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? 發問於 科學及數學其他 - 科學 · 1 十年前

resistivity

1.Compute the voltage drop along a 28

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length of household no. 14 copper wire (used in

15-

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circuits). The wire has diameter

1.628

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current.

Express your answer using two significant

figures.

2.The filament of a light-bulb has a resistance of

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when hot.

a.Calculate the temperature of the filament when it is hot, and take into

account the change in length and area of the filament due to thermal

expansion (assume tungsten for which the thermal expansion coefficient

is

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,

assuming an average

temperature coefficient of resistivity

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.

b.In this temperature range, what is the percentage change in resistance

due to thermal expansion?

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=___%

c.What is the percentage change in resistance due solely to the change in

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?

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=___%

2 個解答

評分
  • 天同
    Lv 7
    1 十年前
    最愛解答

    1. Resistance of copper wire R = (rho).L/A

    where (rho) is the resistivity of copper = 1.68 x 10^-8 ohm-m

    L is the length of wire (= 28 m)

    A is the cross-sectional area of wire (= pi.x (0.001628^2)/4 m^2 ), where pi = 3.14159....

    Thus, R = [1.68 x 10^-8 ] x 28/[pi.x (0.001628^2)/4 ] ohms

    Hence, voltage drop, by Ohm's Law = 12 x R

    2. (a) Usin R = (rho).L/A

    hence, 12 = (rho),L/A

    and 140 = (rho)',L'/A'

    i.e. 140/12 = [(rho)'/(rho)].[L'/L].[A/A'] ------------------------ (1)

    Since (rho)'/(rho) = (1+ 0.0045(T-293))

    L'/L = (1+ 5.5x10^-6(T-294))

    A'/A = (1+2x5.5x10^-6(T-293))

    substitute these values into equation (1) and solve for T

    (b) assume (rho) remains constant

    Ro = (rho).L/A

    R(expansion) = (rho).L'/A'

    hence, R(expansion)/Ro = [L'/L].[A/A']

    using the ratio of L'/L and A'/A is (a), calculate R(expansion)/Ro

    and then find the percentage change in resistance

    (c) R(change in rho) = (rho)',L/A

    hence, R(change in rho)/Ro = [(rho)'/(rho)] = (1+ 0.0045(T-293))

    claculate first the ratio of R(change in rho)/Ro then calculate the percentage change.

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  • 1 十年前

    i have no idea what you wrote on the 2nd question part b

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