Transcript 8.1 Electric and magnetic forces

Chapter Eight: Electricity and
Magnetism
• 8.1 Electricity
• 8.2 Electrical Circuits and Electrical
Power
• 8.3 Properties of Magnets
Investigation 8A
Electricity
• How does electricity work?
8.1 Electric and magnetic forces
• All matter has
electrical (and
magnetic) properties
because the atoms
that make up matter
are held together by
electromagnetic
forces.
8.1 Electric and magnetic forces
• Magnetic forces can
attract or repel because
there are two kinds of
magnetic poles, called
north and south.
• A north pole repels
another north pole but
attracts a south pole.
8.1 Electric and magnetic forces
• Like magnetism,
electric charge comes
in two types. We call
them positive and
negative.
• A positive charge
attracts a negative
charge and vice versa.
• Two similar charges
repel each other.
8.1 Electric charge
• Electric charge, like
mass, is also
fundamental property of
matter.
• Inside atoms found in
matter, attraction
between positive and
negative charges holds
the atoms together.
8.1 Electric charge
• Because ordinary
matter has zero (total)
charge, most matter
acts as if there is no
electric charge at all.
• A tiny imbalance in
either positive or
negative charge is the
cause of static
electricity.
8.1 Electric current
• Electric current is caused by moving electric
charge.
• Electric current comes from the motion of electric
charges that are much smaller than atoms.
• These charges are called electrons.
e-
8.1 Electric current
• Electric current is
similar in some ways to
a current of water.
• Like electric current,
water current can carry
energy and do work.
• A waterwheel turns
when a current of water
exerts a force on it.
8.1 Electric current
• Electric current is
what makes an
electric motor turn
or an electric stove
heat up.
• Electric current is
measured in
amperes honor of
Andre-Marie
Ampere.
8.1 Electric current
• Electric current can carry great deal of energy.
• An electric motor the size of a basketball can do
as much work as five big horses or twenty strong
people.
8.1 Voltage
• When two containers of water are the
same height, no water will flow.
• No current flows when there is zero
voltage difference, which is why a dead
battery won’t light a bulb.
8.1 Voltage
• Current flows from
higher voltage to
lower voltage.
• It is a difference in
voltage that makes
electrical current flow
just as a difference in
height makes water
current flow.
8.1 Batteries
• Since electric current in wires is invisible, think of
a “current” of marchers, each carrying a bucket
of energy.
• The battery refills the buckets with fresh energy
and the bulb uses the energy to make light and
heat.
8.1 Batteries
• A pump is like a battery because it brings
water from a position of low energy to high
energy.
8.1 Batteries
• A battery uses stored
chemical energy to create
the voltage difference.
• Three 1.5-volt batteries can
be stacked to make a total
voltage of 4.5 volts in a
flashlight.
8.2 Electric Circuits and
Electrical Power
• An electric circuit is a complete path through
which electric current travels.
• A good example of a circuit is the one found in
an electric toaster.
8.2 Current in a circuit
• Current only flows
when there is a
complete and
unbroken path, or a
closed circuit.
• Flipping a switch to
the “off” position
creates an open
circuit by making a
break in the wire.
8.2 Current in a circuit
• The amount of
electric current
flowing into any part
of a circuit must be
the same as the
amount flowing
back out.
8.2 Electrical power
• Electrical power is
measured in watts,
just like mechanical
power.
• A 100-watt electric
light bulb uses 100
joules of energy
every second.
8.2 Electrical Power
• To calculate power in an electric circuit
you multiply the voltage and current
together.
What is the
power in this
circuit?
8.2 Resistance
• Resistance (R)
controls how much
current flows for a
given voltage.
• When resistance is
low, current flows
easily.
8.2 Resistance
This light
bulb has a
resistance
of 1 ohm.
• Electrical resistance
is measured in
ohms.
• If a circuit has a
resistance of 1 ohm,
then a voltage of 1
volt causes a current
of 1 amp to flow.
8.2 Ohm’s law
• The current in a circuit depends on
voltage and resistance.
• Ohm’s law relates current, voltage, and
resistance with one formula.
8.2 Ohm’s law
• Voltage and current are
directly related.
• Doubling the voltage
doubles the current.
• Resistance and current
are inversely related.
• Doubling the resistance
cuts the current in half.
• If you know two of the
three quantities, you can
use Ohm’s law to find the
third.