forces  that  attract  molecules  to  each  other.  In  this way, they are somewhat like the rock and the earth we  considered  before.  Molecules  have  energy  of motion (internal kinetic energy) because they are constantly in motion. Thus, the two stored forms of thermal energy—internal potential energy and internal  kinetic  energy—are  in  some  ways  similar to  mechanical  potential  energy  and  mechanical kinetic energy, except everything is on a smaller scale. For  most  purposes,  we  will  not  need  to distinguish   between   the   two   stored   forms   of thermal  energy.  Therefore,  instead  of  referring to internal potential energy and internal kinetic energy, from now on we will simply use the term internal energy. By internal energy, then, we will mean  the  total  of  all  internal  energy  stored  in  the substance or system because of the motion of the molecules and because of the forces of attraction between  molecules.  Although  the  term  may  be unfamiliar  to  you,  you  probably  know  more about internal energy than you realize. Because molecules are constantly in motion, they exert a pressure on the walls of the pipe, cylinder, or other object  in  which  they  are  contained.  Also,  the temperature of any substance arises from, and is directly   proportional   to,   the   activity   of   the molecules.  Therefore,  every  time  you  read thermometers and pressure gauges you are finding out   something   about   the   amount   of   internal energy  contained  in  the  substance.  High  pressures and temperatures indicate that the molecules are moving rapidly and that the substance therefore has  a  lot  of  internal  energy. Heat  is  a  more  familiar  term  than  internal energy,  but  may  actually  be  more  difficult  to define  correctly.  The  important  thing  to  remember is   that   heat   is   THERMAL   ENERGY   IN TRANSITION—that is, it is thermal energy that is  moving  from  one  substance  or  system  to another. An example will help to show the difference between heat and internal energy. Suppose there are  two  equal  lengths  of  pipe  made  of  identical materials   and   containing   steam   at   the   same pressure  and  temperature.  One  pipe  is  well insulated; the other is not insulated at all. From everyday  experience  you  know  that  more  heat  will flow  from  the  uninsulated  pipe  than  from  the insulated  pipe.  When  the  two  pipes  are  first filled with steam, the steam in one pipe contains exactly as much internal energy as the steam in the other pipe. We know this is true because the two pipes contain equal volumes of steam at the same pressure and at the same temperature. After a few minutes, the steam in the uninsulated pipe will contain much less internal energy than the steam  in  the  insulated  pipe,  as  we  can  tell  by measuring the pressure and the temperature of the steam in each pipe. What has happened? Stored thermal   energy—internal   energy—has   moved from one system to another, first from the steam to the pipe, then from the uninsulated pipe to the air.  This  MOVEMENT  or  FLOW  of  thermal energy from one system to another is called heat. A good deal of confusion exists concerning the use of the word heat. For example, you will hear people say that a hot object contains a lot of heat when  they  really  mean  that  it  contains  a  lot of internal energy. Or you will hear that heat is added to or removed from a substance. Since heat is  the  FLOW  of  thermal  energy,  it  can  no  more be  added  to  a  substance  than  the  flow  of  water could be added to a river. (You might add water, and  this  addition  might  increase  the  flow,  but  you could hardly say that you added flow. ) The only thermal energy that can in any sense be added to or  removed  from  a  substance  is  INTERNAL ENERGY. ENERGY   TRANSFORMATIONS The machinery and equipment in the engineer- ing plant aboard ship are designed either to carry energy  from  one  place  to  another  or  to  change a  substance  from  one  form  to  another.  The principles of energy transformations and some of the  important  energy  changes  that  occur  in  the shipboard  propulsion  cycle  are  discussed  in  the following   paragraphs. Conservation of Energy The basic principle dealing with the transfor- mation  of  energy  is  the  PRINCIPLE  OF  THE CONSERVATION  OF  ENERGY.  This  principle can  be  stated  in  several  ways.  Most  commonly, perhaps,  it  is  stated  that  energy  can  be  neither destroyed   nor   created,   but   only   transformed. Another way to state this principle is that the total quantity of energy in the universe is always the same.   Still   another   way   of   expressing   this principle is by the equation, Energy in = Energy out, The energy out may be quite different in form from the energy in, but the total amount of energy input  must  always  equal  the  total  amount  of energy  output. 2-9