HSC M&G.txt

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HSCPhysics
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136043
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HSC M&G.txt
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2012-02-17 18:13:24
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Motors & Generators topic
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  1. Competition between Westinghouse and Edison to supply electricity to cities
    • Edison - planned out DC power grid, advocated DC as a solution for powering cities. He:
    • electrocuted animals with AC
    • supported AC electric chair hoping to prove it was dangerous.
    • Westinghouse - owned the rights to the transformer, advocated AC power.
    • In 1883 - Paris succesful with AC power
    • 1884 - experimental line powering incandescent lights was set up in London, and 1 year later in Italy.
    • 1893 - Chicago World Fair Westinghouse quoted lower power costs using AC electricity and was succesful with many witnesses.
    • 1896 - electric power obtained from Niagara Falls powered Buffalo street railways using AC system and was successful.
  2. How are transmission lines insulated from supporting structures? Why?
    • WHY: In dry air, sparks can jump around 33cm from a 330kV source. So sparks must be prevented from jumping from transmission lines to the metal support towers. Large insulators are used to separate them from each other.
    • HOW: Insulators consist of either of ceramic segments joined together with metal links OR of rubber discs with a fibreglass. Their design reduces possibility of charge leaking through the insulators themselves.
    • Metal linkes in ceramic insulators are isolated from each other, and the fibreglass is a non-conductor, so there is no continuity of conduction.
    • Insulator segments designed to shed water and prevent dust from building up, as either moisture or dust can make a conductive path across the surface of the insulator.
    • The disc-like shape of the segments, whether ceramic or rubber, ensures a long pathway for for the current the traverse, increasing safety.
  3. How are transmission lines protected from lightning strikes?
    • WHY: When lightning strikes, it can strike the metal power towers used to support high voltage power transmission lines.
    • HOW: On power lines there is another line called the shield conductor strung at the very top of power poles, above conducting wires. In the event of a lightning strike, this highest line will be hit. The shield conductor is periodically earthed by having a connection to an earth wire that runs from the top of a power pole right down to the ground and acts as a continuous lightning conductor. If this cable or the tower is struck by lightning, the electricitiy of the lightning will be conducted to the Earth and the transmission lines will not suffer from a sudden surge of voltage that could damage transformers in substations.
  4. How is waste heat produced in transformers? How are difficulties of heating caused by eddy currents in transformers overcome?
    • Principle of transformer: induction of current in secondary coil because secondary coil experiences changing flux. But iron core also experiences changing flux, inducing eddy currents, heating it up due to high resistance of the iron to eddy currents. This represents a power loss and excessive heating can damage a transformer.
    • SOLUTIONS:
    • Reduce size of eddy currents by making the core out of laminated iron (many thin sheets of iron presesd togher with insulation between each layer). This limits the circulation of any eddy currents to the thicknes of one lamina, rather than the whole core, thus reducing overall heating effect. Laminations must not be in the same plane as the coils - but 'slice' this plane as thinly as possible to minimise eddy current formation.
    • Instead of using iron, ferrites, complex oxides of iron and other metals could be used. Ferrites - good at transmitting flux, poor at conducting electricity - so eddy currents and heating are minimised.
    • Use of a coolant to keep coils conducting efficiently as induced currents in the coils causes them to heat up, increasing resistance.
  5. Explain the role of transformer in electricity substations.
    • At the generator - step-up transformer at a substation raises output voltage from 23kV to 330kV.
    • Substations located in urban areas - step-down transformers to reduce voltage for transmission within cities or suburbs.
  6. What are high voltage power lines subject to?
    Arcing - need to be separated, as do substations which can be extremely dangerous for people nearby.
  7. Why are some electrical appliances in the home that are ocnnected to the mains domestic power supply use a transformer?
    • Mains domestic power supply - 240 V AC.
    • Many domestic appliances designed to run efficiently at this voltage - so connected directly to mains supply without need for a transformer.
    • Some appliances designed to run on low DC voltages - usually device using a battery - batteries only capable of providing low DC voltages. Batteries provide portability - laptops & mobile phones can be moved around. Also control & display panel of microwave oven, require low voltages. So they require a step-down transformer - operate best at lower voltage than the mains supply. Also a rectifier to convert AC into DC.
    • Some appliances contain components requiring step-up transformer - operate best at higher voltages above mains supply. E.g. computer monitors and TVs containing cathode ray tubes require large voltages - accelerate electrons towards the screen.
