Magnetism - Hong Kong Polytechnic...
Transcript of Magnetism - Hong Kong Polytechnic...
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Magnetism
Magnesia, province of Greece
Unusual property of lodestone noted over 2000 years ago
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Magnetism
Press SPACE to find out the correct answer.
What substance do the What substance do the paperclip and weight paperclip and weight
contain that make contain that make them attract to them attract to
magnets? magnets? ______________. IRONIRON
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Magnetic Field in a Bar Magnet
• A magnet has two poles, North (N) and South (S)
• Like poles repel, opposite poles attract
• The lines of magnetic field from a bar magnet form closed lines.
• By convention, the field direction is taken to be outward from the North pole and in to the South pole of the magnet.
compass
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The Earth’s Magnetic Field
The spinning iron core of the earth produces a magnetic field.
The magnetic north pole corresponds to the geographic south pole.
The earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the earth.
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Magnetic Field of a Straight Wire
• The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire.
• If a wire is grasped with the thumb of your right hand pointing in the current direction, the fingers encircle the wire in the direction of the magnetic field.
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Magnetic Field of Current Loop
Electric current in a circular loop creates a magnetic field which is more concentrated in the center of the loop than outside the loop.
Magnetic flux Φ measuredin Webers (Wb)
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Magnetic Field in a Solenoid
Stacking multiple loops concentrates the field even more into what is called a solenoid.
Flux density measured in Teslas (T) = (Wb/m2)
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Electromagnets
• Electromagnets are usually in the form of iron core solenoids.
• The ferromagnetic property of the iron core causes the internal magnetic domains of the iron to line up with the smaller driving magnetic field produced by the current in the solenoids.
• The effect is the multiplication of the magnetic field by factors of tens to even thousands.
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Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.
What are magnetic domains?
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An electromagnet can be used to move large quantities of iron. When the current is on the magnet will pick up the iron. When you want to drop it you turn off the power and the electromagnet is disabled and the iron drops.
Electromagnets
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Magnetic Levitation Train (maglev)
Japanese MLX01 can travel at speeds over 500 km/h (310 mile/h)
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Maglev Train “Tracks”
Train moving on Guideway
The Yamanashi Guidewayfor the MLX01
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Maglev Train in Shanghai
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Magnetic Levitation
Some Benefits:
Relatively quiet operating noise
High speed capability: (“268 mph reached by maglev train linking Shanghai’s financial district with airport” in 2004)
Relatively pollution free
Some Drawbacks:
Electrical energy requirement is very great
“Superconducting materials offer the only realistic potential for this means of transportation”
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Magnetic Force on Current-Carrying Wire
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Coil in a Magnetic Field
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DC Motor
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Commutator and Brushes
Commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet. Commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment.
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DC Motor
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DC Motor
The rotating part of the motor, including the electromagnets, is called the armature.
This diagram shows a small battery-powered electric motor and what it looks like inside with one end of the motor case removed.
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As the rotor spins, the three plates come into contact with the positive and negative brushes.
Electric current flows through the brushes into the coils.
DC Motor
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DC Motor
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DC Motor
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Electromagnetic Induction
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Factors Affecting InductionElectricity is induced when a magnet moves inside a coil of wire. When the magnet stops moving, so does the electrical current.What factors affect the size of the electrical current induced?
The speed of the magnet – the faster it moves, the larger the current.
The strength of the magnet – the stronger the magnet, the larger the current.
The number of turns in the coil – the more turns in the coil, the larger the current.
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NS
Electromagnetic Induction
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NS
Electromagnetic Induction
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NS
STRONGER FIELD (B)
Electromagnetic Induction
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NS
STRONGER FIELD (B)
Electromagnetic Induction
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NS
FASTER
Electromagnetic Induction
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NS
FASTER
Electromagnetic Induction
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Electromagnetic Induction
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AC Generator A coil of wire turns in a magnetic field. The flux in the coil is constantly changing, generating an emfin the coil.
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AC generator
ENG327 Basic Electricity & Electronics I 39
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It is possible to predict the direction of the induced current produced by a generator if the direction of the force (or motion) or the magnetic field are known. Fleming’s right-hand rule is used to do this.
Fleming’s Right-hand Rule
First finger = magnetic Field
seCond finger = Current
thuMb = Motion
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Resistor Current & Power
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Time /s
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Ir
/A
Vs /V Pr /W Ir /A
Vs
50Hz220Vrms
r
1kIrrms = 0.22APave = 48.4W
Peak = 311V
Peak = 0.311A
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Vs /V PL /W IL /A
ILrms = 0.7APave = 0
Inductor Current & Power
Vs50Hz220Vrms
L
1H
Peak = 311V Peak = 0.99A
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Capacitor Current & Power
Vs
50Hz220Vrms
c
10uF
Irrms = 0.69APave = 0
Peak = 311V Peak = 0.976A
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Three-Phase AC Generator
Larger generators tend to operate in thee-phases: there are three separate, evenly spaced coils embedded in the stator producing three separate voltage and current waveforms, separated by 120o
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Three-Phase AC Generator
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Steam Turbine and Electric Generator
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Steam Turbine Blades
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Cylindrical-Rotor Synchronous Generator
Stator
Cylindrical rotor
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B-
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C-
A-
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+
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+ +
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+
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A+
C-
B+A-
B-
C+
Salient Pole Synchronous Generator
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Hydro Generator
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How is electrical energy produced?
