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发表于 16-11-2012 13:32:25|来自：新加坡
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本帖最后由 冷风1985 于 16-11-2012 14:37 编辑
Maryguttak 发表于 16-11-2012 12:16 
Hi guys, need some help. Very confused.
Can Q18 be A? or it is B?
Why is Q23 B? I really have no ide ...
Q18.
B
Different temperature is needed on the two ends.
When one end of the wire is at higher temperature than the other end, the electrons have more kinetic energy than the electrons at the cold end. It will produce a net movement of electrons from the hot end to the lower end. Hence, potential difference is produced.
Different metal wires are needed.
If both wires are made of the same material, the potential at both ends of the voltmeter will be equal (zero potential difference across the voltmeter). Hence, voltmeter shows no reading.
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Q23.
B
For total internal reflection to occur, two conditions need to be met: (a) Light ray incidents from optically denser medium to less denser medium; (b) angle of incidence greater than the critical angle.
Reflection always occurs at the same time with refraction. Which means, when refraction occurs, its always accompanied by reflection. x% reflected, (100-x)% will be refracted. Generally, x increases as incident angle increases. Even if incident angle is 0 degree, the will be still a small portion of light ray reflected at the interface.
Q24
C
This question is a bit tricky.
PQ is part of a circle, light source comes from he center of the circle. So incident angle at the interface PQ is 0 degree for all the light rays. Hence, light will not change direction when entering the lens. Answer A and D are wrong.
Answer B is also wrong because at the other interface, the angle of incidence is also 0 degree, the light ray should still go in the same direction. Generally, for either convergent or divergent lens, light passes through the center will go straight, and not be bent.
Q34
A
Property of semiconductor: the effective resistance decreases exponentially as temperature/voltage/light luminance increases. That is due to the special structure of semiconductor: when the semiconductor gains energy, the amount of charge carriers will increase greatly, which effectively reduces its resistance. Typical example: at 20 degree, resistance of a thermistor is 20M ohm, at 80 degree, resistance drops to 2 ohm. (Different design many produce different result.)
In option A, at high temperature, the current increases to very large value. That is correct, because at high temperature, the resistance of the thermistor is almost zero, acting like a wire. It may cause damage to the battery due to large current drawn. In order to prevent that, usually another resistor is connected in series to the thermistor.
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