Download Ch 17: Electricity

CHAPTER 17: ELECTRICITY
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17.1 ELECTRIC CHARGE
AND FORCE
CHAPTER 17: ELECTRICITY
ELECTRIC CHARGE
• Electricity: force created by a difference in charges
(+, -) due to gained or lost electrons.
• Protons (+) are trapped in the nucleus, so moving electrons (-)
are responsible for electricity
• Electric Charge: an electrical property of matter that
creates electric and magnetic forces and interactions
• Like energy, electric charge is never created or destroyed
• The SI unit of electric charge is the coulomb, (C)
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TRANSFER OF ELECTRIC CHARGE
• When charges between two points
differ because electrons build up, the
electric charge can be transferred
• This can happen through induction,
conduction, or friction
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TRANSFER OF ELECTRIC CHARGE
• Induction: presence of (-) charge forces
electrons to move away (no contact
involved)
• Conduction: when a charged object
touches a neutral object and the
charges transfer. Often called charging
by contact
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TRANSFER OF ELECTRIC CHARGE
• Friction: when objects are rubbed
together and one material gains
electrons, becoming negatively charged,
and the other loses electrons, becoming
positively charged
• Static electricity happens when the
charges build up, but don’t move
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TRANSFER OF ELECTRIC CHARGE
• Although charge can jump between points, it often travels
through a conductor
• Electrical Conductor: a material in which charges can move
freely, allows charges to flow
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The best conductors are metals because their electrons are not
tightly bound to the nucleus
• Electrical Insulator: a material in which charges cannot move
freely, does not allow charges to flow
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The electrons are tightly bound in insulators, most plastics are
insulators
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ELECTRIC FORCE
• Electric charges exert a force
• Electric Force: the attractive or repulsive
interaction between any two charged objects
• Like charges repel, opposites attract
• This force decreases as the distance between
charges increases
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ELECTRIC FIELD
• Electric Field: the space around a
charged object in which another charged
object experiences an electric force (this is
like gravitational forces!)
• Any charge that is placed in an electric
field will be pushed or pulled by the field
• For example, if you have a + charge, and
place a + charge near it, they will repel
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17.2 CURRENT
CHAPTER 17: ELECTRICITY
LET’S REVIEW!
• Potential Energy: energy stored in an object due
to its position
• Kinetic Energy: the energy an object possesses
due to its motion
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CURRENT
• Electric Current: rate at which charges pass through a
given point (flow of electrons from point A to B)
• Related more to the amount than the speed
• The SI unit of current is the ampere (A)
1 amp = 6.25 x 1018 electrons per sec.
(6,250,000,000,000,000,000!)
• When charges flow, they carry energy that can be used
to do work
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ELECTRICAL POTENTIAL ENERGY
• Electrical Potential Energy: the
ability to move an electric
charge from one point to another
• Charges naturally move from an
area with high electrical PE to an
area with low electric PE
• Electrical PE is measured in volts
(V)
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ELECTRICAL POWER & ENERGY
• When electrons flow, they transfer energy and transform
electrical energy into other forms of energy (sound, light,
heat)
• Electrical Power: The rate at which electrical energy is
converted into other forms of energy
• The SI unit for power is the watt (W)
• Electric power is calculated by multiplying the total current,
I, by the voltage, V, in a circuit
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power  current  voltage
P = IV
VOLTAGE
• Voltage: The difference in charges between
two points, also called potential difference
• Voltage sets charges in motion
• Think of a battery - there is voltage across
the terminals
• One terminal is positive and the other is negative
• Electrons will always flow from (-) to (+) when
they are connected
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DIRECT CURRENT (DC)
• Flow of a charge always in one direction
• The amount of current can change, but it will
always flow in the same direction
• For example – a battery
• In metals, moving electrons make up the
current
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DIRECT CURRENT (DC)
• Lightning is a form of DC caused
by static electricity in clouds.