  8. Impact of development of transformer to society
    • Uptake of electricity RAPID as transmitting electricity efficiently over long distances was viable.
    • Remote communities have access to grid-supplied high voltage electricity - raised living standards through provision of electric lighting, refrigeration & air conditioning.
    • Cost of electricity lowered - accessible to almost everyone in economic terms.
    • Made many jobs involving labour redundant due to rapid developoment of technology and computers - impacting families but causing a shift in skills.
    • Power stations located in remote locations not in cities so pollutions has been shifted away from these areas.
    • However large scale use of fossil fuels such as coal to power generators, still pollutes the same world - sulfur and nitrogen oxides as well as increase in carbon dioxide levels contributing to global warming.
  9. Universal AC motor
    • Operates on AC or DC supply
    • Power fed in, runs through electromagnetic stators before entering a commutator.
    • Each brush connected to a wire that comprises one of the field coils, and is also connected to one end of a circuit.
    • With a DC source - commutator switches the current and motor operates.
    • With an AC source - although the direction of current being fed into commutator is varying, the same variations are fed into the field coils, with the net effect that AC oscillation is cancelled out and motors runs.
  10. Describe the main features of an AC induction motor (squirrel-cage motor)
    • Two parts - stator and rotor
    • Torque is produced by the interaction of a rotating magnetic field produced by the stator (AC voltage) and currents induced in the rotor.
    • Stator and rotor are separated by a thin air gap. Also in induction motors, rotor is free to move as not in contact with the rest of the motor - low friction, little wear and tear.
    • STATOR: 6 field coils, each opposite pair fed one phase of triple-phase AC power - sets up rotating magnetic field inside stator.
    • ROTOR: similar to a squirrel-cage - two end rings and aluminium or copper bars connecting the end rings to form a cylindrical shape. Cylinder is encased in a laminated iron armature to decrease heating losses due to eddy currents and to intensify the magnetic field passing through the rotor. Armature is mounted on a shaft that passes out through the end of the motor. Bearings reduce friction & allow the armature to rotate freely.
    • Operation: As the magnetic field rotates, it induces current in the bars of the squirrel cage. This creates a force in the same direction as the rotation of the magnetic field, from Lenz's Law. The squirrel cage then rotates, chasing the changing magnetic field.
  11. Uses of AC induction motor
    • power drills
    • hair dryers
    • food processors
    • vacuum cleaners
  12. Advantages of the AC induction motor
    • very reliable - no brushes & commutators, so little friction
    • simple & cheap to construct
    • economical & efficient
    • self-starting
    • little noise
  13. Disadvantages of the AC induction motor
    • For light loads, mechanical power produced low compared with electrical power consumed
    • fixed-speed machines - in Australia 3000 revolutions per minute
    • starting torque relatively slow - don't get heavy loads moving quickly
    • speed of motor drops with increase in load
  14. Explain what the slip speed means in an AC induction motor
    • difference between the speed of the rotating magnetic field and the speed of the rotor.
    • there must be a slip speed in the AC induction motor i.e. squirrel-cage is constantly slipping behind the rotating magnetic field - if there was no difference in speed, no relative motion exists between magnetic field and bars and hence no induced current, no force, no torque.
    • when any induction motor has a load and is doing work, rotor slows down and always travels at a slower speed than the magnetic field of the stator i.e. slip speed icnreases - so relative movement between magnetic field and conductor bars increase; induced current and magnetic force due to the current are increased.
  15. PRAC: demonstrate the principle of an AC induction motor
    • Principle: Moving magnetic field induces a current in the rotor with a directiont hat, according to Lenz's Law, causes the rotor to spin in the same direction as the magnetic field.
    • Used thin aluminium disk suspended by a string from a clamp on a retor stand - disk free to rotate.
    • Moved a strong ceramic magnet in circles around the circumference of the disk.
    • Induced eddy currents caused disk to rotate in same direction as the magnet, demonstrating the principle.
  16. Conversion of electricity in the home and industry
    • HOME:
    • heaters & toasters - electrical energy into heat
    • speakers - electrical energy into vibrational energy - sound
    • food blender - mechanical energy
    • INDUSTRY:
    • machinary in industry - electrical energy converted into kinetic energy for production of goods.
    • furnaces - heat energy
    • electrolytic cells - chemical potential energy in metal plating
    • BOTH INDUSTRY & HOME:
    • light bulbs - light energy
  17. Describe the main features of a DC motor.
    • Stator (fixed) - provides external magnetic field in which coil rotates. Permanent magnets or electromagnets.
