by virulentor Wed May 26, 2010 3:16 pm
Before I talk to you about my famous paddle wheel experiment, let me say something that is really quite important.
I know that you all use metres and kilograms today. In my day, we used pounds and ounces for weighing and feet and inches for lengths. We also measured temperature using degrees Fahrenheit instead of Celsius. I am going to translate my measurements into your units to make it easier for you.
The experiment that I am going to describe showed that heat is just another kind of energy like kinetic energy or potential energy.
Let us begin by thinking about what scientists mean by energy and work. One kind of work is when we move a heavy object against gravity. That is, we lift it up through a height. This means that it now has a kind of energy that we call potential energy. When we let the object fall down, it can do work for us and changes the work back into moving or kinetic energy.
But what happens when an object hits the ground? Surely, it has stopped moving? Does this mean that it has lost its energy? Has the energy somehow disappeared?
The answer must be no, because it is an important rule that energy cannot be destroyed. So, what happens to the energy?
[size=12]The answer is that it changes back into heat and the object heats up. Now, this is a nice idea, but can we prove it? Well, the problem is that the change in temperature is not very large.
I decided that I would use these ideas and try to measure this change in temperature. Instead of just showing that the work became energy, and then heat, I wanted to measure exactly how much heat came out of a certain amount of work.
The first step is to think of a way of doing work in a way that can be measured. The clue to this is in my earlier comments.
[size=12]I decided that, if I dropped an object of known mass through a measured height, I could calculate the work it did and the (potential) energy it used or gave up. Then, I had to decide what I wanted it to do. It seemed to me that an easy thing to make it do would be to heat up some water and then I would know how much the energy of the water had changed.
The problem was how to heat the water this way. Then I had an idea: if I used the energy to stir the water around, maybe that would heat it up. You can see that the heating must be very small because you would never try to heat up water this way.
This is how I came upon the idea of a container with all those paddles in it to stir the water. I still had to make the falling object stir the water but that is quite easy. All I needed was some ropes and pulleys and the job was done.
So far then, I had some falling objects that turn the paddles and this would stir the water and heat it up. I could measure the weight and the height and then I would know how much work had been done. This would tell me how much energy the water was gaining.
I still had to measure the heat that had been given to the water.
I did this by weighing the water before I put it into the apparatus. Then, I needed to measure the change in temperature. I knew that the temperature rise would be very small, in fact it turned out to be less than one degree, so I needed very accurate thermometers. I was lucky to have someone who could make me special thermometers so that I could measure the temperature to the required precision.
You must remember that I had been doing experiments for many years, so I had lots of practice in reading instruments. This allowed me to measure my temperatures extremely accurately. Even so, it was difficult to get good results. I repeated the experiments many times and also improved it to increase the temperature rise. I let the weights fall, then quickly wound them up again and let them fall a second and a third time. This meant that they did more work.
Because the temperature changes were so small, I also had to be careful to make sure that there were no other ways that the water could get warmer.
This is why I did the experiments in a cellar, where the temperature was constant and I was away from other sources of heat. I even shielded myself so that the heat of my body didn't affect the results.
Finally, I worked out how much heat was generated from a given amount of work. The important result was that it was always the same conversion factor, so now I had shown that heat was just another form of energy like kinetic energy and potential energy. I should remind you that I didn't do all this work by myself. I had lots of help in making the apparatus and in doing the experiments.
Things Joule has ignored in this simple approach
The heat capacity of the paddles and container. The experiment with the lead shot should allow us to estimate how much error is introduced by neglecting the thermal capacities of the containers. Also we have ignored the small amount of kinetic energy that the weights had as they reached the bottom.
Remember that Joule performed his experiments using old-fashioned units. 1 pound (lb) falling through a height of 772 feet (ft.) in Manchester would heat 1lb of water through 1degree Fahrenheit.
As well as using old-fashioned units Joule, along with his contemporaries, had different words for some quantities and properties. For example, the word energy was not used at that time. Even in the present day, we seem to be reluctant to use the unit of the Joule. It is true that it appears on all our food packets but we still persist in using the calorie, especially when we talk about diets. One calorie is the same as 4.2 Joules.[/size][/size]