Download Handout 5

Science Olympiad
Physics Lab
Week 5
Alternative Energy
Fischer-Tropsch Process: In this process steam is blown over coal. This forms
synthesis gas, a mix of hydrogen (H2) and carbon monoxide (CO). This gas is used to
produce hydrocarbons and carbon monoxide. Physics involved: The process occurs at
high temperature and pressure. It is an endothermic process as higher energy molecules
are being formed. It requires an input of energy to occur.
Concentrating solar power: The sun’s rays are focused using mirrors and/or lenses to
create high temperature. Any difference in temperature can be used to run a Rankine
cycle steam turbine which can spin a generator. Question: What is the theoretical
temperature of the focal point of a mirror or lens that is focusing the rays of the sun?
Physics involved: Optics of mirrors.
Concentrating PV solar power: Mirrors and/or lenses are used to focus the sun’s rays
on to PV arrays. This increases the number of photons reaching the PV array which is
the most expensive component of the system. Physics involved: Optics of mirrors and
the photoelectric effect.
Question: A violet photon with wavelength 430nm strikes a PV cell made of silicon.
What energy can be delivered to the circuit of the band gap in Si is 1.11 eV? Answer:
2.85X10-19J
Helpful info.: 1V = 1J/C
96,485C = 1 mole of electrons
6.02X1023 electrons = 1 mole of electrons so,
1eV = 1.602X10-19J
Also, E = hν, E = hc/λ
Question 2: Diamond has a band gap of 5.5eV. Find the longest wavelength photon that
can work.
OTEC (Ocean Thermal Energy Conversion): Cold deep ocean water is pumped to the
surface and used as a cold sink while hot surface ocean water is used as the hot source to
enable a Rankine cycle turbine. Physics involved: the second law of thermodynamics,
pressure.
Question: What is the theoretical efficiency of an OTEC plant that operates off Hawaii
drawing deep water at 4 Celsius and hot water at 78F?
How much energy could be extracted, in theory, if 1 ton of water is used? The heat
capacity of water is 4.18J/gK. What is the maximum depth from which this water could
be drawn if the pump is 100% efficient?
Geothermal energy: Hot steam from the earth is used as the hot source for a Rankine
cycle turbine. Alternatively, water could be pumped deep into the earth which is hot,
returned to the surface, and used as the hot source. Physics involved: Second Law,
radioactive decay.
Question: A turbine runs at 100C and has a cold sink at 20C. If the system draws 10
tons of steam which all condenses to water to deliver heat with no temperature change,
then what theoretical work can be done by the turbine assuming theoretical efficiency?
The heat of vaporization of water is around 40,000J/mole.
Question 2: Write nuclear equations for the following processes:
A. Uranium-238 undergoes alpha decay
B. Cesium-137 undergoes beta decay
C. Aluminum-26 undergoes electron capture
D. Potassium-40 undergoes positron emission
Question 3: Uranium-238 eventually decays to form lead-206. It does not do this in one
step. The intermediates in the process are: thorium-234, protactinium-234, uranium-234,
thorium-230, radium-226, radon-222, polonium-218, lead-214, bismuth-214, polonium214, lead-210, bismuth-210, and polonium-210. Tell what type of decay occurs in each
step of the conversion of uranium-238 to lead-206.
Nuclear fusion: Light nuclei are fused to produce heavier nuclei. The heavier nuclei are
lighter than the overall than the nuclei they were made from. The mass lost is converted
to energy (E = mc2). Physics involved: energy-mass equivalence, strong nuclear force,
binding energy, mass defect.
If that seems confusing, look at the exact masses for this process:
2
H + 3H = 4He + 1n
2
H: 2.01355321270 amu
3
H: 3.0160492 amu
4
He: 4.002602 amu
1
n: 1.00867 amu
Calculate the mass defect for the process, then calculate the energy released.
If a proton has a mass of 1.00728, calculate the binding energy per nucleon for deuterium
and for tritium. Which is more stable?
The mass lost is converted to energy (E = mc2). Physics involved: energy-mass
equivalence, strong nuclear force, binding energy, mass defect.
Wind energy: Wind turbines spin generators. Physics involved: Aerodynamics.
Thermophotovoltaics: PV cells where the band gap corresponds to the energy of an IR
photon.
What is the band gap associated with a 5 micron IR photon? Give the answer in joules
per mole of photons and eV.
Thermoelectric: A process where heat is directly converted to a voltage difference.