Chapter 21 – Electricity and Magnetism at Work

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Transcript Chapter 21 – Electricity and Magnetism at Work

Chapter 21 – Electricity
and Magnetism at Work
• All magnetism is created by a
moving charge
• Electrons moving through a wire
(electricity) created a magnetic
field.
• A magnet can make the wire move
by repelling or attracting the
electricity’s magnetic field.
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• The commutator in an electric
motor changes the direction of the
electric current every ½ turn to
keep the coil turning (see pg. 678)
• The brushes are the contact point
to the commutator.
• The many coils around an iron
core is called an armature.
• Galvanometer – used
to measure small
currents. It uses the
same principle as an
electric motor.
• Electric motor – uses current to turn an
axle
• Electric motor – converts electrical
energy into mechanical energy
• Mechanical energy – is the energy of
an object due to its movement or
position.
• Work energy is mechanical energy
Generators
• Turning a coil of wire through a magnetic field
causes electrons in the wire to move
• Turning the shaft of an electric motor will
make electrons move through the wire
• This is generating electricity
• Electromagnetic induction – the process of
generating an electric current from the motion
of a conductor through a field.
• Generators convert mechanical energy into
electrical energy
• Alternating current – as the coil is turned, the
electrons pulse back and forth as they are
attracted to one magnetic pole and then the
other.
• How can electrical contact be maintained to a
turning shaft (by slip rings) see page 684 +
brushes
Generating Electricity On a Big
Scale (electrical power plants)
• Need a major power source such
as water, wind, coal or nuclear
• Wind turns
blade on a
windmill and
water turns a
turbine
• Coal and
nuclear make
steam which
turns the turbine
– (see energy
sources pg.
688)
• Renewable Resource – one that is
replaceable by nature (wind,
water, wood, solar)
• Non-renewable energy source –
one that is not replaced quickly
(coal, petroleum, uranium)
Coal Mining
Uranium Sample
Energy and Power
• Power - how fast energy is being
supplied or consumed
P = V (I)
I=P/V
watts
volts
amps
1000 watts = 1 kilowatt
Energy = power (time)
Energy = kilowatts (hours)
How much energy does a 300 watt light
use if it is on for 10 hours?
E = P (time)
Energy = .3 kilowatts (10 hours)
Energy = 3 kilowatt hours
Transporting electricity – It is first
brought to a very high voltage to
reduce line loss, then it is brought to
120 volts for home use.
Step-up transformer – increases the
voltage
Step-down transformer – decreases the
voltage
Electrochemical Cells and Batteries
• Metals tend to give up
electrons
• Non-metals and positive ions
attract electrons
• All chemical reactions are
competitions for electrons
• In a battery the metal that
provides electrons is kept
apart from the materials that
accept electrons
• The electrons must travel
through a wire to get from one
to the other.
• A metal and a chemical that attracts electrons
make a cell
• The voltage of the cell is the difference in
attraction between the metal and the other
compound
• The voltage of a cell is small (about 1.5 or 2
volts).
• Cells are added in series to make a battery of
higher voltage
• A 12 volt car battery is made from six two volt
cells.
pg 691 practice problems 1 & 2
(Read War of the Currents on
page 694)
page 696 questions 1,2,4
page 701 questions 1-4