Joule & Energy - The Museum of Science & Industry in Manchester

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Educational Resources for teachers and students

Experiments to try in school: For young pupils

Just rub two pieces of wood and observe that they get warmer.

Why not simply rub the hands together to see that they get warm?

Experiments to try in school: For older pupils under supervision

This activity covers the following National Curriculum areas:

Key Stage 3 Science Sc4 Physical Processes: Energy resources and energy transfer: conservation of energy

Key Stage 4 Science Sc4 Physical Processes: Energy Resources and energy transfer: work, power and energy

1. Simple friction to generate heat and even ignite wood.

How hot does a match have to be before it ignites? Can we measure this?

Slowly raise the temperature of a match and monitor the temperature until it ignites.

2. Take a tube about one or two (or h) metres long and put in about half a kilogram (or mkg) of lead shot.

Allow both the lead shot and the water to come to room temperature and then keep the water in a thermos flask (use a thermos carefully).

Invert the tube n times. Each time it is inverted the lead shot falls through a height h and the loss of energy is equal to the total loss of potential energy or the work done (nmgh).

Quickly put the shot into the water and measure the rise in temperature. Remember that the experiment is rather an approximate one, so we are justified in ignoring the thermal capacity of the containers.

Let the specific heat capacity of lead be C, the initial and final temperatures of the water be t_i and t_f and the temperature of the lead after falling through the height nh be t.

Therefore nmgh = JCm(t - t_i)

To calculate the temperature of the lead t we say that:

Cm (t - t_i) = M (t_f - t_i) where M is the thermal capacity of the water.

This experiment is reminiscent of a well-known story about Joule. It is said that while on his honeymoon he tried to measure the temperature rise experienced by water as it fell from the top of a waterfall to the bottom. There is no record of his wife's comments on this behaviour!

3. Joule's experiments showed the equivalence of heat and other forms of energy, so perhaps it is worth reminding ourselves of a simple experiment involving electrical heating. We measure the current and voltage of the heater and also the temperature rise and look at the equivalence of the heat energy and the electrical energy.
Previous Ideas about Heat:
This activity covers the following National Curriculum areas: Key Stage 4 Science Sc1 Scientific Enquiry: Knowledge, Skills and Understanding: Ideas and Evidence in Science
When Joule began his work, people had a very strange idea about heat. They thought it was a kind of invisible fluid - a bit like a gas. Their idea was that when you heat a solid you drive this fluid into the solid and it heats up and expands. Then, when you cool it down, the fluid is squeezed out of the solid and disappears into the surroundings. Although this might sound very odd, it was what everybody thought. However, the idea that friction can produce heating was not new. Even before Joule did his work the American, Count Rumford, had shown that by trying to drill into a brass canon he could produce an enormous amount of heat. A modern example of this can be seen when we use an electric power drill. If we are not careful, it is possible to melt the end of the drill bit - and certainly to make it too hot to touch - by continuous drilling. Another example of this is provided by skating on ice. It is often stated that we can skate on ice because the pressure of the skater lowers the melting point of the ice, thus allowing the skates to bite into the (otherwise solid) ice. However, this is not the reason. Whilst the lowering of the melting point does occur, it is not large enough to produce this result. The real reason is that friction heats, and thus melts, a thin layer of ice below the skate.

For A-level teachers: some more comments about the idea that heat is a kind of invisible fluid

We might think it rather strange, almost childish, to think that heat is an invisible fluid (called caloric) that can flow into and out of bodies. However, before we condemn it, let us look at this idea more closely. We often have problems trying to think in abstract terms and we therefore invent more practical models to represent the world around us. For example, when Maxwell developed the theory that electromagnetic waves were propagated through space with the speed of light, he used a model of the medium which was mechanical. Before Einstein produced his theory of relativity, it was thought that a vacuum was filled with an invisible aether, again to support the propagation of light. Even today, we have similar concepts. In nuclear physics, we talk of the vacuum levels being filled with electrons (particles) and of positive electrons (antiparticles) corresponding to vacancies in these levels. Finally, if we treat the caloric as representing entropy rather than heat itself, then it is possible to present a reasonable case for its adoption.

The paddle wheel experiment introduced by James Joule:

This activity covers the following National Curriculum areas:

Key Stage 4 Science Sc1 Scientific Enquiry: Knowledge, Skills and Understanding: Investigative Skills: Planning, Obtaining evidence, Evaluating

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?

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.

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.