Questions & explanations
1. Compare a superheterodyne receiver with a direct conversion receiver. What are the trade-offs?
A superheterodyne receiver uses an intermediate frequency (IF) for filtering and amplification, giving high selectivity and sensitivity. A direct conversion receiver (also called homodyne) mixes the incoming signal directly to baseband (zero IF) without a separate IF stage. Direct conversion is simpler and cheaper, with no image frequency problem. However, it suffers from DC offset, flicker noise, and I/Q imbalance, which can degrade performance. Superheterodyne receivers have better overall performance but are more complex and require image rejection. For modern digital communications, direct conversion is common due to integrated circuits, but superheterodyne is still used where high dynamic range is needed.
2. What is the purpose of vaccine safety monitoring systems like VAERS?
Vaccine safety monitoring systems, such as VAERS (Vaccine Adverse Event Reporting System) in the US, collect reports of health problems that happen after vaccination. Their purpose is to detect rare or unexpected side effects that may not have been seen in clinical trials. This helps regulators quickly identify potential safety issues and take action if needed. For example, if many people report the same problem after a vaccine, it can be investigated further. These systems rely on voluntary reports from doctors and the public, so they are not perfect but are useful for early warnings. Overall, they help ensure vaccines remain safe by continuously monitoring their use in millions of people.
3. What is cyclic voltammetry and what does the Randles-Sevcik equation describe?
Cyclic voltammetry is an electrochemical technique where the voltage of a working electrode is swept linearly with time, then reversed. The current is measured, producing a voltammogram. The Randles-Sevcik equation relates the peak current (ip) to the scan rate (v), diffusion coefficient (D), concentration (C), and electrode area (A): ip = 0.4463 nFAC(nFvD/RT)^(1/2). For a reversible system, the peak current is proportional to the square root of scan rate. For irreversible systems, the peak current is lower and shifts with scan rate. You can tell apart reversible and irreversible systems by checking if the peak separation is about 59 mV per electron (reversible) or larger (irreversible).
4. Compare chronoamperometry with cyclic voltammetry for determining diffusion coefficients.
Both techniques can determine diffusion coefficients (D). In cyclic voltammetry, D is obtained from the slope of peak current vs. square root of scan rate using the Randles-Sevcik equation. In chronoamperometry, D is obtained from the slope of current vs. inverse square root of time using the Cottrell equation. Chronoamperometry is simpler because it requires only a potential step and a single transient. However, it is sensitive to capacitive current and convection. Cyclic voltammetry provides additional information about reversibility and kinetics. Both require knowledge of concentration and electrode area. Chronoamperometry is often more accurate for D if the system is well-behaved.
5. Compare monocrystalline, polycrystalline, and thin-film solar cells in terms of efficiency and cost.
Monocrystalline solar cells are made from a single crystal of silicon, giving them the highest efficiency (around 18–22%) and a uniform black appearance. They are more expensive to produce. Polycrystalline cells are made from multiple silicon crystals, with efficiency around 15–18% and a blue, speckled look; they cost less. Thin-film solar cells use layers of materials like cadmium telluride or amorphous silicon, with lower efficiency (10–12%) but are cheaper to manufacture and flexible. Thin-film works better in low light and high temperatures. For space-constrained installations, monocrystalline is preferred; for large farms, polycrystalline or thin-film may be more cost-effective.
6. What is the main difference between storing hydrogen as a compressed gas and as a liquid?
Compressed gas stores hydrogen under high pressure (like 350-700 bar) in strong tanks, so the tank is heavy. Liquid hydrogen is cooled to very low temperature (-253°C) to become a liquid, which takes less space but needs special insulation to keep it cold. Compressed gas is simpler but has lower density by volume, meaning you need a bigger tank for the same amount of hydrogen. Liquid hydrogen has higher density by volume, so it fits in a smaller tank, but it loses some hydrogen over time as it boils off. Metal hydrides are a third method where hydrogen bonds with a metal powder, storing it safely at lower pressure, but the material is heavy, so the weight per stored hydrogen is high.
