Answers to last issue's Do You Know

1. How are buildings designed to withstand earthquakes?

Ans: Earthquake or seismic performance defines a structure's ability to sustain its due functions, such as its safety and serviceability, at and after a particular earthquake exposure. A structure is normally considered safe if it does not endanger the lives and well-being of those in or around it by partially or completely collapsing. A structure may be considered serviceable if it is able to fulfill its operational functions for which it was designed.

Basic concepts of earthquake engineering, implemented in major building codes, assume that a building should survive a rare, very severe earthquake by sustaining significant damage but without globally collapsing. A scaled model of the structure is placed on a shake-table that simulates the earth shaking and its behavior is observed.

2. Can a blood test reveal if one has cancer ?

Ans: Early Detection of cancers is a very active area of research, since it is our best chance for a cure. If we can detect tumours early, it's possible to surgically remove them and treat the patient with localized radiation therapy.

The biggest challenge is to identify and validate new blood-based diagnostic tests. Ideally, we need methods to allow us to measure all human proteins in many clinical samples, rather than trying to pick a couple of proteins that might work.

The Fred Hutchinson Cancer Research Center in the USA has been developing some blood based tests that seem to be quite successful, from early indications. They have come up with specific blood-screening tests but as of now they are very expensive to perform. As of now, they are not definitive, but there is some hope that soon we will have a combination of blood tests and imaging technologies that together detect cancer.

3. If your broken arm or leg has been in a plaster cast for a long time, the limb feels weak when the plaster is removed. Why?

Ans: The arm has a complex system of muscles that work together as a group to flex the arm, elbow and wrist, enabling you to perform precise movements such as picking up an object or writing. If the arm has been in a cast intended to protect broken bones for a significant amount of time, it can affect the muscles in a variety of ways.

A cast is used to immobilize the arm when one or more bones have been broken, as well as holding the bone in position. The cast also reduces muscle contractions to allow the arm to heal, particularly after surgery. Casts may be made of plaster or fibreglass. There are three types of cast: A short-arm cast covers the forearm form the elbow to the hand. A long-arm cast covers the upper arm to the hand. A cylinder cast covers the upper arm to the wrist.

The length of time your cast is left on depends on the age of the person, the type of injury and the type of surgery. It can be anywhere from six weeks to several months. The longer the cast is in place, the longer the arm muscles go without exercise. Gradually, the muscles begin to atrophy. Actually, there is a decrease in the mass of the muscle, with a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass.

Muscle atrophy that is caused by lack of use, rather than disease or neural injury, can be reversed.

4. I understand that water boils at 100 degrees C and becomes vapour.  But why does water, in the form of falling rain, turn into vapour at a much lower temperature after it strikes a hot surface, such as a paved road during hot summer?

Ans: Liquid water can evaporate to form gaseous water (steam) at any temperature, not just at its boiling temperature of 100 C.  The difference between normal evaporation and boiling is that below water's boiling temperature, evaporation occurs primarily at the surface of the liquid water whereas at or above water's boiling temperature, bubbles of pure steam become stable within the liquid and water can evaporate especially rapidly into those bubbles. So boiling is a just a rapid form of evaporation.

What you are actually seeing when raindrops land on warm surfaces is tiny water droplets in the air, a mist of condensation. Those droplets happen in a couple of steps.  First, the surface warms a raindrop and speeds up its evaporation. Second, a small portion of warm, especially moist air rises upward from the evaporating raindrop.  Third, that portion of warm moist air cools as it encounters air well above the warmed surface. The sudden drop in temperature causes the moist air to become supersaturated with moisture--it now contains more water vapour than it can retain at equilibrium. The excess moisture condenses to form tiny water droplets that you see as a mist.

This effect is particularly noticeable when it's raining because the humidity in the air is already very near 100%.  The extra humidity added when the warmed raindrops evaporate is able to remain gaseous only in warmed air. Once that air cools back to the ambient temperature, the moisture must condense back out of it, producing the mist.

5. When electricity comes out of the wall and through a lamp, where does the circuit loop complete? Does the circuit go all the way back to the power plant?

Ans: The electric circuit that powers our lamp extends only as far as a nearby transformer. That transformer is located somewhere near our house, probably as a cylindrical object on a telephone pole down the street or as a box a few houses away.

A transformer conveys electric power from one electric circuit to another. It performs this feat using several electromagnetic effects associated with changing electric currents  -- changes present in the alternating current of our power grid. In this case, the transformer is moving power from a high-voltage 
neighborhood circuit to a low-voltage household circuit.

For safety, household electric power uses relatively low voltages, typically 220 volts. But to deliver significant amounts of power at such low voltages, you need large currents. It's analogous to delivering hydraulic power at low pressures; low pressures are nice and safe, but you need large amounts of hydraulic fluid to carry much power.  There is a problem, however, with sending low voltage electric power long distances: it's inefficient because wires waste power as heat in proportion to the square of the electric current they carry. Using our analogy again, sending hydraulic power long distances as a large flow of hydraulic fluid at low pressure is wasteful; the fluid will rub against the pipes and waste power as heat.

To send electric power long distances, you do better to use high voltages and small currents (think high pressure and small flows of hydraulic fluid). That requires being careful with the wires because high voltages are dangerous, but it is exactly how electric power travels cross-country in the power grid: very high voltages on transmission lines that are safely out of reach.

Finally, to move power from the long-distance high-voltage transmission wires to the short-distance low-voltage household wires, they use transformers. The long-distance circuit that carries power to our neighbourhood closes on one side of the transformer and the short-distance circuit that carries power to our lamp closes on the other side of the transformer. No electric charges pass between those two circuits; they are electrically insulated from one another inside the transformer. The electric charges that are flowing through our lamp go round and round that little local circuit, shuttling from the transformer to our lamp and back again.