Transcript Chapter 8 Section 3

Section 8.3: Physical Science
From Mechanical to Electrical
Energy
 A magnet can produce an electric current Moving a loop of wire through a magnetic field
causes electric current to flow in the wire
 Moving a magnet through a loop of wire has the
same effect
 http://micro.magnet.fsu.edu/electromag/java/farada
y2/
 How can a magnet produce a current in a wire?
 ME due to the motion of the wire or magnet is
converted to electrical energy (motion of charges)
Electromagnetic induction &
Generators
 Electromagnetic Induction
 Producing electric current by moving a loop of wire
through a magnetic field

Or moving a magnet through a wire loop
 Generators
 Machines that produce electric current by rotating a coil
of wire in a magnetic field
 Wire coil is wrapped around an iron core and placed
between the poles of a permanent magnet.
 http://www.walter-fendt.de/ph11e/generator_e.htm
Generators
 Coil is rotated by an outside source of mechanical energy
 Examples: Steam, wind, water
 As the coil turns in the magnetic field, electric current flows
through the wire
 When the coil turns so that the ends move past opposite ends of
the magnet:
 The current reverses direction
 The current changes direction twice with each revolution
 A generator that is used in a car is known as an alternator
Electric Generators
 Electricity in the home comes from a power plant
with huge generators
 When the coil is fixed and the magnet rotates, the
current is the same as if the coil rotates and the
magnet is fixed.
 Construction of a generator in a power plant
 Electromagnets contain coils of wire wrapped around
iron cores
 Rotating magnets connect to a turbine
 Turbine: large wheel that rotates
 Hoover Dam and Hydroelectric Power

Sources of electricity: http://www.teachersdomain.org/resources/phy03/sci/phys/energy/energysource/index.html
Direct and Alternating Currents
 Compare and contrast current in batteries and
generators:
 Compare: Both devices move electrons through the wire
 Contrast: Batteries have DC. Generators have AC.
 DC- Direct Current- Flows in only one direction.
 AC- Alternating current- reverses the direction of
current flow regularly.
 In N America, generators produce alternating
current at a frequency of 60 cycles per second, or
60 Hz
 A 60-Hz alternating current changes direction 120
times per second
Alternating Current
 http://schoolforchampions.com/science/ac.htm
Transformers
 To make electric energy safe for your home you must:
 Use a transformer- increases or decreases voltage
 Transformers are made up of 2 coils (primary and
secondary) wrapped around an iron core.
 Alternating current in a primary coil creates a
changing magnetic field around the iron core
 Which induces an alternating current in the
secondary coil
Transformers
 Step – up transformer increases voltage.
 The secondary coil has more turns than the
primary coil does
 Step-down transformer decreases voltage.
 The secondary coil has fewer turns than the
primary coil does
 http://www.school-for-
champions.com/science/ac_transformers.htm
Electricity
 Power carried in power lines as high as 750,000
volts is reduced by step-down transformers to
household current (AC) of 120V
 What happens to electrical energy as it passes
through a wire?
 Some is lost as heat due to resistance of the wire
 Why is electrical energy transmitted at high
voltages?
 To prevent loss of energy as heat