Mr. Rogers' IB Physics Topics

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Topic 8

IB Physics Standards: Items directly related to the standards are shown in blue

Topic 8: Energy, power and climate change

Objectives

Essential Question: Assuming no friction, is it possible to convert heat into mechanical energy with 100% efficiency?

Energy degradation and power generation

  1. State that thermal energy may be completely converted to work in a single process, but that continuous conversion of this energy into work requires a cyclical process and the transfer of some energy from the system. Place a leak-proof friction free piston at the center of a perfectly insulated infinitely long cylinder, so that there is gas trapped behind the piston. Continuously heat the trapped gas so that it maintains a constantly elevated pressure that pushes the piston forward; 100% of the heat energy could be converted into work. However, in the real world, at some point the piston reaches the end of the cylinder. The high pressure gas has to be released from the cylinder--essentially dumping the energy contained in it--and work then has to be done to return the piston to its original position so that the cycle can be repeated. With a continuous cycle, 100% of the heat energy can NEVER be converted to work.

  2. Explain what is meant by degraded energy. When thermal energy is converted to mechanical or electrical energy, part of the thermal energy has to be expelled into the environment. This energy is considered degraded. Degraded energy still exists but essentially can no longer be converted into mechanical or electrical energy. In other words, degraded energy can no longer do work.

Of course, the degraded energy could, in theory, be sent through another heat engine and then its degraded energy sent through yet another an infinitum, but each time the degraded energy is expelled it has to be expelled to a lower temperature. To convert 100% of the thermal energy into mechanical energy would require that the last heat engine in the series would have to expel its energy at a temperature of absolute zero--an impossibility.

Heat engine: Heat engines are devices used to continuously convert thermal energy into mechanical energy. Steam engines, gas turbines, internal combustion engines in cars, and diesel engines are all heat engines. If thermal energy could be continuously transferred into a gas making it expand and  push a piston in an infinitely-long friction-free cylinder, 100% of the thermal energy could theoretically be converted into mechanical energy. In the real world however all heat engines must go through a thermodynamic cycle that results in part of the input heat being expelled to the environment.

Carnot efficient: is the maximum theoretical efficiency a friction-free heat engine could have. It's always less than 100% and for real heat engines may be less than 80%. Add friction and other losses and the actual efficiency is typically less than 50%.

 

Essential Question: How does the world power itself?
  1. Construct and analyze energy flow diagrams (Sankey diagrams) and identify where the energy is degraded. The above Sankey diagram above shows the energy flows through a steam engine. Ein represents the heat put into the heat engine and Eout the work output. The energy flows identified as lost energy are no longer available for doing work and are hence degraded energy. As can be seen in the illustration, most of the energy input into a heat engine becomes degraded energy.

  2. Outline the principal mechanisms involved in the production of electrical power.

 

Essential Question: How does the world power itself?

World energy sources

  1. Identify different world energy sources and the relative proportions of different energy sources that are available.

 

Essential Question: Why is electrical energy so useful?
  1. Explain why, for humanity, electrical energy is the most useful form, followed by mechanical. Explain why electrical energy is also the most expensive in terms of the total energy and equipment required to produce it.

The ability to do useful work and or carry information increases as a quantity of energy moves higher on the energy pyramid. For thermal energy, a higher level on the pyramid represents a higher temperature and, hence, a higher possible Carnot efficiency for conversion to the mechanical energy.

When attempting to convert a quantity of energy to a form that's higher on the energy usefulness pyramid, invariably a large amount will be degraded. This degraded energy still exists but is in thermal form at such a low level on the pyramid that it essentially can never move to a higher level. Going down the pyramid is another matter: 90% or more of electrical energy can be converted directly to mechanical form and 100% to thermal, 100% of mechanical energy can be converted to thermal form, and 100% of high level (thermal energy at a high temperature) can be converted to low level thermal energy (thermal energy at a low temperature).

 

 

  1. Outline and distinguish between renewable and non-renewable energy sources.

Source Original Source Available Supply Type Available  
Renewable        
wind turbine present-day Sun lifetime of Sun mechanical Conceivably, the entire Earth's surface can act as a solar collector with energy harvested by wind turbines.

photovoltaic cell

present-day Sun lifetime of Sun electrical Available power limited by the area of the collector system that's ultimately limited by available land area.

solar heating panel

present-day Sun lifetime of Sun thermal Available power limited by the area of the collector system that's ultimately limited by available land area.
hydro present-day Sun lifetime of Sun mechanical Energy is stored in the water as gravitational potential energy, hence the height of the dam is a major factor and helps limit the number of potential hydro sites
geothermal (volcanic activity) volcanic activity Thousands of years + thermal Few usable sites are available
tidal orbital motions of Earth and Moon millions of years mechanical In most areas the change in height of the water level with changing tide is so small that it would require a huge area to gain any useful energy
Non-renewable        
fossil fuels ancient Sun hundreds of years thermal  
fission (uranium) formation of Solar Syst. hundreds of years thermal  
fusion (hydrogen) formation of  Universe millions of years thermal  

 

 

Essential Question: Why is energy density such an important issue?
  1. Define the energy density of a fuel.

