Transcript Slide 1
Environmental Physics
Chapter 11:
Electromagnetism and Electricity Generation
Copyright © 2008 by DBS
Introduction
• •
Electrical power grid consisting of thousands of km of transmission lines (E, W and TX) Deregulation in 1990’s opened up the grid to anyone with electricity to sell
•
Large grids are prone to blackouts
Introduction
Introduction
• • • •
August 14, 2003
NE blackout: largest in US history occurred in 2003 due to grounding of several transmission lines “Cascading outage” 265 power plants shut down Loss of transportation, all electrical devices CNN
Introduction
Magnetism
• • • 500 BC: First observation of magnetism would have been a lodestone (magnetitie / iron oxide) Can be used to magnetize iron Chinese invention of the compass
19-02
Magnetism
• A magnet is a metallic object (Fe, Co, Ni, alloys) that produces a magnetic field – Every magnet has at least two poles, N and S (dipole) – Like magnetic poles repel each other, while unlike poles attract – Force between two magnets varies inversely as the square of the distance between them
19-01
Demos Shielding 19-20 Curie Point 19-24
Presence of iron in the Earth creates a magnetic field
Magnetism
Earth's present
19-01
north
is thus actually its
magnetic south
Figure 11.5:
The earth’s magnetic field and the Van Allen belts. The Van Allen belts are collections of charged particles that are trapped in doughnut shape of the earth’s field.
shaped shells by the earth’s magnetic field. The particles spiral in these shells from N to S to N, being reflected at each end as if by “mirrors” as a result of the
Magnetism
• • • Cosmic rays (charged particles) provide majority of ionizing radiation we receive Deflected by Earth’s magnetic field Trapped in Van Allen belts – Aurora borealis
Magnetism
• • Unit of magnetism is the gauss (G) (magnetic field strength or magnetic flux density) – Earth’s magnetic field = 0.5 G – Iron magnet = 500 G – Neodymium magnet = 2,000 G – Electromagnet = 15,000 G In SI units 1 Tesla = 10,000 gauss
Magnetism
• Common features of electricity (charged bodies) and magnetism: – Both created forces that could act in a vacuum – Forces were inversely proportional to the square of the distance between the two charges or poles – Forces were both attractive and repulsive • Electromagnetism: Link between electricity and magnetism is attributed to Danish physicist Oersted in 1820 A magnetized needle experiences a force when placed near a current carrying wire Conclusion:
electric currents produce magnetic fields
19-8
Figure 11.2:
Oersted’s experiment showed that a current-carrying wire gave rise to its own magnetic field. The arrows indicate the magnetic field lines as a result of the current-carrying wire.
Magnetism
• Electromagnet: A coil of wire (solenoid) with a current passing through it acts as a magnet Coil acting as a magnet Strength can be increased by wrapping around an iron core since the magnetic field of the solenoid induces magnetism in the iron nail
19-12
Magnetism
• • •
Motion of Charged particles in a Magnetic Field: Cosmic Rays, Magnetic Bottles
Current experiences an opposite force (Newton’s 3 rd Law) Charged particles experience a force when moving at an angle to a magnetic field but not if parallel to the field The force will deflect the particle in a direction perpendicular to its original motion
Figure 11.4a:
A charged particle (e – ) experiences a force
F
when moving in a magnetic field
B
. The direction of the force is perpendicular to the velocity
v
and the magnetic field
B
.
11.4b:
A beam of electrons in the evacuated tube is deflected by a magnetic field (due to the U-shaped magnet above the tube). The beam is visible as it hits a fluorescent screen.
Magnetism
• • Magnetic bottles (boxes without physical walls) used for the confinement of particles in thermonuclear fusion reactors Used to confine a plasma (high temperature ionized gas)
Figure 11.6:
Fusion Test Reactor at Princeton University: the “Tokamak.” In the center is an evacuated ring (torus) containing an ionized gas of deuterium and tritium at a very high temperature. The gas is held in place by the electromagnets surrounding the torus. This test facility was closed in 1998. It achieved 10 million watts of fusion power.
Magnetism
•
Electric Motors
A current carrying wire placed in a magnetic field experiences a force, useful work can be obtained from this motion Force on the wire is opposite due to different current directions
Figure 11.7:
Schematic of a DC electric motor. For the continuous rotation of the shaft, the current in the armature windings must be alternating. With a DC source such as the battery, current reversal is possible with a “commutator”—two connecting rings on the armature shaft that make contact alternately with the positive and negative terminals of the battery.
