Electromagnetism

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Electromagnetism

  1. 1. 1. Magnetic Field Around A Wire <ul><li>A wire contains current going into the page as shown. What will the magnetic field look like? </li></ul><ul><li>A wire contains current going right as shown. What will the magnetic field look like? </li></ul>
  2. 2. 1. Magnetic Field Around A Wire <ul><li>A wire contains current going into the page as shown. </li></ul><ul><li>A wire contains current going right as shown. </li></ul>
  3. 3. 2. Magnetic Field Inside a Solenoid <ul><li>This (“rule 2”) is just an application of Rule 1. </li></ul><ul><li>The fingers follow the current (positive to negative) and the thumb represents the field INSIDE the solenoid. </li></ul>Recall x = field ‘into page’ and a dot = ‘out of page’
  4. 4. “ The Trick” <ul><li>Start with your hand ‘clenched’. </li></ul><ul><li>Run your fingers around in the direction of the current. </li></ul><ul><li>Think “ Does it go right or left ” </li></ul><ul><li>Does it go into or out of the page first? </li></ul>http://www.waowen.screaming.net/Maghandrules.htm
  5. 5. Solenoid / Permanent Magnet comparison <ul><li>The magnetic field comes OUT of the North pole and goes to the south pole OUTSIDE. </li></ul>http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/imgmag/barsol.gif
  6. 6. Therefore <ul><li>Find the direction of the magnetic field. </li></ul><ul><li>Will it attract or repel the permanent magnet? </li></ul>N S + -
  7. 7. Therefore <ul><li>Find the direction of the magnetic field. </li></ul><ul><li>Will it attract or repel the permanent magnet? </li></ul>N S + -
  8. 8. S N - +
  9. 9. S N - +
  10. 10. N S - +
  11. 11. - + N S - http://www.pschweigerphysics.com/images/rhr2.jpg
  12. 12. Magnetic Force on a Current <ul><li>If a wire carries a current through a magnetic field, it creates a force on the wire. The force depends on: </li></ul><ul><li>The current </li></ul><ul><li>The length of the wire </li></ul><ul><li>The strength of the magnetic field. </li></ul><ul><li>The most common application is a motor. It uses lots of wire loops to create more force. Only one wire is shown here. </li></ul>http://www.bbc.co.uk/scotland/education/bitesize/standard/img/physics/electricity/movement/motor.gif
  13. 13. Right Hand Slap Rule <ul><li>Three dimensional </li></ul><ul><li>Fingers – magnetic field </li></ul><ul><li>Thumb – current </li></ul><ul><li>Slap  Force on the electric current </li></ul>Force Current (+-> -) Current http://commons.wikimedia.org/wiki/File:Right-hand-rule.jpg
  14. 14. N S Current into page
  15. 15. N S Current into page
  16. 16. Another One
  17. 17. Another One
  18. 18. Another One The current is parallel to the magnetic field, so no force results.
  19. 21. Questions <ul><li>Pages 163 (all). </li></ul>
  20. 22. New Term Review (not covered with all classes, but good practice if you haven’t seen it) <ul><li>What is a solenoid? What is it used to make? </li></ul><ul><li>Explain the right hand rule (magnetic field around a wire carrying an electric current) and the RHR to determine the magnetic field inside a solenoid. </li></ul><ul><li>How can the strength of an electromagnet be increased? </li></ul><ul><li>Explain the “motor effect” (magnetic force on a current). You should explain how to determine the direction of the force in relation to the magnetic field and current direction. </li></ul><ul><li>Explain how to increase the strength on a current-carrying wire in a magnetic field. How do motors achieve strong forces? </li></ul>
  21. 23. Magnetic Forces Magnetic Fields between Permanent Magnets: http://www.acecrc.sipex.aq/access/page/?page=75ee44de-b881-102a-8ea7-0019b9ea7c60
  22. 24. What causes the motor effect? As explained in class, the field lines interact with each to produce a pattern like that shown below right. Reminder: field lines can never cross. The stronger magnetic field above pushes the wire downwards. http://en.wikibooks.org/wiki/GCSE_Science/The_motor_effect
  23. 25. Another One As explained in class, note that the field lines interact with each other but do not look like this, as field lines cannot cross. The stronger magnetic field on the left pushes the wire to the right.