Most methods of producing electricity involve converting movement (i.e. kinetic energy) into electrical energy.
Coal, oil, gas and nuclear power stations convert kinetic energy from high-pressure steam into electrical energy.
Wind turbines convert kinetic energy from wind into electrical energy.
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Black Point and Castle Peak Power Station
Natural Gas Coal
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Nordjyllandsværket, unit 3
The world’s most efficient coal-fired unit with 47% efficiency
Nordjyllandsværket, unit 3Net output : 385 MWSteam parameters : 300 bar/585 °C/585 °CMaximum district heating : 460 MJ/sCommissioning year : 1998
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Power Plant / Power StationFossil Fuel Power Plant – Coal
Waste Heat
Flue
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Natural Gas Power Station
Waste heat:
a. Cooling Towers:Released to atmosphere
b. Combined Cycle:Recycled in secondary boiler
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Gas Turbine Components
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Nuclear Power Plant
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Pressurized Water Reactor (PWR)
Thermal Turbine
G
Generator
CoolingTower
Condenser
Pump
Pump
Body of waterReactorVessel
SteamGenerator
Pump
Stea
m
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Hydroelectric Power Plant
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Hydroelectric Power Plant
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Propeller turbine for low head plant.
Hydroelectric Power Plant
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Hydroelectric Power Plant
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Types of Hydropower Turbines
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Francis turbine in Hydro power station
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The next part of the story is that a changingmagnetic field produces an electric current in a loop surrounding the field
Transformer
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Transformer
A current in the primary coil produces a magnetic field, like a solenoid.
The magnetic field couples around through the secondary coil.
A voltage is induced in the secondary coil.
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Small transformer construction
a) Lamination, b) Iron core with winding
Iron core
Terminals
Secondarywinding
Insulation
Transformer
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Typical Transformers
Primary-Secondary Terminology
90 V
Secondary
Load
Transformers are bidirectional devices.
However, either winding can be used as the primary.
120 V
Secondary
Load
This transformer was designed to step 120 V down to 90 V.
120 V
Primary
Source120 V
Source90 V
Primary
90 V
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Transmission of Electric Power
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Three-phase Transformer
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Electricity Supply Network
GenerationGeneration 400kV Overhead Line
EHV Substation
Area
Primary
Substation
132kV132kV
Transformer
Room
Transformer
Room
Industrial Commercial Residential
Village Overhead lines
Underground Cable
Customer
Substation
11kV Ring Network11kV Ring Network
Transformer Room
11kV/380V
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Power System
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500 kV TransmissionPower Plant
Generation
TransmissionSystem
DistributionSystem(12kV)
UndergroundDistribution Transfomer
ResidentialCustomer
Commercial/IndustrialCustomer
ResidentialCustomer
OverheadDistributionTransformer
UrbanCustomers
69 kV Sub-transmission
230 kVTransmission
Distribution Substation(69/12 kV)
High-Voltage Substation(230/69 kV)
Extra-High-Voltage Substation(500/230 kV)
Distribution Line
Underground Cable
To OtherHigh-VoltageSubstations
Electrical Power System
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Transmission structures
three-phase “live” wires
500,000 230,000 138,000 69,000 7–13,000long-distance neighborhood
to house
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Control Centre
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Tariff
The rate of electrical energy at which it is sold to the consumers is called tariff .The supply companies invest money to generate, transmit and distribution of electrical energy, a tariff is fixed .The cost of generation depends upon the magnitude of energy consumed by the consumers and his load conditions. Therefore, due consideration is given to different types of consumers (e.g. domestic, commercial and industrial) while fixing a tariff .
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Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in?
1 unit of electricity = 1 unit of electrical energy
Example:
How many units of electricity is 17.6 kWh? 17.6 units
The amount of electrical energy (i.e. the amount of electricity)used by an appliance depends on its power and how long the electricity is used for.
electrical energy = power x time
Calculating the Units of Electricity
= 1 kilowatt hour (kWh)
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Electronic Meter
AC Energy Meters
The meters record how much electricity is used in a house in units of electrical energy.
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The cost of electricity is the number of units of electrical energy multiplied by the cost per unit.
Example:
How much would 10 units of electricity cost at a price of 9p per unit?
cost = 10units x 9p/unit
cost = number of units x cost per unit
How much does electricity cost?
= 90p
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An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p per unit. How much does it cost for the electricity used by the iron in that time?
Number of units:
Cost of electricity:
Buying Electricity
= 12units
= 12kWh
= 3kW x 4h
number of units of electricity = number of kilowatt hours
= 96p
= 12 units x 8p/unit
cost = number of units x cost per unit
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Tariff Structure – Hong Kong Electric
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(a) Basic Charge
(b) Fuel ClauseThe fuel clause charge is 22.4 cents per unit.
Domestic Tariff – CLP Power
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Comparison based on monthly domestic customer consumption of 275 units (3,300 units p.a.), tariffs and exchange rates are as at Nov 2013
Tariff Comparison