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State PCS PYQPrelims 2019

UPPCS · Question 50

A liquid remains hot or cold for a long time in a thermos flask because there is no loss or gain of heat by

  1. Conduction
  2. Convection and radiation
  3. Both (a) and (b)
  4. None of the above
Show answer & explanation

Answer: (C) Both (a) and (b)

Explanation

A vacuum flask's double wall with a vacuum stops conduction and convection, and the silvered surfaces cut radiation. So heat is not gained or lost by either (a) or (b).