“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.”
— Richard Feynman
Archive for September 2010
Girls have always gone through puberty at varying ages. When I was 11 years old, I looked like a flat-chested scrawny little girl. Meanwhile, my best friend Judy at that age looked like a grown woman, basically the same as when she graduated from high school. This was a real problem for large-chested Judy because older men frequently hit on her, probably having no idea that she was only 11 years old and unprepared to cope with their advances.
Early maturation in girls is associated with lower self-esteem, less favorable body image, and greater rates of eating problems, depression, suicide attempts and risky behavior. Beyond the emotional issues, girls that go through puberty early are also at higher risk for some medical problems such as breast cancer, endometrial cancer, pre-diabetes and elevated blood pressure. These emotional and health concerns appear to worsen as the age of puberty onset lowers.
Although the timing of puberty always varies between different girls, the average age when girls enter puberty has fallen in the past two decades. A lot of reports and controversy have surrounded this finding, starting with a study published in 1997 in Pediatrics. Why this is happening is not fully understood. Ongoing studies are trying to determine whether this trend is continuing or whether the age of puberty onset for girls has stabilized.
The results of a new study on the timing of breast development in girls were just reported in Pediatrics by a research team led by Dr. Frank Biro, director of adolescent medicine at Cincinnati Children’s Hospital Medical Center. Dr. Biro and his colleagues studied 1239 girls ages 6 to 8 who were recruited from 3 diverse sites: East Harlem in New York, Cincinnati metropolitan area, and San Francisco Bay Area. The recruited group was 34% white, 31% black, 30% Hispanic, and 5% Asian. The data came from interviews with caregivers and physical examinations of the girls. Great care was taken to ensure that the examinations were performed by only well-trained certified staff, using identical well-established guidelines for determining the onset of puberty.
The researchers found that more girls are starting puberty at the age of 7 or 8 than previously reported 10 to 30 years earlier. At 7 years, 10.4% of white, 23.4% of black, and 14.9% of Hispanic girls had enough breast development to indicate the beginning of puberty. At 8 years, 18.3% of white, 42.9% of black and 30.9% of Hispanic girls had sufficient breast development. In comparison, the 1997 study found only 5% of white girls and 15.4% of black girls to have entered puberty at the age of 7.
So the new study shows that the age of entering puberty is continuing to fall for young girls, especially white girls. However, black and Hispanic girls still mature at younger ages than white girls. The cause of this concerning trend is not fully understood. Increased rates of obesity are thought to play a significant role, because body fat can produce sex hormones. Environmental chemicals are also suspected, since they might mimic effects of estrogen and speed up puberty, but this is still under study. Genetics may also play a role.
Breast Cancer and the Environmental Research Centers (BCERC) were established in 2003 as a consortium to study some of these issues, in partnership with the National Institute of Environmental Health Science (NIEHS) and National Cancer Institute (NCI). As Dr. Biro summarizes, “I think we need to think about the stuff we’re exposing our bodies to and the bodies of our kids. This is a wake-up call, and I think we need to pay attention to it.”
“I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.”
—Thomas Edison, in conversation with Henry Ford (1931)
Everyone loves the idea of solar power — heating and cooling your home using the sun as a clean, free source of power. It sounds like the ultimate way to lower your carbon foot print! However, solar cells are expensive and typically only about 15% efficient, as I discussed in an earlier blog.
In order to make solar power more practical on a wide scale, a lot of research is underway to increase solar power efficiency. Stanford researchers have just reported a significant breakthrough in such solar power research, as described in their new paper in Nature Materials. They have developed a novel solar technology that uses both the light and heat of the sun to generate electricity. This new technology could double solar power efficiency and make it more affordable.
When most people think of solar power, they think of rooftop solar panels. These sort of solar panels (or arrays of photovoltaic solar cells) use expensive semiconductor materials to convert photons of light into electricity. The photons from sunlight are absorbed by the semiconductor material, so the energy from the photons is given to the electrons in the semiconductor. The energy given to an electron can “excite” it from the valence band to the conduction band, where it is free to move around within the semiconductor to produce electricity. Solar panels basically convert solar energy into direct current electricity. However, these types of solar panels aren’t very efficient. If an excited photon doesn’t absorb enough energy, then it can’t make it to the conduction band to produce electricity. On the other hand, if an excited photon absorbs more energy than needed (to make it to the conduction band) then the excess energy is lost as heat. In silicon solar panels, half of the solar energy that hits the solar panel is lost due to these two processes. Ideally you would like to somehow harvest the energy that is lost as heat, in order to make solar cells more efficient.
Solar power can also be generated by a thermionic energy convertor, which directly converts heat into electricity. A thermionic converter produces electricity by causing a heat-induced flow of electrons from a hot cathode across a vacuum gap to a cooler anode. However, only a small fraction of the electrons gain sufficient thermal energy to generate this kind of electricity, and very high temperatures are needed for efficient thermionic conversion.
The Stanford researchers have recently developed a new process that exploits the benefits of both solar and thermal cell conversion. The research was led by Nicholas Melosh, as a joint venture of Stanford and SLAC National Accelerator Laboratory. Melosh’s group coated a piece of semiconducting material with a thin layer of metal cesium, demonstrating that this allowed the material to use both light and heat to generate electricity. This new PETE (photon-enhanced thermionic emission) device used the same basic architecture as a thermionic converter except with this special semiconductor as the cathode.
Although the physical process of this PETE device is different than the standard solar cell mechanisms, the new device gives a similar response at very high temperatures. In fact, the PETE device is most efficient at over 200 C. This means that PETE devices won’t replace rooftop solar panels, since they require higher temperatures to be efficient. Instead, they could be used in combination with solar concentrators as part of a large scale solar power plant, for instance in the Mojave Desert.
Melosh’s initial “proof of concept” research was performed with the semiconductor galium nitride to demonstrate that the new energy conversion process works, but galium nitride isn’t suitable for solar applications. They plan to extend their research to other semiconductors, such as gallium arsenide which is commonly used in household electronics. Based on theoretical calculations, they expect to develop PETE devices that operate with a 50 percent efficiency at temperatures exceeding 200 C. They hope to design the new PETE devices so they can be easily incorporated into existing solar power plants, significantly increasing the efficiency of solar power to make it competitive with oil.