Electromagnetism

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GCSE Physics double award notes

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  • http://www.tutorvista.com/content/physicscontent.php
  • Demo with large horse-shoe magnet, wire passing through connected to a power pack. When the power is switched on the wire flicks forward… The combination of magnetism and electricity can cause motion In fact whenever two of the three from this group are present, the other one will be generated…
  • Dynamo – Electricity, magnetism and motion all interacting In a power station, be it coal-fired, hydro or wind, large turbine spin a magnet around within coils of conducting wire which then cause a current to flow
  • The sensitive ammeter shows a deflection when the magnetic comes close to the coils of wire This means a current is flowing in the wire When the magnet stops moving, the current stops flowing The faster the magnet moves, the higher the current flows When the magnet moves out of the wire the current flows in the opposite direction When the magnet is turned around and put close to the wire a current flows again in the opposite direction The more coils of wire the larger the current generated It doesn’t matter which of the two objects are moving – the coil or the magnet, as long as one of them is, the current will flow MOTION ELECTRICITY MAGNETISM (MEM!)
  • ‘ inducing’ a current – persuading or compelling it to happen!
  • Demo set up – sensitive light ammeter, horseshoe magnet and three electrical leads joined together
  • What are some of the behaviours of ‘inducing’ a current when using this setup? Recalling what we know from the last investigating, what must happen? How can we affect the size and direction of the current? Need to MOVE the wire, the DIRECTION of the movement is connected to the direction of the current, SPEED of the movement effects the SIZE of the current. One new observation – if the wire moves parallel rather than perpendicular to the field lines, no current is induced. The field lines need to be cut through (break through, dissect!). It requires work to be done (energy transferred, similar to working against gravity) for the electricity to be generated.
  • Pupil handout for notes.
  • Java applet of investigation of magnet motion in a coil
  • When the poles of the magnet are turned around, the current will be induced in the opposite direction
  • Ratio of turns = Ratio of voltages Set up the demo as shown and apply a voltage of 5 V across primary, roughly twice the voltage is induced in the secondary. Increase the voltage to 8 V, again double the voltage is induced in the secondary. This represents a step up transformer. Why does it not work when D.C. is supplied? Provide a voltage across the coil with a larger number of turns to represent a step down transformer. Repeat the process but notice that the ratio is reversed and each time half the voltage is induced.
  • Try questions 7 - 10
  • Animation of the turbine rotating within a magnetic field and the change is direction and magnitude of the current
  • The most important use of transformers is in the routing of mains electricity. The power dissipated by a cable = I2R, where I is the current and R is the resistance. There is little that can be done to lower the resistance, so in order to lose as little energy as possible in heating up the cable, the current needs to be kept as low as possible. This is done using a step up transformer which increases the voltage to many thousands of volts. Because power is voltage times current, a high voltage means a low current for a given power value.(Notice that transormers do not obey Ohm's law). At the other end of the distribution network, the voltage is stepped down using another transformer so that the voltage going into homes and factories is not excessively high. Using transformers Transformers are used to lower voltage to be safer to use in your house. You may also use an adapter to lower the voltage even more for some devices you use. DC transformers are now available, but they won't replace AC transformers. House voltage Normally, the current in the electrical lines outside your house are around 1100V AC. The reason it is so high is that the electricity travels more effectively over long distances at higher voltages. High voltage lines carry up to 10,000 volts. The transformer near to top of the electrical pole changes the voltage to a safer 110V for your house. Adapters Most people use adapters when they power devices that also use batteries. An adapter is a transformer that changes the 110V AC house current to 12V DC or 9V DC that is used by the device. It also changes the AC to DC, because the device works on batteries. Changing AC to DC is done by electronic circuitry called a rectifier. It essentially chops off 1/2 of the AC current to make it similar to DC. Some of the lost AC current is turned into heat. That is one reason your adapters sometimes get warm. DC transformers You can see that DC voltages could not be changed with the configuration of the transformer. This is because the DC current would not be changing the magnetic field the way AC current does. And this was the reason that AC won over DC when electricity started to be used around the world. Since then, electrical engineers have developed DC transformers, primarily using special circuitry. Since most everyone now uses AC, it is too late to change the system.