  24. 26. The DC Motor <ul><li>A DC motor applies the motor effect (strangely) to make wire spin inside a magnetic field. </li></ul><ul><li>A commutator is necessary to keep it spinning in one direction. </li></ul><ul><li>The force on the wire (and therefore the torque) changes as the position changes. </li></ul><ul><li>Usually many loops are used (coils) to increase the force; this is necessary for a real-world motor to work. </li></ul><ul><li>A motor can (usually) be used as a generator if forced around - more on this soon. </li></ul><ul><li>An AC motor is quite different (and not to be covered now!). </li></ul>
  25. 27. The DC Motor http://www.walter-fendt.de/ph11e/electricmotor.htm http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=912.0
  26. 28. Problems <ul><li>Electric motor page 165 </li></ul>
  27. 29. Electromagnetic Induction <ul><li>If a wire is moved through a magnetic field, the magnetic field ‘pushes’ the electrons around the circuit. This creates a voltage and (if a circuit is connected) a current. </li></ul><ul><li>The voltage (and current) can be increased by increasing the length of the wire, the speed of the movement or the strength of the magnetic field. </li></ul><ul><li>The wire must ‘cut’ magnetic field lines, or else no voltage will be produced. </li></ul>
  28. 30. Right Hand Rule <ul><li>The diagram showed that: </li></ul><ul><li>FINGERS = magnetic field </li></ul><ul><li>THUMB = movement </li></ul><ul><li>Slap = induced voltage, or the direction conventional current (positive to negative) will flow if a circuit is connected. </li></ul>
  29. 31. Electromagnetic Induction Example Conductor (normally wire) v The “positive charges” move up and the negative charges move down. This generates a voltage difference across the wire. + -
  30. 32. Electromagnetic Induction Example Calculate the current in the circuit. The “positive charges” moved anticlockwise in the animation. 10 Ω + + + + +
  31. 33. Motor Effect and Electromagnetic Induction <ul><li>MOTOR EFFECT </li></ul><ul><li>Fingers = Field </li></ul><ul><li>Thumb = current </li></ul><ul><li>Slap = Force </li></ul><ul><li>EM INDUCTION </li></ul><ul><li>Fingers = Field </li></ul><ul><li>Thumb = movement / force </li></ul><ul><li>Slap = induced Voltage (EMF) </li></ul>Cause Effect Always Magnetic Field http://commons.wikimedia.org/wiki/File:Right-hand-rule.jpg
  32. 34. Another One
  33. 35. Another One The current is parallel to the magnetic field, so no force results.
  34. 37. Generators and Alternators <ul><li>A DC motor produces direct current (electricity which flows in one direction only) when it is turned, however the flow is uneven. </li></ul><ul><li>Most generators produce alternating current, which flows backwards and forwards. The frequency is the number of times the current flows each way per second, which is determined by the number of times the generator turns per second. These generators are called alternators. </li></ul>
  35. 38. Japan’s Electricity <ul><li>Mains electricity is A.C. </li></ul><ul><li>Some countries use 50HZ (eg ________). </li></ul><ul><li>Other countries use 60Hz (eg _________). </li></ul><ul><li>Japan is unusual because it uses two frequencies: 50Hz in Tokyo (and north) and 60Hz in Osaka (and south). </li></ul><ul><li>The reason is historical: generators were purchased from the US (60Hz) and Europe (50HZ) in the nineteenth century before the entire country was connected to the mains electricity (known as the ‘national grid’). </li></ul>
  36. 39. Questions <ul><li>Page 169 & 171 </li></ul>
  37. 40. Transformers <ul><li>An electromagnet produces a m_______ f______. </li></ul><ul><li>If the voltage is increased, the current increases, which in turn increases the magnetic field. This has the same effect as moving a magnet into the coil. </li></ul><ul><li>If the voltage – and therefore the current – is changed, the magnetic field changes. This changing magnetic field can induce a voltage – and therefore a current – in another (“secondary”) coil of wire if the magnetic field flows through it as well. </li></ul><ul><li>If the number of turns in the primary and secondary coils are different, the voltages in each coil will be different. Transformers use two coils with different numbers of turns to transform voltage. Transformers only work with _______. </li></ul>
  38. 41. Transformer Diagram <ul><li>Ignore the writing – unless you can read it! Please note that a transformer is made of two coils with different numbers of turns, connected by an iron (why?) core. </li></ul>http://commons.wikimedia.org/wiki/File:750px-Transformator.png
