Conservation of Energy

1. Conservation of Energy windmill pumping water for cows – west Texas

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3. Fossil Fuels Burning of natural gas: Energy conversions: chemical thermal fluid mechanical electrical pumpjacks in West Texas coal fired power plant in Arizona 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

4. Wind Power 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 How it Works: wind causes turbine to turn turning turbine generates electricity

5. Hydroelectricity Energy conversions: fluid mechanical electrical 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 Hoover Dam – Colorado River – Lake Mead

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

7. Conservation of Energy • change in energies: • internal energy change (temperature) • Example: • kinetic energy change • Example: • potential energy change • Example: • energy coming in and going out of system: • heat transferred to or from a system • Example: • Example: • work done to or by a system • Example: The first law is often written as follows: heat transfer to the system work done by the system Energy can change form but can not be created or destroyed Within an isolated system, energy is constant First Law of Thermodynamics

8. Fishtank System Application - + • ΔEInternal energy change (temperature) • change in energy of the water due to temperature change • 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 • zero • There is no mechanical work being done resistor • 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

9. Fishtank System Application 0 Heat transfer from heater Change in energy of water due to temperature change - + = heat capacity = density = volume = change in temperature = electric voltage = electric current = time Note: and for water at room temperature

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

11. Solution: • Things to know: • Density of water = 1000kg/m3 • 0.001 m3 = 0.264 gal 0 Class Problem: A one gallon (0.003788m3) 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.

12. Things to know: • Density of water = 1000kg/m3 • 0.001 m3 = 0.264 gal - + 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?