physic 電幾題

不想單身

2013-02-18 6:43 am

1. P = VI, P = V^2 / R, 咁P係正比 V, 定正比 V^2 ?? +解釋

2. 30W 既車頭燈有 12V 既 p.d.
what is the work done (作的功) by each coulomb of charge (電賀) flowing through the lamp??
( take e=1.6 x 10 ^ -19 C ) +步驟+解釋 ^_^

3. 兩條圓柱金屬棒, X同Y, 同物料製造, 同質量, X長Y 1倍, 流過X既電流係 1A,
流過Y既電流係 3A, 咁 power dissipation (功率消耗) 的比是? (X比Y)

回答 (1)

天同

2013-02-18 3:51 pm

✔ 最佳答案

1. Both equations are right.
P = V.I when R is kept constant
P = V^2/R when I is kept constant
They apply in different situations.

2. The "volt" is defined as the work done in Joule by a coulomb of charge through a potnetial difference of 1 volt. Hence, 1 volt = 1 J/C

Hence, work done by 1 coulomb of charge through a 12 v lamp
= 12 J

3. Since the masses of the two conductors are the same, then
L.A = L'A'
where L and L' are the lengths of X and Y respectively
and A and A' are the cross-sectional areas of X and Y respectively
Hence, L.A = (L/2)A'
A'/A = 2

Let resistance of conductor X = R
R = pL/A where p is the resistivity of the material
Power dissipation on X, P = 1^2.R = R = pL/A

Resistance of conductor Y, R' = pL'/A'
Power dissipation on Y, P' = (3^2).R' = 9R' = 9pL'/A'
Hence P/P' = (pL/A).(A'/9pL') = (1/9).(L/L').(A'/A) = (1/9).(2).(2) = 4/9
i.e. P:P' = 4:9

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physic 電 幾題

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physic 電幾題