2009 Autumn Mid-term Exam Answers


(1) What is the rationale behind the cross-bar structural reinforcement used in the bookcases?

Metal bar are easier to be bend than to be pulled longer or compressed shorter. If a bookcase/bookshelf has only vertical bar to support it, it is less resistant to the force applied to it from sides, which may cause the bar to bend. With additional cross-bar structure, those bar divert the horizontal forces into forces making the bar to lengthen or to shorten, which is not easy. Thus the cross-bars make the bookshelf more rigid.


(2) What are the superiority using concrete/stone to build arch bridges and archway?

Those structure has everywhere compression deformation for the whole structure. Stone and cement are materials good in taking pressure (it not not change shape or brake under high pressure). Using these types of construction materials is a good choice for building arch bridges and archways.


(3) Why is steel tougher than iron?

Steel is consist of mostly iron and with a a small fraction of carbon. The pure iron is is plastic (easy to change shape) under pressure, this is because the gaps between metallic atoms allow atoms to flow/move from their original location. When a small fraction of carbon is mixed into iron, the gaps between iron atoms are locked and the metallic atom is less easy to flow around, therefore resulting a more rigid body.


(4) Why must alum (Aluminium Sulfate) be added in the purification processes? What is the purpose of adding chlorine to tap water?

When mixed into water, Aluminium Sulfate floc like tiny object that will stick impurities in water as well as sticking together. This form a bigger particle and will lead to sedimentation of those flocs. This is an efficient way to capture and remove the small particles and microorganism in the untreated water.

Adding chlorine into treated wither is kill bacteria.


(5) Why is it not a good idea to discharge water into rivers without removing nutrients via treatment?

Home sewer has nutrition in it, that can be used by (aerobic) bacteria, which will use up the oxygen in reason and kill all living things in the river.


(6) Why do we need to convert the voltage output of generators into very high-voltage electricity for distribution?

Larger the current, greater the loss of electrical energy during the transmission. (This loss of energy can be regarded as the friction force occurred when electrons moving inside a conductor. To deliver same amount of energy, it is therefore a better idea to use very high voltage, so that only small current is needed to send over the same amount of electrical energy.


(7) What cause the temperature increase in the microcosmic world?

Atoms (molecules) move faster. (or has larger transnational kinetic energy.)


(8) Try to explain what Resonance is. Write down some examples.

When external driving force has the same frequency as the natural oscillating frequency of a system, the oscillation (vibration) is multiplied/accumulated to a great extend, this is call resonance. Pushing swing for little children and avoid marching in the same phase when crossing a suspension bridge are two examples.


(9) Explain how dehumidifiers work.

A dehumidifier, like a refrigerator, has refrigerant, pipes, expansion valve, and compressor, so with electric power input, it can create a very hot tube region and very cold tube region. In addition, it based on the fact that warm air is more capable to carry water than cold air. A dehumidifier suck in air and let the air to flow through the cold part of tube, then water precipitated (separated) from air; before the dryer is release back to room, it is warmed up by forcing the air passing through the hot part of the tube. This cycle therefore keep removing water from air.


(10) How can an amplifier magnify the amplitude of signals (of electrical current or water flow)?

The general principle of an amplifier is the following : Some huge power output is very sensitively controlled by a low-power input. This mean small force/energy can guide the release of much larger force/energy. In this way the signal is magnified but in fact the energy is conserved as required by Physical law.