  • Exam questions – Page 165 onwards, Q2 a) and b), Q3 b) – d)
  • Electromagnetism

    1. 1. Electromagnetism GCSE Physics
    2. 2. Learning Intentions <ul><li>Recall the significant link between electricity and magnetism </li></ul><ul><li>Observe how a magnetic field can interact with a conducting wire </li></ul><ul><li>State how electricity can be generated </li></ul>
    3. 3. Electrical Generators
    4. 4. N S
    5. 5. Investigate… <ul><li>How does a magnet affect the loop of conducting wire? </li></ul>
    6. 6. What has to happen for the current to flow? How is the direction of the current flowing related to the magnet? How is it possible to increase the amount of current ? Is there more than one way of doing this? How is it possible for current to flow without moving the magnet ?
    7. 7. The Not-so-Missing Link <ul><li>Magnetism is closely related to Electricity . These two characteristics combine to make one of the fundamental forces in physics known as Electro magnetism . </li></ul><ul><li>Where there is a magnetic field there is the potential to make an electric current flow </li></ul><ul><li>Where there is a flow of an electric current there is an associated magnetic field . </li></ul>
    8. 8. Uniform Magnetic Field between poles
    9. 9. Move the wire through the magnetic field
    10. 10. Making Electricity <ul><li>Move the wire down cutting through the magnetic field </li></ul>N S 0 Induces a current
    11. 11. Making Electricity <ul><li>Move the wire up cutting through the magnetic field </li></ul>N S 0 Induces a current in the other direction
    12. 12. N 0 N S 0 No Current Induced when… … the movement of the wire does not cut through the magnetic field lines … the wire does not move S
    13. 13. Electromagnetic Induction (pg156) <ul><li>This time use a wire coil (solenoid) and move the magnet in and out </li></ul>0 0 N N Magnet moved in Magnet moved out
    14. 15. Induced Current <ul><li>Move the magnet into the coil and a current is induced in one direction </li></ul><ul><li>Move the magnet out of the coil and a current is induced in the other direction </li></ul><ul><li>As soon as the motion stops the current will stop </li></ul><ul><li>For a current to be induced the magnet field lines need to continually be cutting through the wires </li></ul>
    15. 16. Increase the SIZE of current/voltage <ul><li>Increase the speed of the movement of the magnet towards/away from the coil </li></ul><ul><li>Use a more powerful magnet (more magnetic field lines produced) </li></ul><ul><li>Increase the number of coils in the wire (more wire being cut through by magnetic field lines) </li></ul>
    16. 17. For current to flow… <ul><li>The magnetic field lines must continually be cut through by the wire (through the lines of force) </li></ul><ul><li>Work is done against the force of the magnet and an energy transfer occurs </li></ul><ul><li>Kinetic Electrical </li></ul>
    17. 18. Learning Intentions <ul><li>Recall the significant link between electricity and magnetism </li></ul><ul><li>Recognise how a solenoid can induce a current in a coil of conducting wire </li></ul><ul><li>State the need for AC supplied to a solenoid to continually induce a current </li></ul><ul><li>Understand how a transformer can be used to change voltage between two circuits </li></ul>
    18. 20. How does it work? <ul><li>Alternating Current (AC) , one way and then another </li></ul><ul><li>The magnetic field lines are cutting through the wires in the secondary coil- inducing a current ! </li></ul>
    19. 21. Primary Coil Secondary Coil Pass a current through a coil of wire and a magnetic field is created. When a magnetic field is at right angles to a wire, a current is induced in that wire
    20. 22. Primary Coil Secondary Coil Increase the Current and the associated magnetic field becomes stronger A stronger magnetic field will induce a larger current in that wire
    21. 23. Primary Coil Secondary Coil Decrease the Current and the associated magnetic field becomes weaker. A weaker magnetic field will induce a smaller current in that wire
    22. 24. Primary Coil Secondary Coil A current which is kept constant in one direction is known as Direct Current (D.C.) If the magnetic field does not vary then the current will no longer be induced and will not flow
    23. 25. Primary Coil Secondary Coil A current which flows in one direction and then in the other direction is called an Alternating Current (A.C.) If the magnetic field is changing all the time then a current will continually be induced.
