
– Thin wire doesn’t allow electrons to move
fast and has high resistance.
Ohm’s Law
•
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Ohm’s Law states that
I = V/R
V =IR
R = V/I