Electricity presentation (Grade 10)


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This presentation is a Grade 10 MYP Science course on electricity.

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Electricity presentation (Grade 10)

  1. 1. Electric Charge & The Atom  An object is charged if it has gained or lost electrons. http://commons.wikimedia.org/wiki/File:Stylised_Lithium_Atom.png
  2. 2. Like Charges Repel & Opposites Attract http://www.datasync.com/~rsf1/eas.htm http://blog.taser.com/how-does-a-taser-work-electricity-101/ The removed images show that two positive charges attract, two negative charges attract, while a positive and a negative charge will attract each other. There is no need to load the image or read the pages linked below.
  3. 3. Electric Fields  An electric field is a region of space in which electrical forces act.  Electric charges (eg ____________) create electric fields around them. http://www.madsci.org/posts/archives/2004-09/1095325697.Es.r.html The removed image showed a woman standing on top of a mountain with her hair standing up. This was because there was a strong electric field between the ground and the clouds. Moments after her and her brother left the spot it was struck by lightning. So NEVER stay around if your hair stands up – go somewhere safe!
  4. 4. Lightning  Lightning occurs during a thunderstorm when cloud movement causes a negative charge to build up in the cloud, which 'induces' a positive charge beneath it on the ground.  If these charges become large enough, the electrons will 'jump' to the Earth, causing lightning.  Thunder is the sound of lightning, and occurs at the same time, but is heard separately since sound travels so much slower than light.  Most lightning occurs within or between clouds. http://upload.wikimedia.org/wikipedia/commons/thu Lightning_hits_tree.jpg
  5. 5. Is this a Lightning Rod?
  6. 6. Lightning Rod?  A lightning rod provides an 'easy' way for the electrons to pass through a building or other structure, instead of it passing through the material it's made of, which often results in fires.  Many famous structures, including Osaka castle (twice in the 1600s) have been destroyed by lightning. The Eiffel tower lost its top in 1902.  One man has survived seven lightning strikes.  Never shelter under a tree during a thunderstorm.
  7. 7. Lightning Videos Watch the slow motion lightning strike here: http://en.wikipedia.org/wiki/Lightning
  8. 8. Creating a Current  Instead of passing through the atmosphere, electricity can run through a conductor.  Electrons are very small, so physicists measure them in Coulombs. http://commons.wikimedia.org/wiki/File:Stranded_lamp_wire.jpg
  9. 9. How do we Measure Charge  Charge is measured in Coulombs.  One Coulomb (C) = 6 * 1018 electrons (but not by definition). 1. How many electrons in half a Coulomb? 2. What is the elementary charge (on one electron)?  The unit of the Coulomb was used before the elementary charge was known. Image: Charles-Augustin de Coulomb http://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Charles_de_coulomb.jpg/250px
  10. 10. Electric Current  Current is a measure of how much charge are flowing past a given point in a conductor per second.  Its units are Amperes.  One Amp = 1 C/s. 1. If five Coulombs flow past a point in a wire in ten seconds? a) what is the current in the wire? b) how many electrons flow past in two seconds? Image: Charles-Augustin de Coulomb http://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Charles_de_coulomb.jpg/250px
  11. 11. 1) 12 * 1018 electrons pass a point in one second. What is the current in Amps? 2) 3 * 1018 electrons pass a point in one second. What is the current in Amps? 3) How many Coulombs pass a given point in three seconds if the current is five Amps? Harder: how many electrons is this? 4) Draw a diagram to explain an analogy between electric current and either a) a bakery delivering bread to a grocery store b) cars on a freeway c) water flowing down a river 5) An average lightning bolt carries 30 000 Amps and transfers 15 Coulombs. a) How many electrons is this? b) How long does the lightning bolt last? 6) A new Macbook Air contains a 7150mAh battery. If it takes five hours to charge the battery, calculate the average charging current. Optional extension: Do some research and explain why we calculated the 'average' current. Sketch a charging curve for a (lithium ion) battery.