Forms of Stored Energy

Type of energy storage Energy density by mass (Mj/kg) Energy density by volume (Mj/l) Renewable Comments
Fuels        
  hydrogen gas (burned in air) 143 0.01079 storage* There's no source of free hydrogen. More energy is needed to produce the hydrogen than can be recovered from it. Hydrogen must be highly compressed or liquefied to qualify as a transportation fuel.

When hydrogen is burned in a heat engine, the usable energy is limited by Carnot efficiency. When hydrogen is consumed in a fuel cell, the efficiency can be higher.

  hydrogen gas compressed at 700 bar = 10,200 psi (burned in air) 143 5.6 storage* Compressing hydrogen to 10,200 psi involves a significant amount of energy. At these pressures, catastrophic failure of the storage tank or even a leak is extremely dangerous.
           
  coal anthracite 32.5 72.4 no Its high energy density reduces the cost of shipping from the mine to the power plant
  gasoline 46.4 34.2 no Excellent as a transportation fuel because of its high energy density and the speed of refilling its storage tank. However, usable energy is typically limited by Carnot efficiency since gasoline is usually used in heat engines.
  diesel fuel 46.2 37.3 no Excellent as a transportation fuel because of its high energy density and the speed of refilling its storage tank.. However, usable energy is limited by Carnot efficiency.
  natural gas (compressed) 53.6 10 no Lowest CO2 emissions of all fossil fuels. The primary component is methane
  methane (1.013bar, 15°C, from organic source) 55.6 0.0378 yes Methane is produced by the anaerobic digestion (by bacteria) of organic waste such as manure. (Methane is also the key component of natural gas, described above.)
  ethanol 30 24 yes Typically produced from corn or sugarcane.
  methanol 19.7 15.6 storage* The energy stored in methanol is not renewable when made from fossil fuels like natural gas but is renewable when made from biomass.
 

biodiesel (vegetable oil)

42.20 33 yes Biodiesel is very inexpensive when made from waste cooking oil but the supply is then highly limited.
           
Miscellaneous        
  body fat 38 35 yes Similar in energy storage to gasoline.
  TNT 4.610 6.92 storage* Surprisingly, has only about 1/10 the energy per kilogram as gasoline.
  Water @ 100 m dam height 0.001 0.001 storage* Rain water collecting behind a dam is renewable. In pumped storage schemes, water is pumped from a lower reservoir to a higher one at night using excess electric generating capacity. The water is then released, as needed, the next day to generate electricity for spikes in demand. Pumped storage is a very efficient form of energy storage since it is not limited by Carnot efficiency.
  Clock Spring 0.0003 0.0006 storage*  
           
Electrical        
  lead acid batteries 0.14 0.36 storage*  
  lithium ion battery 0.46-0.54 0.83-0.9 storage* Gasoline has an energy density about 10 times higher and a storage tank for it can be refilled about 100 times faster than a battery can be recharged. However, energy removal from a battery is not limited by Carnot efficiency.
  ultracapacitor 0.0206 0.050 storage* Can be charged and discharged almost instantaneously.  A great choice of energy storage for regenerative braking and quick acceleration. Ultra capacitors will likely be used in combination with batteries.

* These are forms of energy storage. Whether they are renewable or not depends on whether the energy stored in them came from a renewable source.

 

  1. Discuss how choice of fuel is influenced by its energy density.

For transportation applications energy density is a major consideration. It determines the following for a vehicle:

  • size  - energy density by volume is key here.

  • mass - the higher the energy density the lower the mass when the fuel supply is fully loaded.

  • acceleration - While energy density is not the only consideration, anything that increases mass tends to decrease acceleration.

  • range - the higher the energy density, the greater the range or number of miles driven for a given fuel capacity.