    • Coil(s) - coils which cary a direct current interacts with the magnetic field, experience a force to produce torque.
    • Armature - made of laminated soft iron, around which coil(s) are wound. The armature and coil(s) together are known as the rotor, which rotates on an axle. The axle protrudes from motor casing, enabling the movement of the coil to be used to do work.
    • Split ring commutator - reverse the current being fed into the coil every half turn to maintain a constant direction in torque.
    • Brushes (graphite) - maintain electrical contact between commutator and external circuit.
    • DC power supply - provides current for motor effect.
  18. What is an advantages of electromagnets over permanent magnets?
    • Electromagnets - winding coils around iron cores and passing current through the coils to produce a magnetic field.
    • The magnetic field produced is strong than permanent magnets.
  19. THE GALVANOMETER: Function? How? What is so special about the radial magnetic field?
    • FUNCTION: Uses the motor effect to measure the magnitude of small electric currents
    • HOW: Current passed through coil of wire wrapped around an iron core placed in a radial magnetic field, producing torque on the coil which is proportional to the magnitude of the current.
    • Radial magnetic field allows for a constant torque.
    • A spring is attached to the solenoid. The motor effect on the coil opposite by the spring who restoring force is proportional to the angle of rotation. When the motor effect is balanced by the restoring force of the spring, a needle attached to the coil indicates the magnitude of the current on a linear scale.
    • Radial magnetic field - magnetic field lines always parallel to plane of the coil so that torque equation is just T = nBIA as theta = 0 i.e. torque is uniform. Hence torque acting on the coil increases in direct proportion to the current fed into the coils, and so the scale on the meter can be uniforom in the size of divisions.
  20. THE LOUDSPEAKER: Function? How?
    • transforms electrical energy into sound energy
    • consnists of a circular magnet with one pole outside and the other inside.
    • voice coil sits in the space between the poles; connected to the output of an amplifier
    • amplifies provides a current that changes direction at the same frequency as the sound that is produced; changes magnitude in propotion to the amplitude of the sound.
    • voice coil caused to vibrate by the motor effect.
    • voice coil attached to a large cone, causing it to vibrate and the air around it, creating pressure waves in the air, which is heard as sound.
  21. Motor effect
    Moving charge in a magnetic field experiences a force. Hence wires or conductors carrying current in a magnetic field also experience a force.
  22. Magnetic flux density's other name; Measuring units
    • Magnetic field strength
    • Teslas or Webers per square metre
    • how much magnetic flux is passing through a unit area
  23. What is the difference between magnetic flux and magnetic flux density?
    • Magnetic flux density - how much magnetic flux passing through a unit area
    • Magnetic flux - how many magnetic field lines pass through a any given area.
    • Φ = BAsinθ. Hence magnetic flux is at a maximum when plane of coil is perpendicular to the magnetic field.
  24. Ways in which to increase an induced EMF i.e. increase rate at which flux is cut or changing with time.
    • decrase the distance between conductor and magnetic field - flux lines closer together nearer the magnet
    • increase the strength of the magnet - more flux lines in the same space in a stronger field than in a weaker fifeld
    • increase speed of relative motion between conductor and magnetic field - conductor can cut more flux lines per unit time
    • increase the angle between the direction of motion of the conductor and the direction of magnetic field from near zero towards 90 degrees - conductor cuts maximum number of flux lines per second when its motion is at right angles to field.
  25. Michael Faraday's discovery of the generation of an electric current using a moving magnet
    • Ørsted's discovery - moving charged produced a magnetic field
    • Faraday wondered if reverse was possible - moving magnetic to produce electromotive force (EMF)
    • Faraday initially built a CRUDE TRANSFORMER running a current through primary coil and checking for current in secondary coil that was immsered in a magnetic field produced by the primary coil.
    • He found when primary coil was connected and disconnected to a battery, galvanometer needle moved slightly, but only momentary.
    • Faraday then took an IRON RING, wound a wire around it. Moved a magnet in and out of the ring, a galvanometer indicated a constantly changing current. Also noticed magnitude of deflection on galvanometer greater when magnet moved more quickly.
    • CONCLUSION: moving magnet could be used to generate electrnic curret.
  26. Back EMF in electric motors
    • Back emf reduces net emf i.e. Net voltage = suply emf - back emf.
    • When motor is running slowly, back emf is small. i.e. net voltage large; may cause large current to flow and potentially damage coil windings i.e. series resistor used to limit the current.
