Question

As a reward for completing your PChem homework you fix yourself a fruit smoothie in a

blender. While using your high-end blender with a vacuum filled double wall jar, you cannot help but

ponder the thermodynamics of your tasty beverage. Explain the following.

(a) Has heat entered the beverage while in the blender jar?

(b) Did the temperature of your beverage rise as a result of the blending?

(c) Did you do work on the beverage?

(d) Have you changed the internal energy of the beverage?

Answer 1

When you push the start button on the blender to make fruit smoothie, motor begins to rotate the blades. The circular motion of the rotating blades creates a vortex or a spiral movement in the fluid. Inside the blender, the fluid contains both liquid friut beverage and air. The whirlpool causes a vacuum at the center of the jar, which pulls the fruit toward the middle, like a tornado does.

(a) No, heat has been added to the beverage. A thermos bottle contains friut beverage, but as the thermos bottle is insulated from its surrounding, therefore ΔQ = 0. Thus, heat has not been entered to the friut beverage. So the sign of the work done will be negative.

(b) Yes, its temperature rises. Since the thermos bottle (vacuum filled double wall jar) is vigorously shaken, the kinetic energy of molecules in the friut beverage will be increased. As the temperature is directly proportional to the average kinetic energy of the molecule, therefore temperature of the friut beverage will be increased.

(c) Yes, work has been done on the system as heat is added to the friut beverage. As, the thermos bottle is vigorously shaken, therefore work is done on the friut beverage which is against the viscous force.

(d) Yes, its internal energy has changed. Heat is the part of internal energy which is transferred from one body to another an account of the temperature difference. So the internal energy of a body was acquired by heat transfer.

In accordance to the first law of thermodynamics, ΔQ = ΔU + ΔW. From the above discussion, as, ΔQ = 0 and the sign of the work done will be negative, therefore ΔU should be positive. Thus internal energy of the friut beverage will be increased.

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