Problem 2

In an insulated vessel,

m_h=0.20\,\mathrm{kg}

of water at

80^\circ\mathrm C

is mixed with

m_c=0.30\,\mathrm{kg}

of water at

20^\circ\mathrm C

. Water has specific heat

c=4200\,\mathrm{J\,kg^{-1}K^{-1}}

; neglect the vessel heat capacity and heat loss. After mixing, the water reaches one uniform final temperature, with no evaporation or phase change. Heat is exchanged only between the two portions of water; the insulated vessel takes up no energy. Define heat as positive when the portion considered gains energy and negative when it loses energy. (a) Find the equilibrium temperature

T_f

. (b) Calculate the signed heat transferred by the hot water. (c) Find the heat received by the cold water and check energy conservation. (d) State the ordering of

T_c,T_f,T_h

problems.proof.stepOf

problems.proof.prevStep problems.proof.nextStep

Q_c=m_cc(T_f-T_c)=30240\,\mathrm J

problems.proof.analysis

The cold water warms, so its signed heat is positive. Equal masses are not assumed; its value follows from its own mass and temperature rise.

problems.proof.pitfall. The hot water’s signed heat is negative because it loses energy; use one consistent sign convention in the heat balance.