  • fuel efficiency - While energy density is not the only consideration or even the main consideration, anything that increases mass will have less fuel efficiency (miles traveled per unit of fuel) because it will take more energy to accelerate the vehicle

  1. Discuss the relative advantages and disadvantages of various energy sources.

 

 

Fossil fuel power production

  1. Outline the historical and geographical reasons for the widespread use of fossil fuels.

  2. Discuss the energy density of fossil fuels with respect to the demands of power stations.

  3. Discuss the relative advantages and disadvantages associated with the transportation and storage of fossil fuels.

Fossil Fuel Storage Distribution  to customer Transport from well/mine to distribution network
natural gas pressurized tanks: These have thick walls and are relatively expensive to build. To get a higher density (thus reducing tank size) natural gas is often liquefied. This requires cryogenic temperatures and specialized materials.
  • pipeline these operate with relatively high pressures and although generally safe, have caused major disasters when they've ruptured. all pipelines suffer from right-of-way and land use issues.
  • Across oceans: Due to low energy density cryogenic storage on tanker type ships is required.
  • Across land: pipeline
propane pressurized tanks: These have thick walls and are relatively expensive to build.
  • truck
 
butane pressurized tanks: These have thick walls and are relatively expensive to build.
  • truck
 
gasoline atmospheric pressure tanks: These have relatively thin walls and are relatively inexpensive to build.
  • pipeline
  • truck
 
diesel / fuel oil atmospheric pressure tanks: These have relatively thin walls and are relatively inexpensive to build.
  • pipeline
  • truck
 
coal piles: requires no tank, just some land area essentially no expense to build
  • Railroad
  • slurry pipeline--consumes large quantities of water
  • truck
 
       
       
       

 

  1. State the overall efficiency of power stations fuelled by different fossil fuels.

  2. Describe the environmental problems associated with the recovery of fossil fuels and use such as electricity generation, transportation, and heating.

 

Type Definition / Significance Origen

Air Pollution

 
CO2 A major exhaust component from burning fossil fuels is also a greenhouse gas that contributes significantly to global warming. use
Ground level ozone Ozone in the upper atmosphere helps reduce the amount of harmful UV reaching Earth from the sun. However, at ground level ozone is a major health hazard. use
NOx (pronounced knocks) When burning a fossil fuel using air, a small amount nitrogen will combine with oxygen forming various nitrogen oxides. Low level ozone is formed when NOx and  VOCs react in the presents of ultraviolet light (from sunlight). Low level ozone is a major lung irritant.

Note that NOx is also naturally formed by lightning. Here, however, the NOx generally dissolves in rain water and acts as a type of nitrogen fertilizer.

use
Particulates Dust or smoke particles in the air. Particulates reduce visibility and are a lung irritant. use
SOx (pronounced socks) Various sulfur oxides are formed when sulfur containing fossil fuels are burned. SOx comes primarily from high sulfur coal burning and is a major cause of acid rain. use
VOC VOCs  or volatile organic compounds refer to fossil fuel components that evaporate or escape into the air. VOCs react with NOx in the presents of sun light producing smog and ground level ozone a major lung irritant. VOCs themselves can create significant health problems. For example, benzene is a major carcinogen (cancer causing compound), is volatile, and present at about 9% in gasoline. Some types of VOCs such as methane can act as powerful greenhouse gasses. recovery and use
Smog

A fog-like combination of all forms of visible air pollution.

use

Ground and Surface Water / Soil Contamination

 
Heavy Metals

mercury emissions

  recovery and use
Organic compounds   recovery and use
     
Destruction of Habitat and Land Use Issues
Solid waste Ash from coal burning can be radioactive (low level)  and contain heavy metals. It often ends up in land fills. recovery and use
Strip Mining   recovery
Land Subsidence   recovery
     

 

 

 

Non-fossil fuel power production

  1. Describe how neutrons produced in a fission reaction may be used to initiate further fission reactions (chain reaction).

  • low-energy neutrons (≈ 1 eV) favor nuclear fission.

  • critical mass.

  1. Describe what is meant by fuel enrichment.

  2. Describe the main energy transformations that take place in a nuclear power station.

  3. Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fission reactor.

  4. Discuss the role of the heat exchanger in a fission reactor.

  5. Describe how neutron capture by a nucleus of uranium 238 (238U) results in the production of a nucleus of plutonium 239 (239Pu).

  6. Describe the importance of plutonium 239 (239Pu) as a nuclear fuel.

  7. Discuss safety issues and risks associated with the production of nuclear power.

  • the possibility of thermal meltdown and how it might arise

  • problems associated with nuclear waste

  • problems associated with the mining of uranium

  • the possibility that a nuclear power program may be used as a means to produce nuclear weapons.

  1. Outline the problems associated with producing nuclear power using nuclear fusion.

 

Solar power

  1. Distinguish between a photovoltaic cell and a solar heating panel.

  2. Outline reasons for seasonal and regional variations in the solar power incident per unit area of the Earth’s surface.

