PracticalElectricityUnit 21x x

OutlinePowerElectrical energy transferResistive dissipationSumming powerHeating effectPaying for electricity

POWER!!!!

Work donePotential difference = ?

Work donePotential difference = Work done per unit chargeW = V Q

PowerPower = Work done / unit time= V Q / t= V I

Power: Electrical Energy TransferP = I V

Watt is the unit?

Power: Resistive DissipationP = I VUse Ohms Law:P = I2 RP = V2 / R

Power: Electrical Energy Transfer vs. Resistive DissipationTitle too long

A questionA 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 unchangedI doubled, R unchangedR doubled, V unchangedR doubled, I unchanged ab/d/c

Another questionYou 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

Another questionYou 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?

Yet another questionA 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)

Heating Effect of Electricity

Heating effect videoHigh / low resistance?Why doesnt it melt / explode?

Characteristics of heating elementNichromeCoiled around some kind of insulating, fire-proof materialSilica, mica, RI boysHigh melting pointHigh resistance

Applications of heating effectElectric kettle: heating element enclosed in a metal tube, water gets heated by conduction and convectionElectric iron: heat generated by the heating element is spread evenly over a metal base (chromium plated)

Applications of heating effectFilament/incandescent lamp: Tungsten (why?) coiled up (why?) in glass filled with argon/nitrogen (WHY?!), casts sharp shadowsFluorescent lamp: No filament (WHYY?!?!1one), vapour emits UV light (isnt that invisible?), casts soft shadows

Incandescent light bulb vs. fluorescent lampBoth are rated at 40 W. Which puts out the most light?

Thick and ThinTwo 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?

60 W bulb vs. 100 W bulbIf connected in series, why is the 60 W bulb brighter? (Higher power rating = higher or lower resistance?)

Paying for electricity

Kilowatt-hourEnergy used by a device at a rate of 1000 watts in one hour

SingPower ~ 16 cents per kWh

Kilowatt-hour 1 kW h= (1000 W) (3600 s)= 3600000 W s= 3600000 J

Question:Paying the billsHow 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?

ConclusionHeating effectPowerElectrical energy transferResistive dissipationSumming powerPaying for electricity