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Mechanical Equivalent of Heat

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  • 0:01 What is Heat?
  • 1:33 Early Historical Views of Heat
  • 2:59 Link Between Work & Heat
  • 5:16 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

We all know when we feel heat, but do you know how or when it was discovered? In this lesson, we learn about the revolutionary findings of James Prescott Joule who discovered how to get heat out of mechanical work.

What is Heat?

You have just made yourself a delicious hot cup of coffee. You put it on the table in front of you and wait for it to cool down. You can see the steam coming off the top and you can see it is clearly too hot to drink right now. The coffee cup may even be too hot to touch. If you measured the temperature of the coffee, it will be about 75 degrees centigrade and you know that is pretty hot. The room you are sitting in measures around 25 degrees centigrade, and over time, both the coffee cup and the coffee will cool down - soon it will be cool enough to drink.

Now, you also know if you don't drink all of your coffee, it continues to cool down to room temperature. Urgh cold coffee! So while you are waiting for you coffee to cool down, watch what is going on at the molecular level. The decreasing temperature of both the coffee and the cup are a sign that the average kinetic energy of the particles are decreasing - the molecules are slowing down. Both the coffee and the cup are losing energy. Energy is being transferred as heat to the surrounding air.

We can say that heat is simply the transfer of energy from a hot object to a colder object. Sounds simple, doesn't it? But, you may be surprised to learn that it wasn't until the middle 19th century that heat was understood in terms of energy.

Early Historical Views of Heat

Fire and heat has always fascinated man, and even in the old days, they knew that fire and heat were different. They noticed that after the fire had gone out, the embers still gave out heat. However, it took until the 1700s for scientists to start to explain what heat actually is. The Scottish chemist Joseph Black was the first modern chemist to explain heat. He noticed that a kettle filled with water and ice placed over a fire did not change temperature until all of the ice had melted. Black explained his observation by suggesting that heat flows like a liquid.

The famous French scientist Antoine Lavoisier took these observations a step further and developed the heat-fluid concept called the caloric theory. This theory described heat as a tasteless, odorless, invisible and weightless fluid, called a caloric fluid. It also stated that hot objects contain more of this fluid than cold objects. Caloric was also thought of as a weightless gas that could pass in and out of pores in solids and liquids. This was a commonly held theory until the end of the 18th century. But very soon this theory was challenged and later became obsolete.

Link Between Work and Heat

It was during the 19th century that the field of thermodynamics started to take off. And it was during this time that our understanding of work and energy developed. In 1843, the English physicist James Prescott Joule calculated the mechanical equivalent of heat in a series of experiments. His most famous piece of apparatus, now called the Joule apparatus, is shown here.

joule apparatus

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