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NIS – PHYSICAL SCIENCE
Lecture 60 – Lecture 61
Producing Electric Current
Ozgur Unal
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From Mechanical to
Electrical Energy
In 1831, Michael Faraday and Joseph Henry independently
found that:
moving a loop of wire through a magnetic field causes an electric
current to flow in the wire.
moving a magnet through a loop of wire produces a current.
In both cases, the mechanical energy is converted into
electrical energy.
The generation of a current by a changing
magnetic field is electromagnetic induction.
http://www.wainet.ne.jp/~yuasa/flash/EngLenz_law.swf
http://micro.magnet.fsu.edu/electromag/java/faraday2/
http://www.educypedia.be/electronics/javamagnetics.htm
http://new.wvic.com/index.php?option=com_content&task=view&id=9&Itemid=46
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Generators
Most of the electrical energy you use every day is provided
by generators.
A generator uses electromagnetic induction to transform
mechanical energy into electrical energy.
Check out Figure 17 carefully!
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The Electromagnetic Force
There is a relationship between electricity and magnetism
A changing magnetic field can cause electric charges to
move.
A moving charge produces a magnetic field.
This connection occurs because the electric force and the
magnetic force are two different aspects of the same force 
electromagnetic force
An electromagnetic force exists between
all objects that have electric charge.
Like electric and magnetic forces, the
electromagnetic force can be attractive
of repulsive.
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Generating Electricity for Your Home
Do you have a generator in your home?
Electrical energy in your home comes from a power plant
with huge generators.
The coils in these generators have many coils of wire
wrapped around huge iron cores.
The rotating magnets are connected to a turbine, which is a
large wheel that rotates when pushed by water, wind or steam.
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Generating Electricity for Your Home
How many types of power plants can you name?
Power plants can use:
wind,
water,
fossil fuels,
nuclear energy .... to generate mechanical energy
Check out Figure 20 to
compare generators and
electric motors.
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Direct and Alternating Current
There are two kinds of current flow in a circuit: Direct
Current (DC) and Alternating Current (AC)
A battery produces a direct current.
Direct current flows only in one direction through a wire.
http://www.koehler.me.uk/animation/e_and_m_ac_gen_2.htm
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Direct and Alternating Current
Alternating current reverses the direction of the current in a
regular pattern.
In North America, generators produce alternating current at
a frequency of 60 Hz.
The electric current produced by a generator changes
direction twice during each cycle of the coil  current changes
direction 120 times each second
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Transmitting Electrical Energy
Once electricity is produced by
generators in the power plants, how
is it transmitted to our houses?
When electrical energy is transmitted
along power lines, some of the electrical
energy is converted into heat. Why?
Since power lines have resistance, some portion of the
electrical energy is lost as heat as the wires get hot.
Since the length of the power lines increases the resistance,
large amounts of heat can be produced.
How can we reduce the amount of heat produced?
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Transformers
What is Transformer?
The high voltages should be reduced before the electricity
comes into our houses  safety!!
A transformer is a device that increases or decreases the
voltage of an alternating current.
A transformer is made of a primary coil and a secondary coil.
These current carrying wire coils are wrapped around the
same iron core.
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Transformers
As an alternating current passes through the primary coil, the
coils magnetic field magnetizes that iron core.
The magnetic field in the iron core changes direction at the
same frequency with the alternating current.
This changing magnetic
field in the iron core
induces an alternating
current in the secondary
coil.
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Transformers
The voltage in the primary coil is the input voltage  Vin
The voltage in the secondary coil is the output voltage  Vout
The number of turns in the primary coil  Np
The number of turns in the secondary coil  Ns
Vout / Vin = Ns / Np
Using this relationship we can define two types of
transformers: Step-Up and Step-Down
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Step-Up Transformers
Step-Up transformers increase the voltage so that the output
voltage is greater than the input voltage.
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Step-Down Transformers
Step-Down transformers decrease the voltage so that the
input voltage is greater than the output voltage.
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