Practical Electricity

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PracticalElectricity

• Unit 21

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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 Ohms Law
• P I2 R
• P V2 / R

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Power Electrical Energy Transfer vs. Resistive
Dissipation

• Title 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
• V doubled, R unchanged
• I doubled, R unchanged
• R doubled, V unchanged
• 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 doesnt 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 (isnt 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 Bs filament is thicker than As.
  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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QuestionPaying 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