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  5. The scope and scale of physics

1.1 The scope and scale of physics  (Page 5/12)

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  • How many hydrogen atoms does it take to stretch across the diameter of the Sun?
    (Answer: 10 9 m/10 –10 m = 10 19 hydrogen atoms)
  • How many protons are there in a bacterium?
    (Answer: 10 –15 kg/10 –27 kg = 10 12 protons)
  • How many floating-point operations can a supercomputer do in 1 day?
    (Answer: 10 5 s/10 –17 s = 10 22 floating-point operations)

In studying [link] , take some time to come up with similar questions that interest you and then try answering them. Doing this can breathe some life into almost any table of numbers.

This table of orders of magnitude of length, mass and time has three columns and thirteen rows. The first row is a header row and it labels each column, “length in meters (m),” “Masses in kilograms (kg),” and “time in seconds (s).” Under the “length in meters” column are the following entries: 10 to the minus 15 meters equals diameter of proton; 10 to the minus 14 meters equals diameter of large nucleus; 10 to the minus 10 meters equals diameter of hydrogen atom; 10 to the minus 7 meters equals diameter of typical virus; 10 to the minus 2 meters equals pinky fingernail width; 10 to the 0 meters equals height of 4 year old child, and a drawing of a child measuring himself against a meter stick is included; 10 to the 2 meters equals length of football field; 10 to the 7 meters equals diameter of earth; 10 to the 13 meters equals diameter of solar system; 10 to the 16 meters equals distance light travels in a year (one light year); 10 to the 21 meters equals milky way diameter; 10 to the 26 meters equals distance to edge of observable universe. Under the “Masses in kilograms” column are the following entries: 10 to the -30 kilograms equals mass of electron; 10 to the -27 kilograms equals mass of proton; 10 to the -15 kilograms equals mass of bacterium; 10 to the -5 kilograms equals mass of mosquito; 10 to the -2 kilograms equals mass of hummingbird; 10 to the 0 kilograms equals mass of liter of water, and a drawing of a balance scale with a liter on one side and a 1 kilogram mass on the other is shown; 10 to the 2 kilograms equals mass of person; 10 to the 19 kilograms equals mass of atmosphere; 10 to the 22 kilograms equals mass of moon; 10 to the 25 kilograms equals mass of earth; 10 to the 30 kilograms equals mass of sun; 10 to the 53 kilograms equals upper limit on mass of known universe. Under the “Time in seconds” column are the following entries: 10 to the -22 seconds equals mean lifetime of very unstable nucleus; 10 to the -17 seconds equals time for a single floating point operation in a supercomputer; 10 to the -15 seconds equals time for one oscillation of visible light; 10 to the -13 seconds equals time for one vibration of an atom in a solid; 10 to the -3 seconds equals duration of a nerve impulse; 10 to the 0 equals time for one heartbeat, and a drawing of the heart with a plot of three pulses is shown. The peak of the first pulse is labeled P. The next pulse is larger amplitude and shorter duration. The start of the second pulse is labeled Q, its peak is labeled R, and its end is labeled S. The peak of the third pulse is labeled T. The entries in the column continue as follows: 10 to the 5 seconds equals one day; 10 to the 7 seconds equals one year; 10 to the 9 seconds equals human lifetime; 10 to the 11 seconds equals recorded human history; 10 to the 17 seconds equals age of earth; 10 to the 18 seconds equals age of universe;
This table shows the orders of magnitude of length, mass, and time.

Visit this site to explore interactively the vast range of length scales in our universe. Scroll down and up the scale to view hundreds of organisms and objects, and click on the individual objects to learn more about each one.

Building models

How did we come to know the laws governing natural phenomena? What we refer to as the laws of nature are concise descriptions of the universe around us. They are human statements of the underlying laws or rules that all natural processes follow. Such laws are intrinsic to the universe; humans did not create them and cannot change them. We can only discover and understand them. Their discovery is a very human endeavor, with all the elements of mystery, imagination, struggle, triumph, and disappointment inherent in any creative effort ( [link] ). The cornerstone of discovering natural laws is observation; scientists must describe the universe as it is, not as we imagine it to be.

Photos of Enrico Fermi and Marie Curie
(a) Enrico Fermi (1901–1954) was born in Italy. On accepting the Nobel Prize in Stockholm in 1938 for his work on artificial radioactivity produced by neutrons, he took his family to America rather than return home to the government in power at the time. He became an American citizen and was a leading participant in the Manhattan Project. (b) Marie Curie (1867–1934) sacrificed monetary assets to help finance her early research and damaged her physical well-being with radiation exposure. She is the only person to win Nobel prizes in both physics and chemistry. One of her daughters also won a Nobel Prize. (credit a: United States Department of Energy)

A model    is a representation of something that is often too difficult (or impossible) to display directly. Although a model is justified by experimental tests, it is only accurate in describing certain aspects of a physical system. An example is the Bohr model of single-electron atoms, in which the electron is pictured as orbiting the nucleus, analogous to the way planets orbit the Sun ( [link] ). We cannot observe electron orbits directly, but the mental image helps explain some of the observations we can make, such as the emission of light from hot gases (atomic spectra). However, other observations show that the picture in the Bohr model is not really what atoms look like. The model is “wrong,” but is still useful for some purposes. Physicists use models for a variety of purposes. For example, models can help physicists analyze a scenario and perform a calculation or models can be used to represent a situation in the form of a computer simulation. Ultimately, however, the results of these calculations and simulations need to be double-checked by other means—namely, observation and experimentation.
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Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
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