A young scientist wants to measure the internal energy of a known fluid. Unfortu

Question

A young scientist wants to measure the internal energy of a known fluid. Unfortunately, he does not have a thermometer

please help with all steps and units.

A young scientist wants to measure the internal energy of a known fluid. Unfortunately, he does not have a thermometer, but he does have a scale, a heating element and an electric power meter. After weighing his sample, he knows that when it starts to boil it will have an internal energy of 135kJ. He places the sample in an insulated box with his heating element. The heating element provides 25W of electrical work to the box for 23 minutes before the scientists observes boiling. What is the original internal energy of the sample before it was heated? Be careful with your units.

Explanation / Answer

>> Let 'U1' be the initial energyof the fluid.

Now, when it starts boiling, it's internal energy becomes U2 and as given ;

        U2 = 135 KJ

>> Now, it requires 23 minutes to start boiling or to take the fluid from state 1 to state 2

>> As, it is achieved b using 25 W Heating Element

>> Total Work required = 25*23*60 J = 34.5 KJ

>> Now, as system is insulated and

according to 1st Law of Thermodynamics,

    dQ = dU + dW

For properly insulated systems, dQ = 0

=> dU = - dW

>> Now, dW will be -ve because it is being provided to the system

=> dW = - 34.5 kJ

=> dU = 34.5 KJ

=> U2 - U1 = 34.5

=> 135 - U1 = 34.5

=> U1 = Initial Internal Energy of fluid = 104.5 KJ   ....ANSWER....

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