# GATE MT 2020 Q23. Two solid spheres X and Y of identical diameter are made of different materials having thermal diffusivities 100 *10-6 m2s-1 and 125 *10 m2s-1 respectively. Both spheres are heated in a furnace maintained at 1000 K. If the centre of the sphere X reaches 800 K in 1 hour. time required for the centre of sphere Y to reach 800) K is

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4 hours

4 hours

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See lessThis answer was edited.Correct answer is option C. @Digbijaya igit described it best. Thank you. Variation of Temperature is dependent on time and distance. 𝑥^2 = 𝛼𝑡, where, α is thermal diffusivity, t is time and x is distance at which temperature has been changed. In both the cases x is center of the sphere. Thus x isRead more

Correct answer is option C. digbijaya igit igit described it best. Thank you.

When the two materials have different diffusivity then how both the spheres have 800K temperature at the centre of the sphere?

When the two materials have different diffusivity then how both the spheres have 800K temperature at the centre of the sphere?

See lessDt= conctant d1t1 = d2t2 100*10^-6 * 1 = 25*10^-6 *t2 t2= 100/25=4 so answer is C

Dt= conctant

See lessd1t1 = d2t2

100*10^-6 * 1 = 25*10^-6 *t2

t2= 100/25=4

so

answer is C