Two straight metallic strips each of thickness t and length L are riveted together. Their coefficients of linear expansion are α1 and α2. If they are heated through temperature Δθ, the bimetallic strip will bend to form an arc of radius
A
t(α1+α2)Δθ
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B
t(α2−α1)Δθ
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C
t2(α1+α2)Δθ
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D
t2(α2−α1)Δθ
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Solution
The correct option is Bt(α2−α1)Δθ Given: Thickness of metallic strips =t Length of metallic strips =L Coefficient of linear expansion of strip 1=α1 Coefficient of linear expansion of strip 2=α2 Temperature change =Δθ To find: Radius of arc of bimetallic strip =r
Before heating -
After heating -
Let the angle subtended by the arc formed at the centre be θ. We know, θ=lR, where l is the length of arc and R is radius of arc. For strip 1, θ=L(1+α1Δθ)r.......(1) [ from formula of linear expansion ] Similarly, for strip 2, θ=L(1+α2Δθ)r+t.......(2) Angle subtended by both the strips at the centre will be equal, so from (1) and (2), L(1+α1Δθ)r=L(1+α2Δθ)r+t ⇒r=t(1+α1Δθ)(α2−α1)Δθ As 1>>α1Δθ [because α<<1] ⇒r=t(α2−α1)Δθ