Act7_sol - Activity 7 Solutions Applications of the Laws of...

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1/23/05 1 Activity 7 Solutions: Applications of the Laws of Thermodynamics 7.1 Conservation of Energy Your instructor will discuss how changes in internal energy U relate to energy conservation. 1) Internal Energy Your instructor will demonstrate two rolling carts colliding with a barrier. Both carts have the same mass and the same frictional force with the table top. a) Which cart has more kinetic energy after it hits the barrier – the cart that rolls back a shorter distance or a longer distance? _ longer distance _ How do you know? The cart that rolls back a longer distance must have bounced off of the barrier with a faster velocity. This cart has more kinetic energy because kinetic energy is proportional to the velocity squared. E kin = ½ M v 2 b) Now watch the carts, without their outer covers, collide with the barrier. How can you explain the difference in the kinetic energy of the carts after they hit the barrier? During the collision, some of the initial kinetic energy of motion of the elastic band cart is used to vibrate the washers. This kinetic energy goes into increasing the internal energy of the cart. The cart with rigid rods rolls farther away from the wall because none of its initial kinetic energy went into vibrating its washers. Since the elastic band cart has less kinetic energy after the collision, it does not move as far away from the wall. c) Explain how energy was conserved when each cart collided with the barrier. If each cart is pushed with the same force, both carts start with the same amount of kinetic energy. Their kinetic energy goes into moving against the force of friction with the table top and into colliding with the wall. Since after the collision the elastic band cart uses energy not only to rebound, but to vibrate the washers as well, it does not roll as far after the collision. d) How does this demonstration illustrate an increase in the internal energy of an object? The carts demonstrate what happens inside of a solid when kinetic energy is converted into the thermal energy of vibrating atoms and molecules. The faster the atoms and molecules of an object vibrate, the greater the internal energy of the object. 7.2 Pressure, Temperature, Volume and the Ideal Gas Law 2) Crushing Cans Your instructor will illustrate the relationship between pressure and volume by changing the internal energy and the phase of water molecules inside cans.
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1/23/05 2 a) What happens to the internal energy and the motion of the water molecules in the can as the water is heated into steam? As their temperature increases, the water molecules have more internal energy. As the water molecules change phase from liquid to steam, they move freely throughout the can with substantial kinetic energy.
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Act7_sol - Activity 7 Solutions Applications of the Laws of...

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