BlogNCERTDegrees of Freedom

Degrees of Freedom


In the formal description of the state of a physical system, it is an independent physical parameter. The number of ways a molecule in the gas phase can move, rotate, or vibrate in space is referred to as degrees of freedom. When utilizing the equipartition theorem to estimate the values of various thermodynamic variables, the number of degrees of freedom possessed by a molecule plays a role. Translational, rotational, and vibrational degrees of freedom are the three forms of degrees of freedom. The number of degrees of freedom possessed by each type of molecule is determined by the number of atoms in the molecule as well as the geometry of the molecule, with geometry referring to the arrangement of the atoms in space.

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    The degree of freedom of a dynamic system is the total number of coordinates required to fully represent the location and configuration of the system or the number of directions in which a particle can freely travel. It is represented by the letters f or N.


    The definition of degrees of freedom is a mathematical equation that is used in statistics, as well as physics, mechanics, and chemistry. The degrees of freedom in a statistical calculation depict the number of values included in a computation that has the ability to vary. The degrees of freedom can be calculated to verify that t-tests, chi-square tests, and even more complex f-tests are statistically valid.

    A gaseous molecule possesses a set of degrees of freedom, including the capacity to translate (move its centre of mass through space), rotate about its centre of mass, and vibrate (as its bond lengths and angles change). The energy associated with each of these modes of motion affects a variety of physical and chemical characteristics. If a molecule has N independent particles, then F=3N is the total degree of freedom in three dimensions of the molecule.

    Translational degrees of freedom: The ability of a gas molecule to move freely in space gives rise to translational degrees of freedom. In a Cartesian coordinate system, a molecule can move in the x, y, and z axes. When a particle’s centre of mass moves from one point to another, we call it translational motion along the x-axis, y-axis, and z-axis. As a result, the translational motion of a gas molecule is connected with three degrees of freedom. This holds true for all gas molecules, whether monatomic, diatomic, or polyatomic because every molecule in three-dimensional space can move freely in all directions.

    Rotational degree of freedom: The number of unique ways a molecule can rotate in space around its centre of mass with a change in its orientation is represented by its rotational degrees of freedom. Because the centre of mass lies squarely on the atom and no rotation that causes change is possible, a monatomic gaseous molecule such as a noble gas has no rotational degrees of freedom. A diatomic molecule lying along the Y-axis can rotate about the mutually perpendicular X-axis and Z-axis passing through its centre of gravity, as shown in the graphic below. This indicates that the linear molecule has two rotating degrees of freedom. Non-linear molecules, on the other hand, have three rotational degrees of freedom.

    Vibrational degrees of freedom: The atoms of a molecule can also vibrate, and these vibrations change the inter-nuclear distances between the atoms of the molecule slightly. The number of vibrational degrees of freedom (or vibrational modes) of a molecule is determined by counting the number of distinct ways atoms within the molecule can move relative to one another, such as in bond stretches or bends. As previously stated, atoms have only one degree of freedom: translation. A diatomic molecule has only one degree of freedom in terms of vibration. During vibrational motion, the molecules’ bonds behave like springs, and the molecule exhibits simple harmonic motion.

    Use of Degrees of Freedom:

    T-tests and chi-square tests are frequently used to compare observed data with data that would be expected to be obtained based on a specific hypothesis.

    The parameters used to describe a physical system, such as chemical composition and pressure, are referred to as degrees of freedom (physics and chemistry). The smallest number of fixed parameters required to define a system or the size of its phase space.

    Degrees of Freedom Example:

    The t-test and chi-squared tests are two examples of how degrees of freedom can be used in statistical calculations. There are several t-tests and chi-square tests that can be differentiated using degrees of freedom.

    Let’s look at an example of the degree of freedom. Assume a medical trial is conducted on a group of patients, and it is hypothesized that the patients receiving the medication will have a higher heart rate than those who did not receive the medication. The results of the test could then be analyzed to determine whether the difference in heart rates is considered critical, and degrees of freedom are factored into the calculations.

    1. To fully specify a free particle moving along the x-axis, only one coordinate is required. As a result, its degree of freedom is one.
    2. A particle moving over a plane has two degrees of freedom as well.
    3. There are three degrees of freedom for a particle moving in space.

    If there are N gas molecules in the container, the total number of degrees of freedom is f = 3N. However, if the system has q constraints (movement restrictions), the degrees of freedom decrease and are equal to f = 3N-q, where N is the number of particles.

    Degrees of Freedom Formula:

    The statistical formula for determining the number of degrees of freedom is straightforward. It implies that degrees of freedom are equal to the number of values in a data set minus one, and it looks like this:

    N-1 = d f

    N denotes the number of values in the data set (sample size). After that, let’s take a look at the sample calculation. If there are six data sets (N=6). Make a list of the values for each data set and call it X.

    For this example data, sample size set X includes the following values: 10, 30, 15, 25, 45, and 55.

    The mean, or average, for this data set is 30. Add the values and divide by N to get the mean:

    (10+30+15+25+45+55)/6= 30

    The degrees of freedom will be calculated using the formula,

    d f =N-1

    In this case, it appears that:

    d f=6-1=5

    This implies that in this data set (sample size), five numbers have the freedom to vary as long as the mean remains at 30. Knowing the degrees of freedom for a sample or population size does not provide us with much useful information on its own. This is because, after computing the degrees of freedom, which are the number of values in a calculation that can be varied, we must use a critical value table to determine the critical values for our equation. It is worth noting that these tables can be found online or in textbooks. The values found in a critical value table identify the statistical significance of the outcomes when using it.

    Also read: Free Path

    Frequently Asked Questions (FAQs):

    Question 1:What is the role of degrees of freedom in advanced statistical techniques?

    Answer: More sophisticated statistical techniques employ more complex methods of counting degrees of freedom. The number of degrees of freedom is calculated using a slightly complicated formula when computing the test statistic for two means with independent samples of n 1 and n 2 elements. It is calculated by taking the smaller of n 1-1 and n 2-1.

    Question 2: What role do degrees of freedom play in standard deviation?

    Answer: Degrees of freedom can also be found in the standard deviation formula. This appearance is not as clear and obvious, but it is discernible if we know where to look. The “average” deviation from the mean is used to calculate the standard deviation. However, after subtracting the mean from each data value and squaring the differences, we find ourselves dividing by n-1 rather than n as we might expect.

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