domestic electricity (urgent)

Zidane

2011-05-05 6:33 am

1. 'All d.c. sources produce constant voltages.' Why is this statement false?

2. 'In case the neutral wire is broken, the earth wire provides a spare wire for the return path to the mains socket.' Why is this false?

3. Why if a fuse is blown, it should not be replaced by a copper wire? Is it becase it has a high melting point and will not be blown easily?

4. The capacity and the voltage of a lithium cell is 800 mA and 3.6V respectively, and it will discharge in 3 days when the mobile phone is in stand-by mode. Estimate the average power of the cell assuming the voltage remains constant during the charge. (Ans: 960mW)

5.
圖片參考：http://imgcld.yimg.com/8/n/HA01097219/o/701105040122213873434430.jpg

ans: A

thanks

更新1:

for 4, why 10368 J is divided by 3 x 3600? There should be 3600 x 24 x 3 seconds in 3 days. thanks

更新2:

ya I know, but why it is divided by 3 x 3600? I suppose '3' refers to 3 days and '3600' refers to the number of seconds in 1 hour, but why '24' (no. of hours in a day) is omitted in the calculation? thanks

回答 (1)

用戶25975

2011-05-05 5:09 pm

✔ 最佳答案

1) D.C. only ensures that current is flowing in ONE direction but NOT with a constant value, hence the power may not be constant.

2) For an appliance in normal status, the earth wire is NOT in contact with any part of the appliance and so when the neutral wire is broken, the circuit is not complete and the appliance is not in operation.

3) Yes, the aim of a fuse is to limit the current flowing into the appliance and since copper can stand a pretty high current, it cannot be used as a fuse.

4) Total charge available = 0.8 x 3600 = 2880 C

Hence total energy stored in the cell = 2880 x 3.6 = 10368 J

So average power = 10368/(3 x 3600) = 0.96 W

5) (1) When X insulation is worn out, the current flows from X to Y directly through the metal case and the current is very high, blowing out the fuse.

(2) When Y insulation is worn out, the current still flows through the heating element and so the fuse won't be blown out.

(3) When the heating element is broken, there's no current flow in the appliance.

2011-05-05 15:11:16 補充：
If divided by 3600 x 24 x 3, then average power = 0.04 W which does not match with the ans you provided.

2011-05-06 17:07:15 補充：
Well, conclusively, I can say that:

If the Q asks "discharge in 3 days", then average power = 10368/(3 x 24 x 3600) = 0.024 W

If the Q asks "discharge in 3 hours", then average power = 10368/(3 x 3600) = 0.96 W

參考: 原創答案

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