• Static is formed when air
molecules move past each other
(like clothes in a dryer), and
negative charges group at the
bottom of the cloud and transfer
electrons to the ground, which
has taken on a positive charge.
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DIRECT CURRENT (DC)
• Battery: device which produces a
continual direct current (DC),
through a chemical reaction.
• When the (+) and (-) terminals are
connected through a device, they
produce a flow of electrons from
negative to positive, which powers
the device.
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ALTERNATING CURRENT (AC)
• Flow of charge back and forth, changing its direction
many times in one second.
• AC power is used all over the world because it’s cheaper
and easier to make devices with AC power
• All of the appliances and lights in your house probably
run off of AC power
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CONVENTIONAL CURRENT
• Electrons move from areas of negative
charge to positive
• Conventional current moves in the other
direction – positive to negative
• The direction of current is opposite to
the direction that the electrons move
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ELECTRICAL RESISTANCE
• Resistance: a material’s opposition to the flow of
electrons
• Resistance is caused by internal friction, which slows the
movement of charges through a conducting material
• All materials have some resistance
• Conductors are low, insulators are high
• Resistor: certain type of conductor used to control
current
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ELECTRICAL RESISTANCE
• Resistance is measured in Ohms (Ω)
• 1 Ω = 1 V/A
• Ohm’s Law: resistance in a circuit equals voltage
difference, divided by current
• Ohm’s Law provides a way to measure the resistance
of objects and materials
voltage
resistance =
current
V
R=
I
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ELECTRICAL RESISTANCE
• Semiconductors: materials that have electrical properties
between those of insulators and conductor, and conduct
under certain conditions.
• Remember your periodic table!
• Superconductors: some metals and compounds that have
zero resistance when their temperature falls below the
critical temperature
• Critical temps range based on the superconductor – can be as low
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as –272 ºC (–458 ºF) and as high as –123 ºC (–189 ºF)
ELECTRICAL RESISTANCE
• Once current is established in a superconductor, it will
continue even if voltage is removed, making them very
useful
• Superconducting magnets are so strong they are used to
levitate trains (MagLev Trains) which reduces the friction
between a normal train and the track
• For any conductor, an increase in temperature will
increase resistance
BILL NYE
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17.3 CIRCUITS
CHAPTER 17: ELECTRICITY
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CIRCUITS
• Electric Circuit: pathway along which an electric
current can travel, en route to a device.
• Closed Circuit: Complete electrical path in
which current circulates
• Open Circuit: An incomplete electrical path
with no charge flow and therefore no current
• Switches interrupt the flow of charges in a
circuit.
• You can use a switch to open and close a
circuit.
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SERIES CIRCUITS
• Series Circuit: only one path from the source through all
of the loads and back to the source
• Current in each device is the same, but resistances and
the voltage across each device may differ
• If one element along the path in a series circuit is
removed, the circuit will not work
• A string of old Christmas lights is an example of a series
circuit
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PARALLEL CIRCUIT
• Parallel Circuit: two or more paths for
current to flow through
• Voltage is the same across each device,
and the sum of all currents equals the total
current
• A break in any one path does not interrupt
the flow of electric charges
• Newer Christmas lights and household circuits
are an example
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CIRCUIT BREAKERS
• When electrical wires carry more than a safe
level of current, the circuit is overloaded.
• Overloaded circuits can cause fires
• Short Circuit: When current stops short of the
intended device
• Circuit Breaker: device which prevents too
much electricity from flowing through a circuit,
can be reset usually with the flip of a switch.
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FUSES
• Fuse: an electrical device that contains a metal
strip that melts when current in the circuit
becomes too great
• Fuses melt to prevent circuit overloads.
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SCHEMATIC DIAGRAMS
• Schematic Diagram: graphical
representation of a circuit
• Uses lines to represent wires
• Uses symbols to represent components
• Schematic diagrams use standard
symbols and can be used to describe all
electrical devices
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