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What is heat measured with?
Question Date: 2012-10-29
Answer 1:

Heat, which is a form of energy that flows from high to low temperature, is measured with a calorimeter.

To understand how a calorimeter works, you need to know about heat capacity:
Heat capacity is the amount of heat energy needed to change the temperature of a certain amount of a substance by a given amount. The units are energy per mass and temperature (eg. Joules/(g•°C). For example, the heat capacity of water is 4.19 J/(g•°C). That means it takes 4.19 Joules of energy to raise the temperature of 1 g of water by 1°C. You could also think of this as 1 g of water stores 4.19 J for every 1°C increase in its temperature.

There are many types of calorimeters, but the general idea behind how they work is that if you know the heat capacities and quantities of the substances in and around the calorimeter, you can measure a change in temperature, and calculate how much heat energy there is.

Let's say you want to measure the heat produced by a reaction. Your calorimeter would be an insulating container, filled with water, with a second container in the water. You would place your reactants in the closed inner container. You would measure the temperature of the system and then start the reaction. After the reaction is complete, you measure the temperature again. If you know the heat capacities of the water and container, you can calculate how much heat was produced.

For an ever more accurate calculation, you would also need to know the heat capacities of the reactants and products

Now, you might be curious as to how we would measure the heat capacities of all of those things in the first place. To do that, you need a different type of calorimetry. Let's say this time you want to measure the heat capacity of water. You would put the water in an insulating container with an electrical heating element and a thermometer. The electrical heating element works by resistive heating, so you can calculate how much heat you are adding by measuring how much current you are using. Then you measure how much the temperature of the water changes. From this information (amount of water, amount of heat (current) added, and temperature change), you can calculate the heat capacity (4.19 J/(g•°C)) of the water. Of course, you can do this for substances other than water as well.

Many calorimeter designs exist but they all operate on similar principles to what I have described here. They often have more complex components to decrease the amount of heat lost to the surroundings, make the calculations of heat simpler, accommodate reactions involving liquids and gases, or use an advanced method for more accurate measurements.

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