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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 2 living with the lab wind power HOW IT WORKS: • wind causes turbine to turn • turning turbine generates electricity energy conversions: fluid → mechanical → electrical wind turbines in California 3 living with the lab 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 4 living with the lab 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 5 living with the lab 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 morgueFile: http://mrg.bz/0hXnhq 6 living with the lab conservation of energy Energy can change form, but it can’t be created or destroyed. Within an isolated system, energy is constant. - + 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 7 living with the lab 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 8 living with the lab - + 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 9 living with the lab 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 10 living with the lab what is heat capacity??? ⁰C 25 - 𝜌 ∙ 𝑉𝑜𝑙 ∙ 𝐶𝑝 ∙ ∆𝑇 = 𝑉 ∙ 𝐼 ∙ 𝑡 𝐶𝑝 = 4180 𝐽 𝑘𝑔 ∙ ℃ 24 23 22 21 - 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 20 19 - 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 18 17 16 15 - 1 kilogram of water 11 living with the lab 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. morgueFile: http://mrg.bz/kp7EGh 12 living with the lab 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℃? + 13
