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Resistive Heating

Resistive Heating

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Resistive Heating

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  1. Resistive Heating April 13th, 2010

  2. Resistive Heating Mark 4th Year Physics student at UBC, Graduated at Riverside Secondary in Port Coquitlam Basketball, Soccer, Music, and Sailing

  3. What Do Each of these Have in Common?

  4. What Do Each of these Have in Common? Burns Bread

  5. What Do Each of these Have in Common?

  6. What Do Each of these Have in Common? Creates Light

  7. What Do Each of these Have in Common?

  8. What Do Each of these Have in Common? Transmits Electricity

  9. What Do Each of these Have in Common? Hot Dog…..

  10. What Do Each of these Have in Common? Hot Dog….. Tastes Delicious 

  11. Resistive Heating…… • Toaster… • Light Bulb… • Power Lines… • Hot Dog… • In this presentation, I plan to explain the concept that connects each one of these four objects

  12. Resistive Heating… • What is Resistive Heating? • Who discovered Resistive Heating? • What causes Resistive Heating? • A little Experiment • Introduction • Experimentation • Calculations • Conclusion • Conclusion

  13. What is Resistive Heating? • “The process by which a current is passed through a conductor and heat is released” Wikipedia

  14. What is Resistive Heating? • “The process by which a current is passed through a conductor and heat is released” Wikipedia • Also Known as: • Resistive Heating • Joule Heating • Ohmic Heating • Electric Heating

  15. What is Resistive Heating? • “The process by which a current is passed through a conductor and heat is released” Wikipedia • Also Known as: • Resistive Heating • Joule Heating • Ohmic Heating • Electric Heating All names mean the same thing but we will be using this name, ‘Joule Heating’ from here on, named after its discoverer.

  16. Who discovered Joule Heating? • First experimented with in 1841

  17. Who discovered Joule Heating? • First experimented with in 1841 • James Prescott Joule • Hence the name: ‘Joule Heating’

  18. Who discovered Joule Heating? • First experimented with in 1841 • James Prescott Joule • Hence the name: ‘Joule Heating’ • What did he do? • Passed a current through a wire of fixed length, • Immersed in a bath of water of fixed volume/mass • Observed the temperature in the water varried with current, length of wire, and time.

  19. Who discovered Joule Heating? • What did he do? • Passed a current through a wire of fixed length, • Immersed in a bath of water of fixed volume/mass • Observed the temperature in the water varried with current, length of wire, and time. • He came up with the relation:

  20. Who discovered Joule Heating? Where: Q = amount of Energy produced (in the form of heat) UNITS: joules 1 joule = kg m /s2 I = Current UNITS: amperes R = Resistance UNITS: ohms t = Time UNITS: seconds

  21. What Causes joule Heating? • In a complete circuit, electrons move around the circuit. • Often these electrons ‘collide’ with ions, and the electrons share kinetic energy with the ions • Ion: a charged atom, ie: NaCl+ (Salt) • The kinetic energy excites the ions, and makes them vibrate. • This increase in kinetic energy displays itself as heat and a temperature increase occurs.

  22. What Causes joule Heating? • In a complete circuit, electrons move around the circuit. • Often these electrons ‘collide’ with ions, and the electrons share kinetic energy with the ions • Ion: a charged atom, ie: NaCl+ (Salt) • The kinetic energy excites the ions, and makes them vibrate. • This increase in kinetic energy displays itself as heat and a temperature increase occurs. • SUMMARY: Energy is transfered from the electrical power supply to the conductor and any materials with which it is in thermal contact.

  23. What Causes joule Heating? • Show “Battery-Resistor Circuit” • What do the blue spheres represent? • What do the green spheres represent? • Why do the green and blue spheres interact? • What changes when the voltage is increased? • WHY?

  24. ?????? • So what does this have to do with: • Burnt Toast • Light Bulbs • Power transmission lines • Hot Dogs???

