Transcript living with the lab

living with the lab
conversion & conservation of energy
© David Hall 2013
windmill pumping water for cows – west Texas
living with the lab
fossil fuels
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
burning of natural gas:
𝐶𝐻4 + 2𝑂2 → 2𝐻2 𝑂 + 𝐶𝑂2 + ℎ𝑒𝑎𝑡
pumpjacks in West Texas
energy conversions: chemical → thermal → fluid → mechanical → electrical
coal fired power plant in Arizona
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wind power
HOW IT WORKS:
• wind causes turbine to turn
• turning turbine generates electricity
energy conversions: fluid → mechanical → electrical
wind turbines in California
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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
energy conversions: radiant → electrical
solar farm in Arizona
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hydroelectricity
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
Hoover Dam – Colorado River – Lake Mead
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nuclear energy
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.
Within an isolated system, energy is constant.
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For our fishtank system, we will run electricity through a resistor
to create heat to increase the temperature of water . . .
. . . converting electrical energy into thermal energy
resistor
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first law of thermodynamics
∆𝐸𝑠𝑦𝑠𝑡𝑒𝑚 = 𝐸𝑖𝑛 − Eout
change 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:
∆𝐸 = 𝑄 − 𝑊
𝑄 = heat transfer to the system
𝑊 = work done by the system
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our application of the first law
∆𝐸 = 𝑄 − 𝑊
𝑸 = heat transfer from heater to water
(we assume no heat is lost by
conduction through the wall of the
pipe or at the surface of the water)
𝑄
𝑾 = zero
(we assume that nothing is moving)
• 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
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our application of the first law
∆𝐸 = 𝑄 − 𝑊
heat transfer from heater
change in energy of water
due to temperature change
𝜌 ∙ 𝑉𝑜𝑙 ∙ 𝐶𝑝 ∙ ∆𝑇 = 𝑉 ∙ 𝐼 ∙ 𝑡
𝐽
𝑘𝑔
3∙
∙
𝑚
∙℃
𝑘𝑔 ∙ ℃
𝑚3
𝐶𝑝 = 4180
𝐽
𝑘𝑔∙℃
zero since our system doesn’t
make anything move
=
𝐽 𝐶
∙ ∙𝑠
𝐶 𝑠
𝜌 = density
𝑉𝑜𝑙 = volume
𝐶𝑝 = heat capacity
∆𝑇 = change in temperature
𝑉 = electric voltage
𝐼 = electric current
𝑡 = time
𝑘𝑔
and 𝜌 = 1000 𝑚3 for water at room temperature
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what is heat capacity???
⁰C
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𝜌 ∙ 𝑉𝑜𝑙 ∙ 𝐶𝑝 ∙ ∆𝑇 = 𝑉 ∙ 𝐼 ∙ 𝑡
𝐶𝑝 = 4180
𝐽
𝑘𝑔 ∙ ℃
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𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
20 19 -
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
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1 kilogram of water
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Example
A one gallon fish bowl contains water at 15℃. 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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