關於物理的簡單問題 - Ans.fyi 參考答案

關於物理的簡單問題

Tieria

2010-04-04 3:14 am

1. A student rubs a plastic ruler so that it becomes negatively charged. When the charged ruler is brought close to some uncharged paper scraps, they are attracted by the ruler. Which of the following about this phenomenon is / are correct?
(1) Positive charges are induced on the paper scraps.
(2) Negative charges are induced on the paper scraps.
(3) If the student rubs a metal ruler rather than a plastic one, the ruler can still attract the paper scraps.
A. (1) only
B. (1) and (2) only
C. (1) and (3) only
D. (2) and (3) only

2. In the circuit shown, bulb X and Y are identical and their corresponding voltages, Vx and Vy, are measured by voltmeters. The e.m.f. of the battery is 4 V. Which of the following statements is / are correct if bulb Y suddenly burns out?
(1) Ammeter reading will be doubled.
(2) Voltmeter reading Vx will change from 2 V to zero.
(3) Voltmeter reading Vy will change from 2 V to zero.
A. (1) only
B. (2) only
C. (3) only
D. (1) and (3) only
http://i617.photobucket.com/albums/tt257/michaelcoco_/3.jpg
3. Two resistors, R1 and R2, are connected in parallel to a battery. The ratio of their resistances is given by R1 : R2 = 1 : 3. Suppose that an amount of energy E is released to R1 whenever an electron passes through R1, then how much energy will be released to R3 whenever an electron passes through R3?
A. (1/3)E
B. (3/4)E
C. E
D. 3E

※試列明計算步驟及對與錯之原因※

回答 (1)

天同

2010-04-04 3:58 am

✔ 最佳答案

1. Statement (3) is wrong because metal is a conductor. All static charges produced on it will be escaped away through the body of the student.

2. When bulb Y has burnt out, there occurs a point of "open circuit" at bulb Y. No current is passed through the circuit, hence the ammeter reads no current. Hence, statement (1) is wrong.

Statement (2) is right. It is because no current passes through the circuit, there is no potential difference across bulb X (Ohm's Law), the voltmeter reading now becomes zero.
Statement (3) is wrong. Since there is an "open circuit" point at bulb Y, the resistance of bulb Y becomes infinite, the voltage of the battery is now all across bulb Y. The voltmeter reading now becomes 4 v.

3. The fall is potential across R1 and R2 are the same because the two resistors are connected in parallel. But the energy released by an electron is only proportional to the potential difference, hence energy released by an electron is the same in R1 and R2.

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