  25. ?????? • Burnt Toast? • Inside the toaster are small wires called ‘filaments’, • The elements heat up when a current is passed through them and so it cooks/burns your toast.

  26. ?????? • Burnt Toast? • Inside the toaster are small wires called ‘filaments’, • The elements heat up when a current is passed through them and so it cooks/burns your toast. • Light Bulbs? • Like the toaster, the light bulb also has a filament. • The filament is made of such a material that it can withstand extreme temperatures and glow brightly. • This material is usually ‘Tungsten’ which has a melting temperature of 3880 C

  27. ?????? • Power Transmission Wires? • The wires are required to transmit large amounts of electricity over long distances • The amount of heat lost due to Joule Heating must be reduced as much as possible to minimize energy lost to heat • We know: V = I R

  28. ?????? • Power Transmission Wires? • So if we maximize V, then we can minimize I, and so minimize Q • Transformers are used to step up the voltage and reduce the current, while keeping the resistance constant Vlarge = Ismall R • If ‘I’ is reduced by ½, then energy lost can be reduced by a 4!!! And so: Q = Ismall2 x R x t

  29. ?????? • Power Transmission Wires? • The power lines used to transport power large distances have VERY large voltages…. 110 kV – 230 kV

  30. Experiment time! • Could you use the concepts of joule heating to cook a hot dog?

  31. Experiment time! • Could you use the concepts of joule heating to cook a hot dog? • OF COURSE!!!...... Why?

  32. Introduction: • Could you use the concepts of joule heating to cook a hot dog? • Hot Dogs are delicious, and so they contain salt: • Remember: Salt = NaCl+ which is an ion!

  33. Introduction: • We will place a Hot Dog in between two electrodes and apply a voltage to cook the hot dog. • We perform 3 quick experiments: • 1) one hot dog • 2) two hot dogs, connected in a parallel circuit • 3) two hot dogs, connected in a series circuit

  34. Introduction: • We will be using “Schneiders All Beef Wieners” • 110 calories, 9g fat, 4g protein, and 380 mg SODIUM  “Taste the difference quality makes!”

  35. Experimentation: • Lets get started: first: cook one hot dog at a time. • We will be timing the procedure, from the moment the voltage is applied, till roughly when the meat stops cooking.

  36. Experimentation: • The meat stops cooking because the connection at the electrodes gets burned out! The moisture in the weiner contains the Ions, NaCl, and when the moisture boils and evapourates, the hotdog stops cooking

  37. V=I R, thus using only the measured current and the known voltage, we can find the resistance • Iave = 2.505mAmps, 0.002505 Amps • V = 120 Volts • V/I = 120V/0.002505 Amps = 47904 Ohms = 47.9 kOhms Thus: The average resistance of a hotdog is ~50kOhms.

  38. Experimentation: For one wiener

  39. The average resistance of a hot dog is 50 kOhms • What does this number depend on?

  40. The average resistance of a hot dog is 50 kOhms • What does this number depend on? • Size of hotdog • Moisture content • Temperature • Fat content (other brands, varieties) • Meat type (Chicken, Beef, Turkey…)

  41. Experimentation: • Parallel Circuits: • Total resistance is calculated:

  42. Experimentation: • Parallel Circuits: • Total resistance is calculated:

  43. Experimentation: • Parallel Circuits: • Total Current is calculated:

  44. Experimentation: • Parallel Circuits: • Total Current is calculated: • This is a different current than when cooking only one weiner. • Does this make sense??? • Itwoweiners, parallel = 4.8mAmps Ione weiner = 2.505mAmps

  45. Experimentation: • Parallel Circuits: • Does this make sense??? Ione weiner = 0.002505Amps We know by the current law for parallel circuits that: Itotal = I1 + I2 So really: each hot dog is receiving the same current as it was before (when there was only one weiner cooking)

  46. Experimentation: • Parallel Circuits: Itotal = I1 + I2 Thus: the current law justifies that the cooking time is the same as the previous experiment…. But what happens when the hotdogs are connected in series?