Transcript Electricity and Electromagnetism

Electricity and
Electromagnetism
Electricity
Magnetism
Electromagnetism
Electricity
• Electricity – the flow of electrons
• Static electricity – buildup of electrical
charges
• Electric current – movement of electrons
from one place to another
• Ampere – the unit used to describe how
much electric current flows through a wire.
Circuit – a path for electric current
• Closed circuit
• A complete, unbroken
path for electric
current
• Open circuit
• An incomplete or
broken path for
electric current
Schematic Diagram
• Schematic diagram – a diagram that uses
symbols to show the parts of a circuit
Schematic Diagram
Conductors and Insulators
• Conductor – material through which
electricity passes easily
– Examples, metals such as copper, gold,
aluminum and silver; solutions that contain
electrolytes
• Insulator – material through which
electricity does not pass easily
– Examples, glass, rubber, wood, and plastic
Resistance
• Resistance – measure of how easily
electric current will flow through a material
• Ohm – the unit used to measure
resistance
• Resistance depends on:
– The material the wire is made of
– The length of the wire
– The thickness of the wire
• The higher the resistance of the current
flow, the greater the heat produced by the
circuit.
Sources of Electric Current
• Electromotive force – the push that keeps
the current flowing in a circuit
• Volt – the metric unit used to measure
electromotive force that tells the amount of
push
• Voltage – the energy that a power source
gives to electrons in a circuit
• When the voltage is high, the electrons
have more energy available to do work.
Batteries
• Battery – common source of voltage
• Dry-cell battery – electric power source
with a dry or paste-like center
• Terminal – points where electrons leave or
enter a battery
• The positive terminal (+) receives
electrons from the negative terminal (-),
which has an excess of electrons
Batteries
• Wet-cell battery – electric power source
with a liquid center
• Made with sulfuric acid and a series of
lead plates.
• A chemical reaction takes place between
the acid and the plates, causing the
electrons to move from one plate to
another.
Batteries
Current
• Direct current (DC) – current that flows in
one direction
– Not the most common type of current. Found
in batteries
• Alternating current (AC) – current that
changes direction regularly
– Electricity produced in homes. Machines
called generators produce AC.
Ohm’s Law
• Power sources provide the push (voltage)
to the current in a circuit.
• The rate at which the current flows can be
measured in amperes.
• Ohm’s Law – current equals voltage
divided by resistance
• Current = electromotive force/resistance
• I = V/R
Ohm’s Law
• What is the current of a circuit that has
electromotive force of 15 V and a
resistance of 3 W ?
• I = V/R
• I = 15 V/3 W
• I=5A
Ohm’s Law
• What is the electromotive force produced
by a circuit with a current of 7 A and a
resistance of 4 W?
• I = V/R
• 7 A = V/4 W
• V = (7 A)(4 W)
• V = 28 V
Ohm’s Law
• What is the resistance of a circuit that has
an electromotive force of 90 V and a
current of 5 A?
• I = V/R
• 5 A = 90 V/R
• R = 90 V/5 A
• R = 18 W
Circuits
• Series circuits – a circuit in which all
current (electrons) flows through a single
path
• Disadvantages – one light is unscrewed or
burns out, all of the other lights will go out.
The circuit becomes open and the
electrons cannot flow.
• The current remains the same in a series
circuit, no matter how many electrical
devices are connected. Adding electrical
devices to the series lowers the voltage
through each device.
Circuits
• Batteries connected in a series circuit
increase the voltage of the circuit. Add the
voltages together to obtain the total
voltage.
• Lights are brighter because batteries are
connected in series circuits.
• Flashlights have batteries connected in
series.
• Fuses or circuit breakers will break the
flow of electrons, which may prevent fires.
Circuits
• Parallel circuits – a circuit in which there is
more than one path for current
• If one bulb burns out, the rest remain on
because the electrons have more than one
pathway to move.
• When several bulbs are connected in
parallel, they are the same brightness.
• However, the more electrical devices that
are connected increases the current,
which increases the heat.
Circuits
• Batteries in parallel circuits provide energy
for a longer period of time.
• Batteries in parallel do not increase the
voltage.
• It will not cause lights to be brighter.
Measuring Electricity
• Light bulbs have the amount of power they
use stamped on them
– 100 W stands for 100 watts
– 50 W uses twice as much power as 25 W
• Electric power – the amount of electrical
energy used in a certain amount of time
• Kilowatt-hour – a unit used to measure
how much electric energy is used; it is
1,000 watts used in one hour
Magnets
• Magnet – an object that attracts certain
kinds of metals, such as iron
• Many types of magnets are man-made,
created by running an electrical current
outside of a bar of metal
• Lodestone is a naturally occurring magnet
• Magnetic pole – the end of a magnet,
where magnetic forces are greatest
– Attract – to pull together
– Repel – to push apart
Magnets
• Magnetic field – area around a magnet in
which magnetic forces can act
• Lines of force – lines that show a magnetic
field
Magnets
• Earth is a giant bar magnet.
• North magnetic pole and the south
magnetic pole attract each other.
Magnetism
• Some atoms have north and south
magnetic poles, but most substances do
not. Therefore, certain substances are
magnetic, while others are not.
• Materials that are not magnetic are wood,
copper, plastic, rubber, gold, and glass.
Also, magnets are not attracted to these
substances.
Magnetism
• Substances that are magnets have their
atoms arranged so that they line up with
corresponding poles, north poles in one
direction and south poles in another
direction. Therefore, these substances
are magnets.
• Substances that can be made into
magnets are iron, nickel, and cobalt.
Magnets are also attracted to these
substances.
Magnetism
• To make a magnet, use another magnet
and stroke it over a piece of iron.
• After four or five strokes, the iron will
become magnetized.
• Even if the iron is cut in two pieces, both
pieces will remain magnetized because
the atoms are lined up with a north and
south pole.
Magnetism
• Two ways to demagnetize a magnet.
• Heating the magnet will rearrange the
atoms in a magnets so they are no longer
lined up.
• Striking the metal with a hard blow will
also rearrange the atoms in a magnet so
they are no longer lined up.
Electromagnetism
• Electromagnetism – the relationship
between magnetism and electricity
• Electromagnet – a temporary magnet
made by passing a current through a wire
wrapped around an iron core.
• The strength of the electromagnet
depends on power sources with high
voltage and the number of turns of wire
around the core.
Electromagnetism
Electromagnetism
• When current is supplied to the electromagnet,
each pole is attracted to the opposite pole of the
permanent magnet. This attraction causes the
electromagnet to turn.
• The turning of the electromagnet causes the
motor to turn.
• As the direction of the current changes, the
electromagnet’s poles are reversed. This
causes a spinning motion, which works the
motor.
• Motors operate cars, refrigerators, electric toys,
hair dryers, air conditioners, and kitchen
appliances.
Electromagnetism
• Speakers, earphones, and telephones use
electromagnets to change electric currents
into sound waves
• Motor – device that converts electrical
energy to mechanical energy
– Permanent magnet cannot move
– Electromagnet that is free to turn
– Device that supplies alternating electric
current to magnetize the magnet