Answers to last issue's Do You Know?

1. Why can you hear the ocean when holding a seashell to your ear?

Every child must have held a seashell to his/her ear. No matter how far away from the ocean you are, you can still hold a seashell up to your ear and hear the roar of the waves rolling onto the shore. The best shells for producing this sound are the large, spiral conch shells.
Some people have suggested that the sound you hear from the seashell is the echoing of your blood rushing through the blood vessels of your ear. That is not the case. If that were true, then the sound would intensify after exercising, since your blood races faster after exercising. However, the sound is the same even after exercising.
Others say that the whooshing sound inside the shell is generated by air flowing through the shell - air flowing through the shell and out creates a noise. You'll notice that the sound is louder when you lift the shell slightly away from your ear than it is when the shell is right against your head. However, this theory doesn't hold true in a soundproof room. In a soundproof room, there is still air, but when you hold the seashell to your ear, there's no sound.
The most likely explanation for the wave-like noise is ambient or background noise from around you. The seashell that you are holding just slightly above your ear captures this noise, which resonates inside the shell. The size and shape of the shell therefore has some effect on the sound you hear. Different shells sound different because different shells accentuate different frequencies. You don't even need the seashell to hear the noise. You can produce the same "ocean" sound using an empty cup or even by cupping your hand over your ear. Go ahead and try it and vary the distance at which you place the cup near your ear. The level of the sound will vary depending on the angle and distance the cup is from your ear.
Noise from outside the shell also can change the intensity of the sound you hear inside the shell. You can look at the shell as a resonating chamber. When sound from outside enters the shell, it bounces around, thus creating an audible noise. So, the louder the environment you are in, the louder the ocean-like sound will be.
2. How does DNA profiling help in proving that someone did (or did not)
commit a crime?
Although 99.9% of human DNA sequences are the same in every person, enough of the DNA is different to distinguish one individual from another. DNA profiling uses repetitive ("repeat") sequences that are highly variable, called variable number tandem repeats (VNTR). VNTRs are very similar between closely related humans, but so variable that unrelated individuals are extremely unlikely to have the same VNTRs.
With current technology it is not practical to look at each difference. Currently in the laboratory we look at 10 different areas of DNA, which are known to vary widely between people. These 10 areas contain short repeating sequences known as Short Tandem Repeats (STR).
Each of the chromosomes in your cells contains sections of non-coding DNA -- DNA that does not code for a protein. The short STRs are found in the non-coding part of DNA.
The number of these repeating sequences varies between individuals. An additional area is also provided which indicates whether the person is male or female. The technique of DNA profiling is centred on analysing and measuring these differences in length.
If the DNA of two people was analysed for 10 different STRs on different chromosomes, there is only one chance in a million that they would have the same number of repeats in all of these STRs. Identical twins are the only exception -- they have identical DNA and identical STRs.
Generating a DNA profile usually involves analysing an individual's DNA for ten different single tandem repeats (STRs) on different chromosomes. Statistically, no two people (except identical twins) are likely to have the same numbers of repeats in all of these STRs. Polymerase chain reaction is used to produce many copies of the ten STRs before they are analysed using electrophoresis. The different lengths will show up as bands at different spots on the electrophoresis gel. The banding pattern produced is called a DNA profile or fingerprint, and can be analysed.
Steps are:
* Dissolve the crime stain
* Separate, clean and measure the quantity and quality of DNA
* Target the specific pieces of interest within the DNA molecule
* Produce multiple copies of these (PCR)
* Sort the pieces of DNA according to size
* Measure the size of the pieces.
If a crime suspect's DNA profile for 10 STRs matches the STR profile of a sample found at the crime scene, there is a very high probability that both lots of DNA are from the same person. However, if the profiles differ for even one STR, this cannot be assumed.
Just like our favourite forensic science TV dramas, forensic scientists can help solve crimes by analysing DNA samples from crime scenes and comparing them to DNA samples from victims and suspects.
However, in reality, forensic techniques are a bit more complicated and take much longer than the ten minutes seen on TV -- more like two weeks, in fact. And, in a real forensic investigation, specialists would be performing specific tasks, rather than one person doing everything.
Disaster victim identification
It is often difficult to identify victims after disasters such as bombing or fires. Forensic scientists are called in to identify the DNA obtained from body parts or teeth.
During the aftermath of the 2002 Bali bombing, relatives of victims were asked to arrange collection of DNA samples from personal items such as toothbrushes or combs. So far, of the 221 missing or deceased in Bali, 182 have been identified. DNA profiling identified 115 people, while fingerprints, dental records and medical records were also used to identify victims.
Forensic experts also assisted in Thailand with the identification of bodies following the 2004 tsunami.

3. How does a solar cell work?
A solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.
Assemblies of cells are used to make solar panels, solar modules, or photovoltaic arrays. Photovoltaics is the field of technology and research related to the application of solar cells in producing electricity for practical use. The energy generated this way is an example of solar energy (also called solar power).
Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. Complementary positive charges, called holes, are also created and flow in the opposite direction to the electrons. The electron flow provides the current, and the solar cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.
So, an array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.
4. In summer, how does a fan make you feel cooler?
The cooling effect is from evaporation. The fan moves an increased amount of air and speeds the evaporation from the skin or other moist surface. A fan blowing on a dry surface will have no cooling effect beyond the average temperature of the air. 
The reasons fans are used for cooling are (a) blowing air over the skin increases evaporation which cools the body, and (b) blowing air replaces the stagnant air in a room by forcing turbulence and perhaps bringing in fresh and cooler air if the outside air is cooler.
When air is moving over the body, it "feels" cooler because the rate of heat dispersion of one's body is increased. Cooling by evaporation can be experienced by inserting a finger in water (or better yet an alcohol) and then blowing on it. One feels the finger getting colder. This is called the wind-chill effect.
This is how the wind increases convective heat loss. By blowing air around, the fan makes it easier for the air to evaporate sweat from your skin, which is how you eliminate body heat. The more evaporation, the cooler you feel. Actually, since the fan contains a motor, it should heat up the room as it works! But it cools you down.