  39. 42. Transformer Equation <ul><li>Example: A transformer consisting of a primary coil of wire with 100 turns and a secondary coil of 25 turns is connected to the Japanese mains voltage (100V). Calculate the voltage of the secondary coil. </li></ul>
  40. 43. <ul><li>A transformer with 50 turns in the primary (input) coil and 200 turns in the secondary (output) coil is connected to a 1.5V (AA) battery (like the one in a remote control). Calculate the output voltage. </li></ul><ul><li>This was a trick question (and practice at the equation). The answer is 0V, since a battery produces DC and transformers need AC. </li></ul><ul><li>A laptop charger has an output of 16.5V. If the output coil has 2000 turns (probably unrealistic), how many turns will the input coil need in: a) Japan (100V) b) Europe (230V) </li></ul>
  41. 44. Free Power? <ul><li>Most transformers lose some power, however if the loss is n egligible , then the input power must be the same as the output power. </li></ul><ul><li>Input * input = output * output voltage current voltage current </li></ul><ul><li>Determine the ratio of the currents in the input and output coils in relation to the number of turns in each coil. </li></ul>
  42. 45. Bookwork <ul><li>Answer problems page 173 & 175 (read 174-5). </li></ul><ul><li>Chapter Summary Pages 178-179 (everything). </li></ul>
  43. 46. Thermal Power Stations <ul><li>Photo of a thermal power station in AiChi (not necessary): </li></ul><ul><li>http://www.thedailystar.net/newDesign/photo_gallery.php?pid=127029 </li></ul>
  44. 47. Thermal Power Stations <ul><li>A thermal power station uses steam to turn a turbine, which then turns a generator to produce electricity. Most produce the steam from burning coal, oil and natural gas, but some also waste incineration or bio-fuel. Nuclear power stations use heat from nuclear reactions and geothermal power stations use heat from the Earth. </li></ul><ul><li>Most thermal power stations produce over 1000MW. The Hekinan Power Station (Aichi) produces 4100MW of power. </li></ul>
  45. 48. Diagram <ul><li>A simplified diagram of a thermal power station: (note that this one burns gas, however it could burn coal, oil or biomass) http://www.bbc.co.uk/scotland/learning/bitesize/standard/physics/energy_matters/generation_of_electricity_rev1.shtml </li></ul>
  46. 49. Efficiency and Waste Heat <ul><li>Laws of Physics (IB) limit the efficiency of thermal power stations; most are between 33% and 48% efficient. The rest of the energy is converted to heat. </li></ul><ul><li>This heat can be used for industrial purposes (eg??), to heat homes, to desalinate seawater, or it can be ‘dumped’ into a source of water or through a cooling tower. </li></ul>
  47. 50. This diagram from Tepco is a little more complicated than is needed at IGCSE, but still worth viewing. Note the transformer to increase the voltage. http ://www.tepco.co.jp/en/challenge/energy/thermal/power-g- e.html
  48. 51. An example of a coal-fired power station with cooling towers, since there isn’t a water supply to absorb the waste heat: http ://forums.xkcd.com/viewtopic.php?f=8&t=32541&start= 280
  49. 52. Pros & Cons of Fossil Fuels <ul><li>Advantages of Fossil Fuels Disadvantages of Fossil Fuels </li></ul>
  50. 53. Pros & Cons of Nuclear Power <ul><li>Advantages of Nuclear Power Disadvantages of Nuclear Power </li></ul>
  51. 54. The Global Energy Crisis <ul><li>The developed world – and our lifestyle – is built on an abundant supply of cheap energy. </li></ul><ul><li>Our energy started with coal during the industrial revolution, and then last century oil and gas became more popular. </li></ul><ul><li>Pollution and climate change, along with the developing third world, are leading people to question how we live and use energy. </li></ul>
  52. 55. Solution to the Energy Crisis <ul><li>The problem will be discussed in the coming unit, but includes pollution (including heavy metals such as mercury from coal), political issues, climate change, risk of nuclear accidents and disposal of nuclear waste. The solution/s is/are A) Stop using nuclear and burn more fossil fuels. B) Stop using fossil fuels and use more modern, cleaner, safer nuclear. C) A combination of A and B D) Reduce our energy consumption so we can use only renewable energy. We will discuss this option more in the next unit. </li></ul>
  53. 56. Questions <ul><li>Page 63 (all) </li></ul><ul><li>Finish EM Revision worksheet from PHETs and save them to your shared Google folder (go to “collections shared with me” then find yoursurname yourfirstname Physics ) </li></ul><ul><li>If time permits, read 64-65 and answer questions on page 65. </li></ul>

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