The difference between hot and cold

Physics of the Mundane

Adam Johnston

Temperature is a measure of the kinetic energy of the particles of a given material.  This energy can be transferred from one material to another via heat.  When two materials are in contact for a long enough amount of time, they will come to thermal equilibrium, a state in which both materials have the same temperature.  Thus, one way of transferring energy from one body to another is to place them in contact.  The warmer body will lose energy to the cooler body in this process.

In this lab, you will use this energy transfer technique in two ways. First, you will use a metal washer that is at the temperature of boiling water (by placing it in a bath of boiling water) to increase the temperature of some tap water in a cup.  Then, you will use the same metal washer, now at an extremely low temperature (by placing the washer in a bath of liquid nitrogen, low temperature alcohol, or low temperature salt solution) to decrease the temperature of some tap water in a cup.  (This procedure will be discussed and demonstrated in class.)

Important note: Metal pieces placed in liquid nitrogen or other low temperature slushes are extremely cold!  Yes, your tongue will stick to a piece of metal at liquid nitrogen temperature; and yes, it would require a trip to the emergency. Please don=t do this. 

By doing this process (probably several times), you should discuss/answer the following:

C     In what direction is energy flowing when the hot metal is placed in your cup of water?

C     In what direction is energy flowing when the cold metal is placed in your cup of water?

C     In which case is the energy transfer the greatest?  (You should be measuring the temperature change of your cup of water with a thermometer.)

C     (Primary task) Determine the temperature of the cold reservoir by comparing the temperature change of the water when the hot washer is placed in it to when the cold washer is placed in it.

You should have the following equipment:

C     thermometer

C     cup

C     water

C     washer with string (the string is crucial B don=t try to hand dip the washer in either boiling water or liquid nitrogen)

C     community shared supplies of cold liquid and boiling water

C     a lab partner to fight with and to keep you from putting your tongue on the metal washer

Discuss your procedure with you lab partner and your instructor. If you are getting incredibly low temperature calculations for the cold reservoir liquid/slush, you are probably on the right track.