practical electricity unit 21 x. outline power »electrical energy transfer »resistive dissipation...
TRANSCRIPT
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PracticalElectricity
Unit 21
x x
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Outline
• Power» Electrical energy transfer» Resistive dissipation» Summing power
• Heating effect• Paying for electricity
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POWER!!!!
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Work done
Potential difference = …?
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Work done
Potential difference = Work done per unit charge
W = V Q
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Power
Power = Work done / unit time
= V Q / t
= V I
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Power: Electrical Energy Transfer
P = I V
Watt is the unit?
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Power: Resistive Dissipation
P = I V
Use Ohm’s Law:
P = I2 R
P = V2 / R
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Power: Electrical Energy Transfer vs. Resistive
DissipationTitle… too… long
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A question
• A potential difference V is connected across a resistance R, causing current I through the resistance. Rank the following variations according to the change in the rate at which electrical energy is converted to thermal energy in the resistance, greatest change first:a) V doubled, R unchangedb) I doubled, R unchangedc) R doubled, V unchangedd) R doubled, I unchanged ab/d/c
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Another question
• You are given a length of uniform heating wire made of Nichrome with a resistance of 72 ohms. At what rate is energy dissipated in the following scenarios:» A p.d. of 120 V is applied across the full length of the wire.
» The wire is cut in half, and a p.d. of 120 V is applied across the length of each half.
• 200,400
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Another question
• You are given a length of uniform heating wire made of Nichrome with a resistance of 72 ohms. At what rate is energy dissipated in the following scenarios:» A p.d. of 120 V is applied across the full length of the wire.
» The wire is cut in half, and a p.d. of 120 V is applied across the length of each half.
• Heat output = power; why not cut?
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Yet another question
• A wire of length L = 2.35 m and diameter d = 1.63 mm carries a current I of 1.24 A. The wire dissipates electrical energy at the rate P of 48.5 mW. What is the resistivity of the material?
• 2.8x10-8ohmmetres (Al)
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Heating Effect of Electricity
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Heating effect video
High / low resistance?
Why doesn’t it melt / explode?
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Characteristics of heating element
• Nichrome• Coiled around some kind of insulating, fire-proof material» Silica, mica, RI boys
• High melting point• High resistance
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Applications of heating effect
• Electric kettle: heating element enclosed in a metal tube, water gets heated by conduction and convection
• Electric iron: heat generated by the heating element is spread evenly over a metal base (chromium plated)
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Applications of heating effect
• Filament/incandescent lamp: Tungsten (why?) coiled up (why?) in glass filled with argon/nitrogen (WHY?!), casts sharp shadows
• Fluorescent lamp: No filament (WHYY?!?!1one), vapour emits UV light (isn’t that invisible?), casts soft shadows
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Incandescent light bulb vs. fluorescent lamp
Both are rated at 40 W. Which puts out the most
light?
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Thick and Thin
Two incandescent bulbs A and B are identical in all ways except B’s
filament is thicker than A’s. If both are screwed into 110V sockets, which
will be brighter? Why?
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60 W bulb vs. 100 W bulb
If connected in series, why is the 60 W bulb brighter? (Higher power rating = higher or lower
resistance?)
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Paying for electricity
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Kilowatt-hour
Energy used by a device at a rate of 1000 watts in one
hour
SingPower ~ 16 cents per kWh
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Kilowatt-hour
1 kW h= (1000 W) (3600 s)
= 3600000 W s= 3600000 J
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Question:Paying the bills
How much would you have to pay the Public Utilities Board if you used two 40 W lamps and a 120 W television for 5 hours a day for the month of March?
(Assume the cost of 1 kWh of energy to be 16 cents.)
Consider: Series? Parallel? How to add?
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Conclusion
• Heating effect• Power
» Electrical energy transfer» Resistive dissipation» Summing power
• Paying for electricity