20-01 20-01 Demos: Electric Motor http://www.youtube.com/watch?v=zOdboRYf1hM 20-09
The Generation of Electricity
• • Electric current creates a magnetic field Moving magnetic field creates electric current
20-11 20-12
Figure 11.9:
Faraday’s law of induction: A moving magnet will induce a current in the surrounding wire.
The Generation of Electricity
• Electric fields and electric currents can be produced by changing magnetic fields – If a loop of wire is spun between the N and S poles of a magnet an electric current is generated in the wire – Mechanical energy used to spin the wire may be provided by water flowing through a dam, steam (burring fossil fuels) or wind driven propellers – Kinetic energy → electrical AC current flows in both directions (alternating current) DC current flows in one direction (direct current) requires a commutator
Figure 11.10:
Simple electrical generator.
20-15 Demos: Electric Generator 20-07
Figure 11.11:
Cutaway view of a modern nuclear steam-turbine generator. On the left (with housing removed) are the turbine blades; the turbine shaft is connected to the generator —the cylinder on the right.
Fig. 11-11, p. 367
• Review End
Transmission of Electrical Energy
• • • Electricity in the home = 120 V Electricity is not transmitted at 120 V due to dangerously high current – If 10 homes required 1000 W each,
P
=
IV
,
I
= 10,000/120 = 83 A – And high energy losses due to resistance leading to lower efficiency How is this overcome?
Figure 11.12:
High-voltage (345-kV) transmission lines. Each system transmits power on three wires, and each set of towers carries two systems.
Transmission of Electrical Energy
• • • Voltage is stepped up for transmission Lower current and reduction of heat losses - due to resistance (
P
=
I
2
R
) Much less power lost to heating, efficiency around 99 %
Transmission of Electrical Energy
• • Transformer: Principle of operation is a corollary to Faraday’s Law of induction
Figure 11.13:
Corollary to Faraday’s law: The current in the primary loop produces a magnetic field that varies with time, since we have an AC source. This varying field induces an alternating current in the secondary loop.
Transmission of Electrical Energy
• • • Better example: A change in current of the primary coil causes a voltage to be induced in the secondary coil Induced voltage depends on coil ratio – Step-up – increase coils in secondary coil – Step-down – increase coils in primary
Figure 11.14:
Transformer model: a device for stepping voltage up or down.
20-22 Demos: Transformer 20-23
Transmission of Electrical Energy
• Power in = Power out
V
in
I
in =
V
out
I
out • Increase in secondary voltage is balanced by decrease in secondary current
V
out =
V
in x (turns in sec. coil / turns in primary coil) =
V
in x (
N
s /
N
p ) Only works with AC current!
• JAVA: http://micro.magnet.fsu.edu/electromag/java/transformer/index.html
Question
An electricity generating plant produces 1000 MW electricity at 24,000 V. To transmit at 360,000 V, what must be the ratio of turns? What current is used?
N
s /
N
p =
V
out /
V
in = 360,000 V / 24,000 V = 15 Power transmitted at 1000 MW
P
=
IV I
= 1000 x 106 W / 360,000 V = 2780 A
Transmission of Electrical Energy
Figure 11.15:
Examples of transformers: the smaller one (a) is for a door bell system (120 V stepped down to 12 V), and the larger one (b) is a step-up transformer at a power plant.
Transmission of Electrical Energy
• • To minimize
I
2
R
power loss on transmission electricity is transmitted at high voltages Transformer sub-stations are used to reduce voltage for use in the home
Figure 11.16:
Electrical transmissions system. Electricity passes through a number of steps on its way from the power plant to the customer.
Transmission of Electrical Energy
• Electricity is usually generated at around 25,000 V (25 kV) and raised to 115 kV, 345 kV or 765 kV
Figure 11.17:
First line crew on a high-voltage transmission line —Niagara Falls to Buffalo, New York, 1896.
Transmission of Electrical Energy
• • •
Environmental and Health Impacts of High-Voltage Lines
Steel-cored bare-aluminum conductors Corona visible at night High frequency buzzing • Should we worry about EM fields?
– Electric component?
– Magnetic component?
Figure 11.18:
Close-up view of a high-voltage transmission line crossing the Hudson River, New York. Shown are the large ceramic insulators.
