conversion & conservation of energy

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© David Hall 2013 windmill pumping water for cows – west Texas

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fossil fuels

coal fired power plant in Arizona

pumpjacks in West Texas

HOW IT WORKS:• chemical reaction (combustion) creates heat• make steam and/or hot exhaust gases• steam or exhaust gas turns turbine• turning turbine makes electricity

𝐶𝐻4+2𝑂2→2𝐻 2𝑂+𝐶𝑂2+h𝑒𝑎𝑡burning of natural gas:

energy conversions: chemical thermal fluid mechanical electrical

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wind powerHOW IT WORKS: • wind causes turbine to turn• turning turbine generates electricity

wind turbines in California

energy conversions: fluid mechanical electrical

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solar energy

HOW PHOTOVOLTAIC CELLS WORK: • sun strikes a semiconductor material• electrons gain energy resulting in a buildup of voltage

between electrodes• this voltage is harnessed to produce electric power

solar farm in Arizona

energy conversions: radiant electrical

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hydroelectricity

Hoover Dam – Colorado River – Lake Mead

HOW IT WORKS: • water behind dam creates a large

pressure differential across turbine• moving water contacts turbine blades,

forcing them to turn• turning turbine generates electricity

energy conversions: fluid mechanical electrical

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nuclear energy

morgueFile: http://mrg.bz/0hXnhq

HOW IT WORKS:• splitting atoms creates heat• heat creates steam• steam turns turbine• turning turbine makes electricity

energy conversions: atomic thermal fluid mechanical electrical

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conservation of energy

Energy can change form, but it can’t be created or destroyed.

For our fishtank system, we will run electricity through a resistor to create heat to increase the temperature of water . . .

Within an isolated system, energy is constant.

. . . converting electrical energy into thermal energy

resistor

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first law of thermodynamics

∆𝐸=𝑄−𝑊

∆𝐸𝑠𝑦𝑠𝑡𝑒𝑚=𝐸𝑖𝑛− Eoutchange in energies:• internal energy change (temperature)• kinetic energy change• potential energy change

energy coming in and going out of system:• heat transferred to or from a system• work done to or by a system

The first law is often written as follows:

heat transfer to the system

work done by the system

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heat transfer from heater to water

∆𝐸=𝑄−𝑊(we assume no heat is lost by conduction through the wall of the pipe or at the surface of the water)

zero (we assume that nothing is moving)

𝑄

• the “system” here is defined by the boundary of the water • the heater, PVC and air above water are NOT part of our system• we apply the first law only to our system, carefully accounting for all energy

crossing the system boundary

our application of the first law- +

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our application of the first law

∆𝐸=𝑄−𝑊zero since our system doesn’t make anything move

heat transfer from heaterchange in energy of waterdue to temperature change

𝜌 ∙𝑉𝑜𝑙 ∙𝐶𝑝 ∙ ∆𝑇¿𝑉 ∙ 𝐼 ∙ 𝑡 = heat capacity = change in temperature

= electric voltage = electric current = time

= density

𝑘𝑔𝑚3

= volume

∙𝑚3∙𝐽

𝑘𝑔 ∙℃∙℃¿𝐽𝐶∙𝐶𝑠∙𝑠

and for water at room temperature

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what is heat capacity???

𝜌 ∙𝑉𝑜𝑙 ∙𝐶𝑝 ∙ ∆𝑇¿𝑉 ∙ 𝐼 ∙ 𝑡 𝐶𝑝=4180𝐽

𝑘𝑔∙℃

25 -

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⁰C

1 kilogram of water

𝟒𝟏𝟖𝟎 𝑱𝟒𝟏𝟖𝟎 𝑱

𝟒𝟏𝟖𝟎 𝑱𝟒𝟏𝟖𝟎 𝑱

𝟒𝟏𝟖𝟎 𝑱

𝟒𝟏𝟖𝟎 𝑱

𝟒𝟏𝟖𝟎 𝑱

Example A one gallon fish bowl contains water at . If you insert a fishtank heater that draws 1A of electric current at 12V, then how long will it take the heater to increase the water temperature to 20? Assume no heat loss or gain through the wall of the bowl or at the surface of the water.

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Class Problem A fishtank is 1.6 inches in diameter and contains water 2 inches deep. If you heat the water using an 24Ω resistor and a 12V power supply, then how long will it take to heat the water up by 1?

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