  3. Solve problems involving specific applications of photovoltaic cells and solar heating panels.

Hydroelectric power

  1. Distinguish between different hydroelectric schemes.

• water storage in lakes

• tidal water storage

• pump storage.

  1. Describe the main energy transformations that take place in hydroelectric schemes.

  2. Solve problems involving hydroelectric schemes.

 

 

Wind power

  1. Outline the basic features of a wind generator.

  2. Determine the power that may be delivered by a wind generator, assuming that the wind kinetic energy is completely converted into mechanical kinetic energy, and explain why this is impossible.

  3. Solve problems involving wind power.

 

Wave power

  1. Describe the principle of operation of an oscillating water column (OWC) ocean-wave energy converter.

  2. Determine the power per unit length of a wavefront, assuming a rectangular profile for the wave.

  3. Solve problems involving wave power.

 

 

Essential Question: Is global warming/climate disruption avoidable?

Greenhouse effect

Solar radiation

  1. Calculate the intensity of the Sun’s radiation incident on a planet.

  2. Define albedo
  3. State factors that determine a planet’s albedo.
  • Students should know that the Earth’s albedo

  • season

  • cloud formation

  • sand

  • latitude.

  • Oceans have a low value

  • snow a high value.

  • global annual mean albedo is 0.3 (30%) on Earth.

 

The greenhouse effect

  1. Describe the greenhouse effect.
  2. Identify the main greenhouse gases and their sources.

  • The gases to be considered are CH4, H2O, CO2 and

  • N2O. It is sufficient for students to know that each

  • has natural and man-made origins.
  1. Explain the molecular mechanisms by which greenhouse gases absorb infrared radiation.

Students should be aware of the role played by

resonance. The natural frequency of oscillation

of the molecules of greenhouse gases is in the infrared region

  1. Analyse absorption graphs to compare the relative effects of different greenhouse gases.

  2. Outline the nature of black-body radiation.

  3. Draw and annotate a graph of the emission spectra of black bodies at different temperatures.

  4. State the Stefan–Boltzmann law and apply it to compare emission rates from different surfaces.

  5. Apply the concept of emissivity to compare the emission rates from the different surfaces.

  6. Define surface heat capacity Cs.
  • Surface heat capacity is the energy required to raise

  • the temperature of unit area of a planet’s surface by

  • one degree, and is measured in J m–2 K–1.
  1. Solve problems on the greenhouse effect and the heating of planets using a simple energy balance climate model.

A planet’s temperature over a period of time is given

by:

(incoming radiation intensity –outgoing radiation

intensity) × time / surface heat capacity.

Students should be aware of limitations of the

model and suggest how it may be improved.

Aim 7: A spreadsheet should be used to show a

simple climate model. Computer simulations could

be used to show more complex models (see OCC

for details).

 

Global warming

  1. Describe some possible models of global warming.

Students must be aware that a range of models

has been suggested to explain global warming,

including changes in the composition of

greenhouse gases in the atmosphere, increased

solar flare activity, cyclical changes in the Earth’s

orbit and volcanic activity.

  1. State what is meant by the enhanced greenhouse effect. -- caused by human activity

  2. Identify the increased combustion of fossil fuels as the likely major cause of the enhanced greenhouse effect.

  3. Describe the evidence that links global warming to increased levels of greenhouse gases.

For example, international ice core research

produces evidence of atmospheric composition

and mean global temperatures over thousands

of years (ice cores up to 420,000 years have been

drilled in the Russian Antarctic base, Vostok).

  1. Define coefficient of volume expansion.
  2. State that one possible effect of the enhanced greenhouse effect is a rise in mean sea-level.

  3. Outline possible reasons for a predicted rise in mean sea-level.

Students should be aware that precise predictions are difficult to make due to factors such as:

• anomalous expansion of water

• different effects of ice melting on sea wate compared to ice melting on land.

  1. Identify climate change as an outcome of the enhanced greenhouse effect.

  2. Solve problems related to the enhanced greenhouse effect.

  • Problems could involve volume expansion, specific

  • heat capacity and latent heat.
  1. Identify some possible solutions to reduce the enhanced greenhouse effect.

Students should be aware of the following:

• greater efficiency of power production

• replacing the use of coal and oil with natural gas

• use of combined heating and power systems

(CHP)

• increased use of renewable energy sources and

nuclear power

• carbon dioxide capture and storage

• use of hybrid vehicles.

  1. Discuss international efforts to reduce the enhanced greenhouse effect.

These should include, for example:

• Intergovernmental Panel on Climate Change (IPCC)

• Kyoto Protocol

• Asia-Pacific Partnership on Clean Development and Climate (APPCDC).

 

 

 

 

 

 

 

 

 


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