2011 GCE O Level Pure Physics P2 Suggested Solutions (Finally Released)

冷风1985

23092009 发表于 4-11-2011 00:31
老师, Q6b) 可以写 because the metal sphere is a conductor, hence it cannot be charged by friction wh ...

Conductors can still be charged. For examples, you should have seen a lot of examples of a metal sphere on an insulator stand being charged.
You see, the key point of this question is that "the metal is hold in hand" which provides a pathway for electrons to be directed to the metal to neutralize the charges.
Hand is is considered as conductor.

冷风1985

宁静、淡然 发表于 4-11-2011 00:16
老师、我Q6(c)(i)答案错了、不过我(ii)&(iii)方法是对的、会不会有ecf呢？谢谢~

By right, you might get 1/2 marks if you get the formulas correct. For example: E=p t.  But if you didn't write any formula, then you will not be given any marks.

❤阿珣

冷风1985 发表于 4-11-2011 08:55
By right, you might get 1/2 marks if you get the formulas correct. For example: E=p t.  But if you ...

老师会出pure biology的答案么？？？

JamesKhoo

Erm for 8c isnt it an AC circuit? So how can we use left hand rule? For question 10bi if we use that formula to count resistance of A, it is different from the one that is given. For question 7 they said that B moves away, so shouldnt B have less magnetic field since it is weaker?

冷风1985

❤阿珣 发表于 4-11-2011 11:08
老师会出pure biology的答案么？？？

I am sorry, I am not really good at biology

JamesKhoo

Sorry teacher but could you clear my doubts as above? ;x

❤阿珣

冷风1985 发表于 4-11-2011 11:23
I am sorry, I am not really good at biology

哦好的没关系谢谢老师:)

宁静、淡然

冷风1985 发表于 4-11-2011 08:55
By right, you might get 1/2 marks if you get the formulas correct. For example: E=p t.  But if you ...

谢谢老师~

冷风1985

JamesKhoo 发表于 4-11-2011 11:22
Erm for 8c isnt it an AC circuit? So how can we use left hand rule? For question 10bi if we use that ...

Left hand rule can be used on any type of current, be it AC or DC.
As long as a current is flowing in the magnetic field, it will experience a force whose direction can be determined by left hand rule.
For this particular case, in a AC generator, although the current is induced by rotating a coil in the magnetic field. Since there is a current, it will still experience a force, opposing the relative motion.
(Many students don't really understand why a current will experience a force when placed in a magnetic field. Here is the explaination: Around the current, there will always be a magnetic field. So, the force is actually between the two magnetic fields - one induced by the current, the other one is the magnetic field that the current is locating in).

For question 10 bi) your answer is also correct. Using different methods will produce different results. It appears to be that there's some mistake in the question. However, actually, it's acceptable. That is because when the electric devices are in actual usage, their resistance value may differ according to its operating temperature. For 10 bi) particularly, the question didn't state very clearly what are the circumstances that has caused the change. Hence, you can use either method. However, the answers will not differ too much.

For 7a) your statement is wrong.
Even if B's magnetic field is weaker than A. According to Newton's third law of motion, action and reaction forces are always of the same magnitude. So the force from B to A is the same as the force from A to B. They just different by direction.
Actually, you have probably misunderstood the question. The emphasize of the question is that B receives both attraction and repulsion force from A, but why it still moves to the right. Rather than your understanding: "B moves to the right, A is stationary."
So the answer should be: because the repulsion force on B is stronger than the attraction force. Because the north pole of A is closer to the north pole of B, further away from the south pole of B.

冷风1985

23092009 发表于 4-11-2011 15:35
老师，还有 Q5c 可以写 Ultrasound is a longitudinal wave whereas electromagnetic waves are transverse ...

Your answer could be wrong. That is not the reason. That is the difference of their properties.
When we want to differentiate two terms, we always need to refer to their definitions.