    • When motor gathers speed, back emf increases, leading to small net current, meaning a series resistor no longer needed.
    • If motor is overloaded, rotates too slowly; back emf reduced and voltage across coil remains high; resulting in a high current through coil; which can burn out the motor.
    • AC motor continuously produces a back emf, and so limits itself without need for series resistor.
  27. Induction cooktops
    • Electromagnetic induction to produce heat
    • beneat cooktop is an induction coil consisting of a solenoid withi circular coils lying horizontally.
    • AC passed through coil, magnetic field formed, aimed straight up through the cooking surface.
    • When saucepan or pot placed on the cooking surface, it experiences a changing flux.
    • Causes eddy currents to be formed in the pan, results in pan heating up due to electrical resistance.
    • Induction cooktop more efficient at converting electrical energy to heat - pan heated directly, rather than heat being lost to surrounding air.
  28. Electromagnetic breaking
    • Used in trains - silently & efficiently slow it down - without noise and wear and tear of conventional friction braking.
    • electromagnet placed underneath train
    • when electromagnet turned on while train is moving, metal rails below experience change in flux
    • Faraday's Law - eddy currents induced in rails
    • Lenz's Law - eddy currents create magnetic field opposing change which produced them
    • i.e. eddy currents repel the electromagnet as it moves, exerting a force opposite to direction it is moving in i.e. a braking force acts against movement of train.
    • hence inducing eddy currents in tracks, allows trains to slow down.
    • EMF dependant on rate of change of flux - train is travelling quickly, greater production of emf, electromagnetic braking most efficient at high speeds which is great because at high speeds the most noise and heat ar eproduced by frictional braking, resulting in wear and tear.
  29. DC Generators - Advantages & Disadvantages
    • DC generators: small-scale applications
    • Advantages - output made smoother by arranging many coils in a regular pattern around armature. each coil connected to 2 segments of a multi-part commutator and the brushes make contact only with the segments connected to the coil, producing the greatest emf at a particular time. Useful for appliances that require constant voltage. Doesn't induce emf in its surroundings when transmitted, less insulation and separation required, DC cable insulation can be lighter, cheaper.
    • Disadvantages - brushes wear out, need to be replaced or maintained regularly because they remain in contact with commutator under spring pressure, constantly striking the leading edge of each successive bar. Commutator wears down and subject to sparking, for same reason, insulating material between it and brushes wears out and prevents brushes making proper contact with the bars, reduces efficiency of generator, regular maintenance needed. current from DC generator cannot be stepped-up using transformers, making them inefficient for commercial power production.
  30. AC Generators - Advantages & Disadvantages
    • Advantages - useful for commercial electricity generation, AC can be transformed, minimising power losses during transmission i.e. AC generators located away from cities. Also AC transformed to suit power requirements of appliances. Induction motors running on AC, much longer lasting, requiring less maintenance compared with universal motors. Slip-rings have continuous, smooth surfaces, allowing brushes to continuously remain in contact with slip-ring surface. Do not wear as fast as DC split-ring commutators therefore less maintenance required. Hence also more reliable than DC.
    • Disadvantages - Many household appliances require DC power so must convert AC supply. Rectifier and a smoothing circuit required. Some power loss in AC as AC current always fluctuating, causing changing flux around wire. Hencing shielding required. Also wires carrying high voltages, subject to arcing, more dangerous to surroundings. Stronger insulation required compared to DC generated power.
  31. What is electric potential difference?
    • Eleectric potential difference or voltage (V) is the change in electric potential energy per unit charge/ per coulumb between any two points.
    • The potential difference (voltage) between the terminals of a power supply is the number of joules of electric potential energy GIVEN to each coulomb of electric charge.

    The potential difference between the ends of a resistor is the number of joules of electric potential energy DISSIPATED for each coulomb of electric charge that passes through the resistor.
  32. Total amount of energy lost in transmission lines
    • In one second the amount of power dissipated P = VI
    • so energy dissipated in a period of time W = VIt where W is the energy transformed from one form into another. This can also be the energy generated in a power supply since you have a current passing through it, a potential difference (voltage) across the power supply and a length of time that the current travels across it.
    • Power (P) = rate at which energy or work W is transformed from one from to another. P = W/t.

    • Power is measured in watts (W)
    • Work is mesaured in joules (J)
  33. Function of a split ring commutator in a DC GENERATOR
    At each half revolution of the coil, the brushes swap the side of the rotating coil they are in contact with so that current supplied to the external circuit remains in the same direction.

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