Sargent-Welch Teacher Resources

Heat of Fusion

Sargent Welch — Mon, 03 Dec 2007 20:53:27 +0000

OBJECT: To measure the heat of fusion of ice, using the method of mixtures.

METHOD: A piece of ice of known mass is dropped into a calorimeter containing a measured quantity of water. The heat given up by the calorimeter cup and its contents (water, thermometer and stirrer) is computed from their combined thermal capacity and the change in temperature. The heat absorbed by the ice is expressed in terms of the mass of ice, the heat of fusion and the change in temperature of the water from the melted ice. The heat given up by the cup and its contents is set equal to the heat absorbed by the ice, and this equation is solved for the latent heat of fusion. A correction is made for the exchange of heat between the calorimeter cup and its surroundings.

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Graphs and Equations

Sargent Welch — Mon, 03 Dec 2007 16:01:02 +0000

INTRODUCTION: Experimental work in science is frequently a study of the relationship between two interacting variables. For example the following questions might be answered from experimental data. How does the velocity of a falling body vary with time? What is the angular distribution of radiant energy transmitted through a small opening? What is the pressure response frequency characteristic of a crystal telephone receiver?

When experiments are performed, the independent variable, in these examples time, angle, and frequency, is progressively changed, and the corresponding values of the resulting dependent variable, velocity, intensity, and response respectively, are measured for a series of tests. These data are appropriately recorded in an organized table, that is, in tabular form.

A display of the data as a graph shows more clearly than the tabular form how the one quantity, or property, is related to the second. The graph also indicates probable experimental errors and provides values intermediate to the several readings.

The most powerful form in which the relationship of the variables can be expressed is a mathematical equation. Such equations permit various mathematical expansions and the deduction of additional information. A straight line curve on a graph may be converted quickly to the equation form. Obtaining a straight line curve may require the selection of suitable conversion factors for the axis values, or a special type of graph paper. These techniques, the basis of this discussion, reduce the laborious matching of curves of empirical equations to a reasonable “match” of the original graph form.

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Graphical Interpretation of Data

Sargent Welch — Mon, 03 Dec 2007 14:53:28 +0000

INTRODUCTION: Many experiments are performed to study the manner in which one property or quantity depends upon or varies with another property or quantity. For instance, how does the frictional force between two surfaces vary with the perpendicular force exerted by one surface on the other? Or, how does the length of a pendulum influence its period? Such variables may conveniently be displayed in the form of graphs that summarize the relationships that were studied in the experiment. This discussion will present in an elementary fashion some of the basic ideas connected with the drawing and interpretation of graphs and their use in developing physical laws that express analytically the relationships between the mutually varying quantities.

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Expansion of Gases

Sargent Welch — Wed, 28 Nov 2007 21:20:25 +0000

OBJECT: To investigate the relationship between the pressure and temperature of a constant volume of gas.

METHOD: A mass of dry air is trapped above a column of mercury in a closed tube immersed in water. The closed tube forms one arm of a mercury manometer. The pressure upon the confined air, and hence its volume, can be regulated by means of a plunger in a mercury reservoir. The value of the pressure is obtained from the difference between the mercury levels in the open and closed arms of the manometer. The temperature of the water bath is altered and a series of observations is made upon the pressure of the confined gas, its volume being maintained constant.

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Errors

Sargent Welch — Wed, 28 Nov 2007 19:19:21 +0000

I. INTRODUCTION: Observations are taken in the laboratory and from these observations certain conclusions are drawn. Since no observation or series of observations is absolutely accurate, it is often desirable to check the dependability of the conclusions by a study of the errors in the experiment.

Suppose that an experiment on the relation between the pressure and volume of a gas is performed in the laboratory and that the conclusion is the statement of the law that the volume is inversely proportional to the pressure. The experiment does not prove that the law is absolutely accurate but only that within certain limits, determined by the accuracy of the experiment, it has been found to be true. Small departures from the law will always be found and it should be possible to determine whether these departures indicate that the law is not exactly true or whether they are due to unavoidable experimental errors. Even if in this experiment no significant departures were found, observations with more refined apparatus might show conclusively that the law was only an approximation to the truth.

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Energy Transformations in Multiple Collisions (Linear Air Track)

Sargent Welch — Wed, 28 Nov 2007 18:37:30 +0000

OBJECT: To study energy transformations in multiple collisions of objects on a linear air track and to relate the rate of energy change to a Die-away Curve.

METHOD: An object floating on a linear air track is made to collide with several different materials. Upon impact the object rebounds and recollides several times, reaching a particular height for each rebound. Heights of successive rebounds are measured, and the coefficients of restitution are computed. Next, Die-away Curves are drawn from these data to show the rate of energy transformation. The curves then are related to other decay processes.

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Efficiency of an Incandescent Lamp

Sargent Welch — Mon, 26 Nov 2007 20:35:05 +0000

OBJECT: To determine by means of a photometer the relationship between the efficiency of an incandescent lamp and the potential drop across it.

METHOD: A standard lamp and a test lamp are placed at opposite ends of a photometer bench. By adjusting the position of a photometer box so that its screen is equally illuminated on the two sides, the candle power of the test lamp is determined. This is repeated for various voltages on the test lamp and the relationship between the voltage and efficiency (candles per watt) determined.

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Dynamical Comparison of Masses

Sargent Welch — Mon, 26 Nov 2007 16:08:35 +0000

OBJECT: To investigate the effect upon the period of a vibrating system caused by varying its mass, and to make a determination of the mass of a body by a dynamical method.

METHOD: An elastic system is so arranged that its mass can be varied. The period is observed for a number of (known) masses and a curve is plotted of the mass versus the square of the period. A body of unknown mass is then added and its mass determined from observations of the period. This dynamically determined mass is then compared with the corresponding mass obtained by weighing.

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The Diffraction Grating

Sargent Welch — Mon, 26 Nov 2007 15:22:45 +0000

OBJECT: To measure the wavelength of light with a diffraction grating.

METHOD: A slit in an opaque screen, illuminated with a sodium flame or other source of bright line spectrum, is viewed through a diffraction grating held near the eye. With the rulings of the grating parallel to the slit, several orders of spectra are seen on either side of the slit. The various spectral images are located by means of a transverse scale mounted beside the slit. From the known value of the grating space and from the measured distances between the slit and the grating and between the slit and the successive spectra, the wavelength of light is calculated.

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Density of Liquids and Solids - Pycnometer Method

Sargent Welch — Wed, 21 Nov 2007 17:16:50 +0000

OBJECT: To determine the density of a liquid and of a solid, using the pycnometer method.

METHOD: The mass of an irregular solid is determined by weighing. When the solid is placed in a pycnometer (Fig. 1) filled with a liquid of known density, the volume of the liquid which will overflow is equal to the volume of the solid. The mass of the liquid which will overflow is determined as the difference between the sum of the mass of the pycnometer filled with liquid plus the mass of the solid and the mass of the pycnometer filled with liquid after the solid has been placed inside. The volume occupied by this mass is determined from the known density of the liquid. It is necessary that the solid be insoluble in the liquid used. The density of the solid is determined from these measurements of mass and volume.

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