Chapter 10

Thermodynamics

Calorimetry with flow rate

Take 10 minutes to prepare this exercise.

Then, if you lack ideas to begin, look at the given clue and start searching for the solution.

A detailed solution is then proposed to you.

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A coil is immersed in a calorimeter.

Is passed through the coil a current of water.

is the specific heat of water.

At the input, the water is at a temperature of , the output is the calorimeter temperature which, through an electric heater, is maintained at .

Calorimetry with flow rate

Question

Calculate the power required from the heating resistance if the water flow rate through the coil is .

Hint

Consider using a similar assessment to that used for flowing gas in thermal machines.

Solution

Is selected as the closed fluid system in the calorimeter and a mass to the input at the instant .

At time , the system consists of the fluid in the calorimeter and a mass at the output, at the calorimeter temperature.

The first law of thermodynamics then gives : (assuming steady state)

Question

The resistance is ; calculate the current intensity .

Hint

Solution

Write :

So :

Question

Passing another liquid in the coil and for the same conditions (temperature and intensity), we must ensure a flow rate of .

Calculate the specific heat of the liquid.

Hint

Solution

We have the equality :

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