< Physics 123, Lecture 6 Warm-Up

Warm-Up Exercise 6

Due 12:15 pm, Mon 10 Sep 2012

Physics 123, Fall 2012

Reading assignment: 21.5,21.6,20.1-20.3

Section 21.5 talks about three different velocities of particles in a gas. Explain in your own words what the difference is between the three. Why is vmp the smallest of the three velocities?

V_ave is just the regular average: add up the speeds and divide by number of particles. V_rms is "root mean square": add up the *squares* of the velocities, divide by the number of particles, then take the square root. It corresponds to the speed of the average kinetic energy. V_mp is the "most probable" velocity: the peak of the speed histogram. V_mp is the smallest because the distribution of speeds is skewed to the right--the peak in Fig. 21.11 (7th edition) is decidedly to the left of center.

In terms that a lay person could easily understand, explain what heat capacity is. Explain the difference between "heat capacity" and "specific heat". Why do we need both?

Specific heat is the amount of energy (per unit mass) absorbed by a material for a unit change in temperature. It depends on the material properties but not the amount of mass in the object. Heat capacity is not a "per unit mass" quantity. It's a total for an object. It depends on the mass and the type of material. It's relevant for an engineering application but not generally what we use in most physics problems.

This image from the text shows the temperature of water as a function of energy added. In region B and D, energy is being added but the temperature is constant. How can energy be added to a system and the temperature not change? Where is the energy going?

The energy is being used to break bonds during a phase change (first from solid to liquid, later from liquid to gas), and so the temperature doesn't change even though energy is being absorbed.

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