    24. 27. Transformers (pg 161) <ul><li>What is a transformer? </li></ul><ul><li>A transformer consists of two coils wrapped around a laminated soft iron core. </li></ul>Primary Coil Secondary Coil
    25. 28. Transformer’s Symbol Iron Core
    26. 30. Types of Transformers <ul><li>Primary coils < Secondary coils </li></ul>Step Up The Iron Core links the two coils magnetically
    27. 31. Step Up <ul><li>A step up transformer has more turns of wire on the secondary coil , which makes a larger induced voltage in the secondary coil. </li></ul><ul><li>It is called a step up transformer because the voltage output is stepped up </li></ul>
    28. 32. Step Down Transformer <ul><li>Primary Coils > Secondary Coils </li></ul>Step Down The Core is made of iron so it can be easily magnetised and demagnetised.
    29. 33. Step Down <ul><li>A step down transformer has less turns of wire on the secondary coil, which makes a smaller induced voltage in the secondary coil. </li></ul><ul><li>It is called a step down transformer because the voltage output is stepped down . </li></ul>
    30. 34. Transformers Equation Voltage Across Primary Voltage Across Secondary Number of turns on Primary Number of turns on Secondary = V p N p V S N S =
    31. 35. Worked Example <ul><li>A voltage of 240V is applied to a primary coil of 200 turns . What is the voltage across the secondary coil if it has 10 turns ? </li></ul>V p N p V S N S = 200 V S 10 = 240 V S = 240 x 10 200 = 12 Volts V S
    32. 36. Learning Intentions <ul><li>Recall the significant link between electricity and magnetism </li></ul><ul><li>Recognise how the electrical power generated by a turbine is A.C. </li></ul><ul><li>Recall how electricity is transmitted through out the country by The Grid </li></ul><ul><li>State the need for step-up and step-down transformers for nationwide electricity distribution </li></ul>
    33. 37. A.C. Generator <ul><li>Alternating Current Generator (alternator) - Page 158 </li></ul>http://www.sciencejoywagon.com/physicszone/otherpub/wfendt/generatorengl.htm
    34. 38. Katie Explains AC generators-
    35. 39. The National Grid (pg 159) <ul><li>Electricity generated a power station is distributed to homes, shops, schools factories etc across the country by a network of cables called the National Grid . </li></ul><ul><li>Step up Transformers are used before transmission on the Grid </li></ul><ul><li>Local Step down Transformers are used before consumption by homes, schools etc </li></ul>
    36. 40. Why use Transformers? <ul><li>Page 160 and 161 </li></ul>
    37. 41. Let’s get active… <ul><li>Match the statements to the pictures to make up the process for which electricity is distributed across our beautiful country… </li></ul>
    38. 42. <ul><li>The power is then supplied to the homes, factories, offices etc for their own consumption. </li></ul><ul><li>The electrical power is connected to the National Grid which is a network of cables stretching over the country. It is important that the voltage is high in these cables. </li></ul><ul><li>A Transformer does not create more energy by stepping up the voltage. The power generated is equal to that consumed. The voltage is increased so that the current is reduced. This means less energy will be lost as heat in the cables (smaller resistance). </li></ul><ul><li>The voltage needs to be stepped down to a lower value by local transformers </li></ul><ul><li>In Power Stations, the alternators are driven by huge turbines, spun round by the high pressure steam. A large current is produced . </li></ul><ul><li>Transformers are used to step up the voltage before the electrical power is distributed across the country </li></ul>1 2 3 4 5 6
    39. 43. To finish with <ul><li>Page 164 </li></ul><ul><li>Questions 11 and 12 </li></ul>

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