Electromagnetic Radiation in Modern Technology

One modern technology that beautifully illustrates the dual nature of electromagnetic radiation is the solar panel (photovoltaic cell).

Explanation using the Particle Model (Photons):

Solar panels operate based on the photovoltaic effect, which is best explained using the particle model of light. According to this model, light consists of discrete packets of energy called photons.

• When photons of sunlight strike the semiconductor material (typically silicon) in a solar cell, they can transfer their energy to electrons within the material.
• If a photon has sufficient energy (greater than the band gap of the semiconductor), it can knock an electron loose from its atom, creating a free electron and a positively charged “hole.”
• The solar cell is designed with an internal electric field (created by doping different layers of the semiconductor) that forces these free electrons to move in one direction and the holes in the opposite direction.
• This directed flow of electrons constitutes an electric current, which can then be used to power devices or charge batteries.

In this process, the energy transfer from light to electrons occurs in discrete amounts corresponding to the energy of individual photons. The wave-like properties of light (like its frequency and wavelength, which are related to the photon’s energy by E=hf, where E is energy, h is Planck’s constant, and f is frequency) determine whether a photon has enough energy to liberate an electron. Thus, the energy of individual photons is key to understanding how solar panels generate electricity, aligning with the particle model.

Links to Good Resources on Solar Panels:

• U.S. Department of Energy – How Solar Panels Work: https://www.energy.gov/energysaver/solar-panels
• National Renewable Energy Laboratory (NREL) – Photovoltaic Research: https://www.nrel.gov/pv/

Research Fusion and Fission Reactions

Part A

The deuterium-tritium (D-T) reaction is considered the most promising nuclear fusion reaction for future energy production for several key reasons:

• High Energy Release: The D-T reaction releases a significant amount of energy (17.6 MeV) per fusion event, which is a relatively high yield compared to other potential fusion reactions.
• Lower Ignition Temperature: Compared to other fusion reactions like deuterium-deuterium (D-D) or proton-boron (p-B), the D-T reaction requires a relatively lower plasma temperature (around 100 million degrees Celsius) to achieve sustained fusion. While still extremely hot, this temperature is more readily achievable with current technology.
• Large Reaction Cross-Section: The D-T reaction has a large cross-section (a measure of the probability of the reaction occurring) at these lower temperatures, making it more likely for fusion to happen within the plasma.
• Manageable Products: The primary products of the D-T reaction are a helium-4 nucleus (an alpha particle) and a high-energy neutron. While the neutron can activate the reactor materials, the helium-4 is an inert and non-radioactive gas.

Part B

Uranium-235 is the most common isotope for nuclear fission in current use in nuclear power generation due to the following factors:

• Fissile with Thermal Neutrons: Uranium-235 is fissile, meaning it can undergo nuclear fission when it absorbs a slow-moving (thermal) neutron. This is crucial for sustaining a controlled chain reaction in a nuclear reactor, as the neutrons produced by fission are initially fast and need to be slowed down by a moderator (like water or graphite) to efficiently cause further fissions of U-235.
• Relatively Abundant (in enriched form): While natural uranium is mostly composed of the isotope uranium-238 (which is not easily fissile with thermal neutrons), uranium-235 makes up about 0.72% of natural uranium. Through the process of uranium enrichment, the concentration of U-235 can be increased to the levels needed for nuclear reactor fuel (typically 3-5%), making it a viable fuel source.
• High Fission Cross-Section: Uranium-235 has a relatively high fission cross-section for thermal neutrons, meaning there is a high probability that a thermal neutron will cause a U-235 nucleus to fission, releasing energy and more neutrons to continue the chain reaction.

Part C

It is advantageous to produce plutonium-239 for several reasons related to nuclear technology:

• Fissile Material: Plutonium-239 is a fissile isotope, similar to uranium-235. It can sustain a nuclear chain reaction and release a significant amount of energy through fission when it absorbs a neutron.
• Produced from Abundant Uranium-238: Plutonium-239 is not found naturally in significant quantities. However, it can be produced in nuclear reactors through a process involving the abundant and non-fissile uranium-238. When a U-238 nucleus captures a neutron, it undergoes a series of beta decays, eventually transforming into plutonium-239. This “breeding” of fissile material from a more abundant isotope extends the lifespan of nuclear fuel resources.
• Fuel for Breeder Reactors: Plutonium-239 can be used as fuel in “breeder reactors.” These specialized reactors are designed to produce more fissile material (like Pu-239 from U-238) than they consume, effectively “breeding” their own fuel and further enhancing the efficiency of nuclear fuel utilization.
• Component of Nuclear Weapons: Historically and controversially, plutonium-239 has also been used as a primary fissile material in nuclear weapons due to its suitable nuclear properties and the relative ease of its production compared to highly enriched uranium-235.