  12. 12. Quick Review One Coulomb is 6*1018 electrons. I = Q/t a) How many Coulombs in 18 *1018 electrons? b) How many Coulombs in 3 *1018 electrons? c) If Three Coulombs pass a point in 1.5 seconds, what is the current in the wire? d) Why do we use Coulombs instead of electrons? Give another example of a unit representing a number of objects.
  13. 13. Conductors and Insulators Make the bulb light up. Materials: 1 wire, 1 bulb and 1 battery. Make a circuit to test whether or not something conducts electricity. A) Draw the circuit in your book (without using symbols). It should be possible for another student to reconstruct it the same way. B) Test the following for conductivity: glass, iron, plastic, aluminium, copper, wood, graphite, zinc. Complete everything on the worksheet.
  14. 14. Voltage Electrons carry electrical energy. Work is done to move electrons through a conductor, so the electrons lose potential energy. Potential difference is the difference in energy between one point and another. Potential difference is measured in Volts. 1 Volt = 1 Joule per Coulomb (J/C or V)
  15. 15. E.M.F e.m.f = electromotive force is the voltage generated by a power source (eg ______). It drives the charge around a circuit. e.m.f and P.D. Measure the same quantity (voltage) so of course have the same units.
  16. 16. Voltmeters Voltmeters measure the potential difference between two points in a circuit. It is difficult for electricity to get through a voltmeter (we say its __________ is high – next class) so that it has minimal disruption on the circuit. This image is worth loading to show how voltmeters are connected. http://www.gcsescience.com/pe5.htm
  17. 17. Resistance Energy is required to push electrons through most conductors. Resistance measures how difficult it is for electrons to get through. Resistance is measured in Ohms (Ω). 1. Calculate the resistance of a light bulb which uses three Amps from a 9V battery. 2. Calculate how many Amps will flow through a heater with an effective resistance of 10Ω when connected to the Japanese 100V mains. Ohm'sLaw :resistance= voltage current Image: Georg Simon Ohm (Wikipedia) http://en.wikipedia.org/wiki/Geo
  18. 18. More Resistance Problems 1. Tammy takes a flashlight to Phuket. It runs on two 1.5Volt AA batteries (total 3V) and draws a current of 0.2A. Calculate the resistance of the LED (light source). 2. A kettle connected to the Japanese mains voltage (100V) draws five Amps. Calculate its resistance. 3. How much current would the kettle use if it were connected to the mains power in Europe (240V). Why might this be a problem? 4. A small solar-powered motor with an effective resistance of 100Ω uses 0.5 Amps. What is the voltage of its power source?
  19. 19. Length and Thickness The resistance of a resistor (or anything else) is greater if the resistor is thinner, since there is less area for the electrons to pass. The resistance is greater if the resistor is longer, because the electrons need to travel further through the resistor.
  20. 20. Example A length of metal has a resistance of 10Ω. It is cut in half, widthwise, and the two ends are placed lengthwise. What is its new resistance?
  21. 21. Pylons Why are pylons made of aluminium, when copper is a better conductor? http://www.flickr.com/photos/italianstylelover/4720689749/sizes/z/in/photostream/ Photo of pylons under Mt Fuji unnecessary.
  22. 22. Series and Parallel Circuits. Current that leaves the battery must all come back. It can not disappear. If 10 Amps leaves, 10 Amps must come back. Voltage is 'used up'. If a Coulomb leaves a battery with 12V, it must 'use it all up' in the circuit. ie. If the EMF of the power source is 12V, the sum of the PD of all components in one circuit must be 12V.
  23. 23. Series Circuit In a series circuit, the electricity can only flow one way. The electricity goes through each bulb. The current is the same everywhere in the circuit. The P.D. Of all the components equals the EMF of the power supply.
  24. 24. Parallel Circuit In a parallel circuit, each bulb has its own path to the power supply. The electricity goes through each bulb only once. The current from the power source is shared between all the different branches. The PD of each component is the same.
