Conservation of Energy windmill pumping water for cows – west Texas

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Fossil Fuels How it works: Burning of natural gas: Energy conversions: 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: 𝐶 𝐻 4 +2 𝑂 2 →2 𝐻 2 𝑂+𝐶 𝑂 2 +ℎ𝑒𝑎𝑡 Energy conversions: chemical → thermal → fluid → mechanical → electrical pumpjacks in West Texas coal fired power plant in Arizona

Wind Power Solar Energy How it Works: Energy Conversions: wind causes turbine to turn turning turbine generates electricity Energy Conversions: fluid → mechanical → electrical wind turbines in California 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

Hydroelectricity How it works: Energy conversions: 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

Nuclear Energy How it works: Energy conversions: splitting atoms creates heat heat creates steam steam turns turbine turning turbine makes electricity Energy conversions: atomic→ thermal → fluid → mechanical → electrical

Conservation of Energy Energy can change form but can not be created or destroyed Within an isolated system, energy is constant First Law of Thermodynamics ∆ 𝐸 𝑠𝑦𝑠𝑡𝑒𝑚 = 𝐸 𝑖𝑛 − E out change in energies: internal energy change (temperature) Example: 𝜌∙𝑉∙ 𝐶 𝑝 ∙Δ𝑇 kinetic energy change Example: 1 2 ∙𝑚∙ 𝑣 2 potential energy change Example: 𝑚∙𝑔∙ℎ energy coming in and going out of system: heat transferred to or from a system Example: 𝑉∙𝐼∙𝑡 Example: 𝜌∙𝑉∙ 𝐶 𝑝 ∙Δ𝑇 work done to or by a system Example: 𝐹∙𝑑 The first law is often written as follows: 𝑄= heat transfer to the system ∆𝐸=𝑄−𝑊 𝑊= work done by the system

Fishtank System Application Run electricity through a resistor to create heat to increase the temperature of water 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 ∆𝐸=𝑄−𝑊 resistor - + ΔE = Internal energy change (temperature) change in energy of the water due to temperature change ∆𝐸=𝜌∙𝑉𝑜𝑙∙ 𝐶 𝑝 ∙∆𝑇 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=𝑉∙𝐼∙𝑡 𝑄 𝑾= zero There is no mechanical work being done 𝑊=0

Fishtank System Application ∆𝐸=𝑄−𝑊 - + Change in energy of water due to temperature change Heat transfer from heater 𝜌∙𝑉𝑜𝑙∙ 𝐶 𝑝 ∙∆𝑇 𝜌 = density 𝑘𝑔 𝑚 3 = 𝑉∙𝐼∙𝑡 𝑉𝑜𝑙 = volume ∙ 𝑚 3 𝐶 𝑝 = heat capacity ∆𝑇 = change in temperature ∙ 𝐽 𝑘𝑔∙℃ = 𝐽 𝐶 ∙ 𝐶 𝑠 ∙℃ ∙𝑠 𝑉 = electric voltage 𝐼 = electric current 𝑡 = time Note: 𝐶 𝑝 =4180 𝐽 𝑘𝑔∙℃ and 𝜌=1000 𝑘𝑔 𝑚 3 for water at room temperature

What is heat capacity, Cp? Number of heat units need to raise the temperature of a substance by one degree. 𝐶 𝑝 𝑤𝑎𝑡𝑒𝑟 =4180 𝐽 𝑘𝑔∙℃ ⁰C 25 - 24 - 23 - 22 - 21 - 20 - 19 - 18 - 17 - 16 - 15 - 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 𝟒𝟏𝟖𝟎 𝑱 1 kilogram of water

∆𝐸=𝑄−𝑊 𝜌∙𝑉𝑜𝑙∙ 𝐶 𝑝 ∙∆𝑇 = 𝑉∙𝐼∙𝑡 Class Problem: A one gallon (0.003788m3) 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. Solution: Things to know: Density of water = 1000kg/m3 0.001 m3 = 0.264 gal 𝐶 𝑝 𝑤𝑎𝑡𝑒𝑟 =4180 𝐽 𝑘𝑔∙℃ ∆𝐸=𝑄−𝑊 𝜌∙𝑉𝑜𝑙∙ 𝐶 𝑝 ∙∆𝑇 = 𝑉∙𝐼∙𝑡 1000 𝑘𝑔 𝑚 3 ∙0.003788 𝑚 3 ∙4180 𝐽 𝑘𝑔∙℃ ∙ 20℃−15℃ =12𝑉∙1𝐴∙𝑡 𝑡=6597.2𝑠=1.83ℎ𝑟𝑠

Class Problem: A fishtank is 1 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℃? Things to know: Density of water = 1000kg/m3 0.001 m3 = 0.264 gal 𝐶 𝑝 𝑤𝑎𝑡𝑒𝑟 =4180 𝐽 𝑘𝑔∙℃ - +