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

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3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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 -
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20 -
19 -
18 -
17 -
16 -
15 -
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
𝟒𝟏𝟖𝟎 𝑱
1 kilogram of water

11. ∆𝐸=𝑄−𝑊 𝜌∙𝑉𝑜𝑙∙ 𝐶 𝑝 ∙∆𝑇 = 𝑉∙𝐼∙𝑡
Class Problem: A one gallon ( m3) 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 = gal
𝐶 𝑝 𝑤𝑎𝑡𝑒𝑟 =4180 𝐽 𝑘𝑔∙℃
∆𝐸=𝑄−𝑊
𝜌∙𝑉𝑜𝑙∙ 𝐶 𝑝 ∙∆𝑇
= 𝑉∙𝐼∙𝑡
1000 𝑘𝑔 𝑚 3 ∙ 𝑚 3 ∙4180 𝐽 𝑘𝑔∙℃ ∙ 20℃−15℃ =12𝑉∙1𝐴∙𝑡
𝑡=6597.2𝑠=1.83ℎ𝑟𝑠

12. 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 = gal
𝐶 𝑝 𝑤𝑎𝑡𝑒𝑟 =4180 𝐽 𝑘𝑔∙℃