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
• Law of conservation of charge, charge
can be transferred from object to object,
but cannot be created or destroyed.
• Whenever an object becomes charged,
electric charges have moved from one
place to another.
Behavior of Charges
• Unlike charges
attract, like
charges repel.
Opposite charges attract
Like charges repel
• Force between any 2
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 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.
Series Circuit
• The current only has 1 loop to flow
through.
• Used in older 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
• Useful device that prevents electric circuits
from overheating.
• Fuses - small pieces of metal that melt if
the current is too high. 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 current and voltage used in
an appliance.
• Measured in Watts or Kilowatts
• P = IV
• Power = Current (amps) x voltage (volts)
Current and Voltage Difference
• Ohm’s Law: 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