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Question

A clock with an iron pendulum keeps correct time at 15C. If the room temperature rises to 20C, the error in seconds per day will be (coefficient of linear expansion for iron is 0.000012/C):

A
2.5sec
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B
2.6sec
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C
2.4sec
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D
2.2sec
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Solution

The correct option is B 2.6sec
The initial time period of a pendulum is Ti=2πlig and final time period is Tf=2πlfg
If α be the coefficient of linear expansion, then lf=li(1+αΔθ) where Δθ= change in temperature.
So, Tf=2πli(1+αΔθ)g=Ti(1+αΔθ)1/2=Ti(1+αΔθ2) [for small value of αΔ, we can neglect the higher order term]
Or, TfTiTi=12αΔθ
Thus, the error in seconds per day =TfTiTi×86400=12αΔθ×86400=0.5(0.000012)(2015)(86400)=2.6sec

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