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Energy Density v. Specific Energy
Energy density (ENERGY / VOLUME) is the amount of energy stored in a given system or region of space per unit volume. - Wikipedia
Specific Energy (ENERGY / MASS) Energy per kilogram. The SI unit for specific energy is the joule per kilogram (J/kg).
General Resources:
General Resources:
Kognity Textbook: Chapter 8 (Use your ACS Login)
IB Physics Site: Topic 8 - Comprehensive notes
IB Physics Site: Topic 8 - More notes
Topic 8 Flashcards - Vocab Devo.
Density of Fuels
Density of Fuels
Charcoal and propane are two common cooking fuels used by campers in the deserts of the UAE. Currently, charcoal sells for 58.00 AED for a 10kg bag and an easily portable 11-kg cylinder of propane sells for 60 AED. The specific energy of charcoal is 30MJ/kg and has an average efficiency of 10% (10% of the heat produced is transferred to the material being cooked). Propane is slightly higher at 50.3 (±0.01) MJ/kg and has an average efficiency of 0.40.
Lary is going camping this weekend and would like to purchase the most cost effective fuel.
Determine the cost per usable megajoule (MJ) of each fuel.
Lary would like to boil 750-ml of water to make a pot of coffee. He collects the water from a local stream with a water temperature of 18˚C. The specific heat of water (C) is 4126 J /kg •˚C. The equation to determine the energy required to heat a substance is Q=mC∆T.
Determine the energy (Q) to bring the 0.750 kg water to a boil (100˚C).
Calculate the cost to boil the water using both fuels.
Gary, Lary's brother, would like to be ecologically friendly and burn the fuel with the lowest carbon footprint. According to the Engineering Toolbox website, for every 1-kg of propane burned 2.99 kg of CO_2 is produce. Where as, 123,300 g CO_2 are produced for every 45-kg of charcoal burned .
Determine the amount of CO_2 produced by each fuel to boil the water.
Despite all of the data, Lary still wants to use charcoal on his camping trip. Construct an argument against using charcoal based on the calculations above.
How to Read a Sankey Diagram
How to Read a Sankey Diagram
The image to the left is a Sankey diagram of a blow dryer. Answer the following questions regarding the energy transfers:
Determine the energy lost due to kinetic energy. 50 J
Calculate the percentage of energy is lost due to sound energy. 20%
The main function of a blow dryer is to create heat. Calculate the efficiency of this blow dryer. 73%
Below is a more complex Sankey diagram involving the heating of a cabin by a wood burning stove.
Determine the amount of wasted heat from the wood. 15
As humans, we are most concerned about the temperature of the air in the cabin. What is the efficiency of the wood burning stove in achieving this? 20.4%
Suppose the Tea Kettle was not placed on the stove (removed from the diagram). How would this affect the remaining values of the diagram, choose the three most effected and explain the impact of removing the kettle from the 'equation'. Waste heat remain constant, Values in cabin would increase accordingly.
Adjust the diagram above by using the sliders.
Sankey Diagram:
Sankey Diagram:
Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.
This Sankey diagram for an electric lamp shows that most of the electrical energy is transferred as heat rather than light.
Sankey Diagram Questions:
Sankey Diagram Questions:
The Sankey diagram represents the energy flow for a coal-fired power station.
What is the overall efficiency of the power station?
A. 0.3 ✔
B. 0.4
C. 0.6
D. 0.7
2. The diagram below show the Sankey diagram for an engine
If 106 J of energy are put into the engine how much energy is converted to
a. Heat due to friction 3/20 = 1.5x106
b. Heat loss 8/20 = 4.0x106
c. Useful work 11/20 = 5.5x106
Make you own Sankey Diagram: Easy to use, just make your changes
Energy Problem Solving
Energy Problem Solving
Two Key Ideas
A kilowatt hour (kWh) is a measurement of energy (1 kilowatt-hour (kWh) = 3.60x106 J). Power companies bill by the kWh (energy), instead of kW (power), they (power companies) only care about the total energy consumed v. the rate at which you consume the energy (power).
Mass - rate or volume - rate, is a way of describing power. As can be seen in the following equations:
Cabin Problem Sankey
Cabin Problem Sankey
(make your own HERE)
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No Variable Problem
No Variable Problem
Upon arriving at a small Alpine cabin and wanting to relax after a long (8hr) hike, you would like to carry the minimum amount of wood into the cabin to warm it to a comfortable temperature and boil a kettle of melted snow to make a pot of tea/coffee. There is a pile of uniform logs outside of the cabin and a wood burning stove in the cabin.
Construct a variable map to illustrate the various variable and their relationship each other.
Identify realistic quantities for the variables you have chosen.
Based on your variable map, solve the problem of the amount of wood you would need to carry into the cabin.
Create a Sankey Diagram showing the energy distribution within your cabin.
Evaluate the reliability of your solution.
The specific energy of Propane (a fuel often used in remote locations) is 50.3 MJ/kg. Determine the ratio of propane to wood you would need to heat your cabin.
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