  25. 25. Current and Voltage A1 A3 A2 V1 V1 12V 3Ω Meter Reading V1 A1 A2 A3
  26. 26. Current and Voltage A1 A3 A2 V1 V1 V2 12V All bulbs are (equivalent) 4Ω Meter Reading V1 V2 A1 A2 A3 A4 A4
  27. 27. Current and Voltage A1 A3 A4 A2 V1 V3 V1 V2 12V All bulbs are (equivalent) 6Ω Meter Reading V1 V2 V3 A1 A2 A3 A4
  28. 28. Resistors in Series Two resistors in series are just like one long resistor. RT = R1 + R2 + R3... Calculate the current in this circuit. http://en.wikibooks.org/wiki/GCSE_Science/Parallel_and_series_circuits 1Ω 2Ω
  29. 29. Resistors in Parallel With two resistors in parallel, there are two paths for the electricity to travel through, so it is easier, so the resistance is lower. Calculate the resistance of a 10Ω, 1Ω and 5Ω resistor, all in parallel. ANS: 0.77Ω. Don't forget the final reciprocal at the end! Note for IGCSE students:IGCSE only requires two in parallel. http://www.learnabout-electronics.
  30. 30. Extension Exercise Five 10Ω resistors are connected in parallel to a 10V power source. A) Calculate the current through each resistor. B) Calculate the current in the whole circuit. C) If all the resistors were to be replaced with one resistor (to save space) what would its resistance be? D) Can you find a connection between the total resistance and the additional resistances? What if they were different resistances?
  31. 31. How is Power Different in Different Countries? http://bionicbong.com/wp-content/uploads/2011/03/socket.jpg http://img.archiexpo.com/images_ae/photo-m/power-socket-50417-2252991.jpg 1. 2. 3. Photos of different power sockets in different countries unnecessary.
  32. 32. Power Five Amps flow through a 12 Volt car bulb. How many Joules flow through the light bulb each second? This is called the power. Power measures how much energy something uses or produces per second. A Joule is a measure of energy. One Joule is about the amount of energy required to lift a calculator out of a bag onto a desk. Image: James Joule (Wikipedia)
  33. 33. Energy and Power Power (Watts) = Voltage (Volts) * Current (Amps) Mr Duffield bought a small heater in Japan. It was labelled “600W”. Japan's mains voltage is 100V. 1) Calculate the current through the heater element in Japan. 2) Calculate the resistance of the heater element in Japan. He then took it to Taiwan, where the voltage is 110V. 3) Assuming the resistance stays the same in Taiwan (?), calculate the new current through the heater. 4) Calculate the new power of the heater in Taiwan. 5) How many extra Joules of energy must be dissipated by the fan in the heater each second in Taiwan (instead of Japan)? Is this likely to be dangerous? 6) How many Joules of heat does the heater produce in Taiwan in ten seconds? 7) Repeat calculations for New Zealand (240V). What will happen if the heater is used there?
  34. 34. Iphones and Fridges Does an iphone use more power than a fridge? Some studies have claimed this, but the results have been disputed. Research this and explain your conclusion. http://theweek.com/article/index/248273/your- iphone-uses-more-energy-than-a-refrigerator
  35. 35. Kilowatt Hours One Kilowatt hour (kWh) is a measure of energy. It is the amount of energy something which has a power of 1kW uses in one hour. It is a useful unit for the general public since electricity is sold in kWh. In Japan 1 kWh costs approximately 20 Yen. How many Joules in 1 kWh? If a vending machine uses 3 000 kWh per year, calculate its power in Watts (assuming it is on all the time). ANS: ~ 340 Watts.
  36. 36. Why Different Voltages? Electric current interferes with the body's nerve system, and as a resistor the body produces heat (thus shocks burn). Assume a person has a resistance of 1000 Ohms (it varies a lot person to person, and depends on the skin in particular). 1. Calculate how much current they will have pass through them in Japan (100V). 2. Calculate how much current the same person would have pass through them if they were electrocuted in Australia (240V). 3. On a cold winter's day a particular house uses 2000 Watts. Calculate the current through the wires coming into the house in Japan and Europe. Why might a higher current be more dangerous? 4. Draw a table to show the advantages and disadvantages of high and low voltage mains electricity.