Transmission of Electrical Energy
• • • •
Environmental and Health Impacts of High-Voltage Lines
Maximum magnetic field below 765 kV line is less than 1 gauss (with a frequency of 60 Hz) Earth’s magnetic field ~ 0.5 gauss (frequency is static) Home 0.1 – 50 mG Drop off rapidly away from sources EM fields blocked easily by vegetation and buildings Magnetic fields not blocked
Transmission of Electrical Energy
• • • • • • Biological effects of EMF’s highly controversial recently! Uncertainty in some studies and non reproducibility of others 1979 Wertheimer study childhood deaths vs proximity to power lines EMF’s do not have sufficient energy to initiate cancer, may be secondary effects 1995 Savitz and Loomis looked at 139,000 utility workers over 36 yrs Suggested weak link to brain cancer 1994 Canadian study of utility workers found statistically significant link between magnetic fields and leukemia • • More interest lately in the use of cell phones and associated microwave radiation EMF ~ 50 - 60 Hz, cell phones 1 – 2 GHz cause warming of internal organs
Radiation: Electromagnetic Spectrum Biological Effects or Non-Ionizing Radiation http://www.mybiopro.ca/Research_Emf_Dangers.aspx?ID=supermind2
Transmission of Electrical Energy
• •
Alternatives to Transmission Lines
Overhead transmission lines are most expensive method for transporting energy (50%) Alternative is underground wires (more expensive)
Standard Steam-Electric Generating Plant Cycle
• • • Mechanical energy → Electrical energy Utilizes electromagnetic induction to produce electricity Source of mechanical energy is falling water, steam (produced from burning fuel) or wind
Standard Steam-Electric Generating Plant Cycle
Figure 11.19:
Cutaway view of a steam turbine, showing 16 blades and the shaft, which connects to the generator at the top of the picture. Steam enters by the small blades and exits at the larger blades.
Figure 11.20:
Electrical power demand during the week. During periods of reduced demand, electricity is used to pump water into a reservoir. The dashed line indicates energy use when this is done. During the day, pumped storage (hydropower) meets the demand shown by the shaded part.
Fig. 11-20, p. 379
Cogeneration
• • • Cogeneration is the production of both electricity and heat from the same fuel source Two types: – Topping Cycle: Electricity is produced from a turbine generator, uses exhaust gases or high pressure steam for process steam or district heating and / or additional electricity – Bottoming Cycle: Steam from an industrial process is run through a low-pressure steam turbine to produce electricity Overall conversion efficiency is increased from 35-40% for standard plant to 55-60 for Co-gen • • Most common fuel for cogeneration is natural gas Solid waste plants (electricity and heat from garbage) are on the rise
Cogeneration
Figure 11.21:
Cogeneration is the simultaneous production of various forms of energy from one power source.
Cogeneration
• •
“Combined cycle”
systems use gas turbines to generate electricity and then either (a) use high temperature exhaust gases (1000-1200 °F) to generate more electricity with a conventional steam turbine or (b) use high pressure steam for industrial processes Popular due to abundance and cost of natural gas and high efficiency e.g. Oswego NY plant generates 1040 MW and is 54 % efficient
Figure 11.22:
Combined-cycle system for the production of electricity by a gas turbine and a steam generator.
Figure 11.23:
Independence Station: This 1040-MW cogeneration power station in upstate New York (Scriba, Oswego) uses natural gas and provides thermal energy to a nearby industry. Water vapor emerges from the cooling towers on the left behind the plant. The four stacks are associated with the gas turbines. In the background is Lake Ontario.
Service nearby Alcan plant
Fig. 11-23, p. 381
Figure 11.23:
This 1040-MW cogeneration power station in upstate New York uses natural gas and provides thermal energy to a nearby industry. Water vapor emerges from the cooling towers on the left behind the plant. The four stacks are associated with the gas turbines. In the background is Lake Ontario.
Fig. 11-24, p. 382
Summary
• • • • • Electricity production accounts for about 25 % of total energy consumed in US Electromagnetism can be summarized as follows: – Electrical current produces a magnetic field – A moving charged particle can experience a force in a magnetic field – A wire moved in a magnetic field has a potential difference induced across its ends Electricity is transmitted at very high voltages in order to reduce power loss due to heating Transformers are used to step-up and step-down thevoltage Cogeneration is the production of both electricity and heat from the same fuel source at increased efficiency