Q139 / 160 · TS EAMCET 2021 (04 Aug Shift 1) · Physics

Two rods whose lengths are l1 and l2 with heat conductivity co-efficients k1 and k2 are placed end to end. The heat conductivity coefficient of a uniform rod of length l1+l2 whose conductivity is same as that of the system of these two rods is

  • Correct answer
    A
    l1+l2k1k2k2l1+k1l2
  • B
    l1+l2k1k2k1l1+k2l2
  • C
    k1l1+k2l2l1+l2k1k2
  • D
    k1l2+k2l1l1+l2k1k2

Solution

When two rods are connected in series then the heat flow per unit time is the same across each rod, it means, H=H1=H2,

⇒kAT1-T2l1+l2=k1AT1-TJl1=k2ATJ-T2l2, first, calculate the junction temperature by the idea,  H1=H2=k1AT1-TJl1=k2ATJ-T2l2⇒TJ=k1T1l1+k2T2l2k1l1+k2l2

it means,
 kT1-T2l1+l2=k1T1-k1T1l1+k2T2l2k1l1+k2l2l1⇒k=l1+l2k1k2k2l1+k1l2

Hence, the correct answer is option: 'A'.

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