It follows that the absolute or total number of heat units contained in any substance, must be determined by some other means than the thermometer, and that a degree on the thermometer cannot be considered a unit of heat. What then is a unit of heat It has been agreed to consider the amount of heat necessary to raise one pound of water from 33 Fah. It is also a correct inference that any particular substance in a uniform state, as regards the cohesive power of its particles, must exhibit the same temperature so long as it maintains that condition, since heat is the opposite force to cohesion.
The more heat the less cohesion, and vice versa. Water, when passing from the liquid to the solid state, maintains such a uniformity of condition ; its temperature may, therefore, be regarded as constant. It also maintains the same uniformity of condition while passing from the liquid state into steam at the boiling point. The freezing and boiling points of water may therefore be considered as the two prominent landmarks of temperature from which the amount of expansion of some uniformly, or nearly uniformly expanding substance, as mercury, immersed in water in the two conditions named, being noted on a scale, divisions may be arbitrarily made each way on the same scale, which will indicate temperatures above or below these points.
The Centigrade scale makes the hight of a mercury column immersed in freezing water, zero, and divides the distance between this point and the hight of the same column immersed in boiling water, into one hundred degrees, while the Fahrenheit Scale makes the first named hight 33 degrees above zero, and divides the space between this hight and the hight at which the mercury stands in boiling water, into one hundred and eighty divisions, or degrees.
How it is possible to determine the amount of heat in any body from thermometric indication next claims our attention. The following law has been established. The total amottnt of heat in any body is the sum of its latent heat and its sensible heat. The latent heat is determined by the known capacity of the body under examination, at given temperatures to absorb heat, or, in other words, to render it latent. This term, latent heat, is not a good one, though we are still obliged to use it for want of a better.
We use it only to distinguish the heat which, acting within a mass of matter, and expending its energy in antagonism to cohesive attraction, cannot be recognized by sensation, Mke the free or sensible heat. The latent, or specific heat of various bodies has been made the subject of careful study, and tables of reference have been constructed to afford a ready means of computation ; but the specific heat of all bodies is changed by any cause which lessens or increases the distance between the particles which make up their mass.
The compression of steam lessens its specific heat while it increases its temperature, and vine versa. Two metal leads of dissimilar metals are placed in close proximity to each other, creating voltage. Changes in voltage correspond to changes in temperature. Thermocouples are used in industry, and are often connected to other devices that turn mechanisms off and on in response to certain temperatures.
Thermocouples are not as accurate as thermometers. Thermocouples are increasingly being replaced by resistance temperature detectors, or resistance thermometers. RTDs are generally more stable and accurate than thermocouples; they use carbon or platinum sensors to detect changes in electrical resistance.
These changes are caused by temperature changes, and the changes are predictable. A consistent light current is passed through the RTD, past the leads, and then resistance can be determined and temperature calculated. A pyrometer measures the surface temperatures of objects.
It is a tool that combines an optic feature with a temperature reader constructed of an ultrathin filament. The pyrometer is aimed at the surface of an object, whereupon the optic device focuses on the thermal signature — or radiated heat — and transfers that signature to the filament reader.
These are especially useful for measuring temperatures on surfaces that are out of reach or too hot to touch, like steam boilers, metallurgy furnaces and hot air balloons. Irving Langmuir was a Nobel Prize winning physicist. Langmuir wanted to learn how to take the temperature of electrons as part of his research to learn about the electrical potential of plasma, a gas-like condition of matter in which some particles lose electrons.
Such state changes are referred to as being endothermic. Freezing, condensation and deposition are exothermic ; energy is released by the sample of matter when these state changes occur. So one might notice that a sample of ice solid water undergoes melting when it is placed on or near a burner. Heat is transferred from the burner to the sample of ice; energy is gained by the ice causing the change of state.
But how much energy would be required to cause such a change of state? Is there a mathematical formula that might help in determining the answer to this question? There most certainly is. The amount of energy required to change the state of a sample of matter depends on three things. It depends upon what the substance is, on how much substance is undergoing the state change, and upon what state change that is occurring.
For instance, it requires a different amount of energy to melt ice solid water compared to melting iron. And it requires a different amount of energy to melt ice solid water as it does to vaporize the same amount of liquid water.
And finally, it requires a different amount of energy to melt The substance, the process and the amount of substance are the three variables that affect the amount of energy required to cause a specific change in state. Use the widget below to investigate the effect of the substance and the process upon the energy change. Note that the Heat of Fusion is the energy change associated with the solid-liquid state change. The values for the specific heat of fusion and the specific heat of vaporization are reported on a per amount basis.
It takes J of energy to melt 1. It takes 10 times as much energy - J - to melt Reasoning in this manner leads to the following formulae relating the quantity of heat to the mass of the substance and the heat of fusion and vaporization.
Values of Q are positive for the melting and vaporization process; this is consistent with the fact that the sample of matter must gain energy in order to melt or vaporize. Values of Q are negative for the freezing and condensation process; this is consistent with the fact that the sample of matter must lose energy in order to freeze or condense.
As an illustration of how these equations can be used, consider the following two example problems. Example Problem 3 Elise places What quantity of energy would be absorbed by the ice and released by the beverage during the melting process? The equation relating the mass Substitution of known values into the equation leads to the answer. Example Problem 3 involves a rather straightforward, plug-and-chug type calculation. Now we will try Example Problem 4, which will require a significant deeper level of analysis.
Example Problem 4 What is the minimum amount of liquid water at The specific heat capacity of liquid water is 4. In this problem, the ice is melting and the liquid water is cooling down. Energy is being transferred from the liquid to the solid. To melt the solid ice, J of energy must be transferred for every gram of ice. This transfer of energy from the liquid water to the ice will cool the liquid down. At this temperature the liquid will begin to solidify freeze and the ice will not completely melt.
The - sign indicates that the one object gains energy and the other object loses energy. We can calculate the left side of the above equation as follows:. The solution is:. On the previous page of Lesson 2 , the heating curve of water was discussed. The heating curve showed how the temperature of water increased over the course of time as a sample of water in its solid state i. We learned that the addition of heat to the sample of water could cause either changes in temperature or changes in state.
At the melting point of water, the addition of heat causes a transformation of the water from the solid state to the liquid state. And at the boiling point of water, the addition of heat causes a transformation of the water from the liquid state to the gaseous state.
These changes in state occurred without any changes in temperature. However, the addition of heat to a sample of water that is not at any phase change temperatures will result in a change in temperature. Now we can approach the topic of heating curves on a more quantitative basis. The diagram below represents the heating curve of water. There are five labeled sections on the plotted lines. The three diagonal sections represent the changes in temperature of the sample of water in the solid state section 1 , the liquid state section 3 , and the gaseous state section 5.
The two horizontal sections represent the changes in state of the water. In section 2, the sample of water is undergoing melting; the solid is changing to a liquid. In section 4, the sample of water is undergoing boiling; the liquid is changing to a gas. So now we will make an effort to calculate the quantity of heat required to change The calculation will require five steps - one step for each section of the above graph.
While the specific heat capacity of a substance varies with temperature, we will use the following values of specific heat in our calculations:. Section 1 : Changing the temperature of solid water ice from Section 3 : Changing the temperature of liquid water from 0.
Section 5 : Changing the temperature of liquid water from That is,. Summing these five Q values and rounding to the proper number of significant digits leads to a value of kJ as the answer to the original question.
In the above example, there are several features of the solution that are worth reflecting on:. This understanding will be critical as we proceed to the next page of Lesson 2 on the topic of calorimetry.
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