PracticalElectricity

Unit 21

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Outline

• Power» Electrical energy transfer» Resistive dissipation» Summing power

• Heating effect• Paying for electricity

POWER!!!!

Work done

Potential difference = …?

Work done

Potential difference = Work done per unit charge

W = V Q

Power

Power = Work done / unit time

= V Q / t

= V I

Power: Electrical Energy Transfer

P = I V

Watt is the unit?

Power: Resistive Dissipation

P = I V

Use Ohm’s Law:

P = I2 R

P = V2 / R

Power: Electrical Energy Transfer vs. Resistive

DissipationTitle… too… long

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

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

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?

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)

Heating Effect of Electricity

Heating effect video

High / low resistance?

Why doesn’t it melt / explode?

Characteristics of heating element

• Nichrome• Coiled around some kind of insulating, fire-proof material» Silica, mica, RI boys

• High melting point• High resistance

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)

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

Incandescent light bulb vs. fluorescent lamp

Both are rated at 40 W. Which puts out the most

light?

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?

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?)

Paying for electricity

Kilowatt-hour

Energy 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 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?

Conclusion

• Heating effect• Power

» Electrical energy transfer» Resistive dissipation» Summing power

• Paying for electricity