7. How did Ehrlich's search for a magic bullet lead to a treatment for syphilis?
Ehrlich tested hundreds of chemicals on rabbits infected with syphilis to find one that killed the bacteria without harming the rabbit. In 1909, his assistant Sahachiro Hata found that compound 606, called arsphenamine, worked. It killed the syphilis bacteria, Treponema pallidum, and cured the rabbits. Ehrlich named it Salvarsan, meaning 'saving arsenic'. It became the first effective drug for syphilis in humans. However, it had side effects because it contained arsenic. Later, better drugs like penicillin replaced it. Ehrlich's method of screening many chemicals was new and influenced how we find drugs today. His magic bullet idea was a big step toward modern chemotherapy.
8. Explain why solar cell efficiency is less than 100% and name one major loss mechanism.
Solar cell efficiency is limited because not all light energy is converted into electricity. One major loss is the bandgap loss: photons with energy less than the bandgap of the semiconductor pass through without being absorbed, while photons with energy greater than the bandgap produce heat as the excess energy is lost. Another loss is recombination, where electron-hole pairs recombine before being collected. Reflection of light from the cell surface also reduces absorption. Additionally, electrical resistance in the cell and contacts causes power loss. The theoretical maximum efficiency for a single-junction silicon cell is about 33% (Shockley-Queisser limit).
9. How does structure from motion differ from traditional photogrammetry?
Structure from motion (SfM) is a type of photogrammetry that automatically finds the camera positions and 3D structure from a set of unordered photos, without needing to know the camera locations beforehand. Traditional photogrammetry often requires known camera positions or control points with known coordinates. SfM uses computer vision algorithms to detect matching features across images, then simultaneously solves for camera motion (where each photo was taken) and the 3D points. This makes it easier to use with casual photos, like those taken with a smartphone. However, it may be less accurate than traditional methods if the photos are not well-conditioned.
10. Before Pasteur, many people thought living things could appear from non-living matter, like maggots from meat. What is this wrong idea called?
This wrong idea is called spontaneous generation. It means that life can arise suddenly from non-living things, like mice from dirty hay or maggots from rotting meat. Pasteur did experiments with swan-neck flasks to show that this does not happen. He boiled broth in a flask with a long curved neck that let air in but kept out dust and microbes. The broth stayed clear, showing that no life appeared. Only when he broke the neck and dust entered did the broth go cloudy. This proved that life comes only from other life, not from non-living matter. His work helped establish the germ theory of disease, which says tiny living things called germs cause many diseases.
11. Explain how a fuel cell produces electricity and give an example of a common type.
A fuel cell is an electrochemical device that converts the chemical energy of a fuel (like hydrogen) and an oxidant (like oxygen) directly into electricity, with water and heat as byproducts. Unlike a battery, it does not store energy; it generates electricity as long as fuel is supplied. A common type is the proton exchange membrane (PEM) fuel cell, which uses a solid polymer membrane as the electrolyte. Hydrogen gas flows to the anode, where it splits into protons and electrons. Protons pass through the membrane, while electrons travel through an external circuit, creating current. At the cathode, oxygen combines with protons and electrons to form water.
12. How is Ehrlich's magic bullet concept different from traditional medicines that treat symptoms?
Traditional medicines often treat symptoms like pain or fever without killing the germ. For example, aspirin lowers fever but does not kill the bacteria causing the infection. Ehrlich's magic bullet aims to kill the specific germ causing the disease, not just relieve symptoms. This is a different approach because it gets to the root cause. For instance, Salvarsan killed the syphilis bacteria, curing the disease. Today, antibiotics like penicillin kill bacteria directly. This targeted approach is more effective because it stops the disease from progressing. Ehrlich's idea shifted medicine from treating symptoms to curing diseases by destroying their cause.