1. Basic Guide

Models, Theories, and Laws

The terms modeltheory, and law have exact meanings in relation to their usage in the study of physics.

Definition of Terms: Model, Theory, Law

In colloquial usage, the terms modeltheory, and law are often used interchangeably or have different interpretations than they do in the sciences. In relation to the study of physics, however, each term has its own specific meaning.

The laws of nature are concise descriptions of the universe around us. They are not explanations, but human statements of the underlying rules that all natural processes follow. They are intrinsic to the universe; humans did not create them and we cannot change them. We can only discover and understand them. The cornerstone of discovering natural laws is observation; science must describe the universe as it is, not as we may imagine it to be. Laws can never be known with absolute certainty, because it is impossible to perform experiments to establish and confirm a law in every possible scenario without exception. Physicists operate under the assumption that all scientific laws and theories are valid until a counterexample is observed. If a good-quality, verifiable experiment contradicts a well-established law, then the law must be modified or overthrown completely.


model is a representation of something that is often too difficult (or impossible) to display directly. While a model’s design is justified using experimental information, it is only accurate under limited situations. An example is the commonly used “planetary model” of the atom, in which electrons are pictured as orbiting the nucleus, analogous to the way planets orbit the Sun. We cannot observe electron orbits directly, but the mental image helps explain the observations we can make, such as the emission of light from hot gases. Physicists use models for a variety of purposes. For example, models can help physicists analyze a scenario and perform a calculation, or they can be used to represent a situation in the form of a computer simulation.


Planetary Model of an Atom: The planetary model of the atom in which electrons are pictured as orbiting the nucleus, analogous to the way planets orbit the Sun


theory is an explanation for patterns in nature that is supported by scientific evidence and verified multiple times by various groups of researchers. Some theories include models to help visualize phenomena, whereas others do not. Newton’s theory of gravity, for example, does not require a model or mental image, because we can observe the objects directly with our own senses. The kinetic theory of gases, on the other hand, makes use of a model in which a gas is viewed as being composed of atoms and molecules. Atoms and molecules are too small to be observed directly with our senses—thus, we picture them mentally to understand what our instruments tell us about the behavior of gases.


A law uses concise language to describe a generalized pattern in nature that is supported by scientific evidence and repeated experiments. Often, a law can be expressed in the form of a single mathematical equation. Laws and theories are similar in that they are both scientific statements that result from a tested hypothesis and are supported by scientific evidence. However, the designation law is reserved for a concise and very general statement that describes phenomena in nature, such as the law that energy is conserved during any process, or Newton’s second law of motion, which relates force, mass, and acceleration by the simple equation F=ma. A theory, in contrast, is a less concise statement of observed phenomena. For example, the Theory of Evolution and the Theory of Relativity cannot be expressed concisely enough to be considered a law. The biggest difference between a law and a theory is that a law is much more complex and dynamic, and a theory is more explanatory. A law describes a single observable point of fact, whereas a theory explains an entire group of related phenomena. And, whereas a law is a postulate that forms the foundation of the scientific method, a theory is the end result of that process.

1. Basic Guide

Physics and Other Fields

Physics is the foundation of many disciplines and contributes directly to chemistry, astronomy, engineering, and most scientific fields.

Physics and Other Disciplines

Physics is the foundation of many important disciplines and contributes directly to others. Chemistry deals with the interactions of atoms and molecules, so it is rooted in atomic and molecular physics. Most branches of engineering are applied physics. In architecture, physics is at the heart of structural stability and is involved in acoustics, heating, lighting, and the cooling of buildings. Parts of geology rely heavily on physics, such as the radioactive dating of rocks, earthquake analysis, and heat transfer in the Earth. Some disciplines, such as biophysics and geophysics, are hybrids of physics and other disciplines.


Physics in Chemistry: The study of matter and electricity in physics is fundamental towards the understanding of concepts in chemistry, such as the covalent bond.

Physics has many applications in the biological sciences. On the microscopic level, it helps describe the properties of cell walls and cell membranes. On the macroscopic level, it can explain the heat, work, and power associated with the human body. Physics is involved in medical diagnostics, such as X-rays, magnetic resonance imaging (MRI), and ultrasonic blood flow measurements. Medical therapy sometimes directly involves physics: cancer radiotherapy uses ionizing radiation, for instance. Physics can also explain sensory phenomena, such as how musical instruments make sound, how the eye detects color, and how lasers can transmit information.

The boundary between physics and the other sciences is not always clear. For instance, chemists study atoms and molecules, which are what matter is built from, and there are some scientists who would be equally willing to call themselves physical chemists or chemical physicists. It might seem that the distinction between physics and biology would be clearer, since physics seems to deal with inanimate objects. In fact, almost all physicists would agree that the basic laws of physics that apply to molecules in a test tube work equally well for the combination of molecules that constitutes a bacterium. What differentiates physics from biology is that many of the scientific theories that describe living things ultimately result from the fundamental laws of physics, but cannot be rigorously derived from physical principles.

It is not necessary to formally study all applications of physics. What is most useful is the knowledge of the basic laws of physics and skill in the analytical methods for applying them. The study of physics can also improve your problem-solving skills. Furthermore, physics has retained the most basic aspects of science, so it is used by all of the sciences. The study of physics makes other sciences easier to understand.

1. Basic Guide

Introduction: Physics and Matter

Physics is a study of how the universe behaves. Physics is a natural science that involves the study of matter and its motion through space and time, along with related concepts such as energy and force. More broadly, it is the study of nature in an attempt to understand how the universe behaves.

What is Physics?: Mr. Andersen explains the importance of physics as a science. History and virtual examples are used to give the discipline context.

Physics uses the scientific method to help uncover the basic principles governing light and matter, and to discover the implications of those laws. It assumes that there are rules by which the universe functions, and that those laws can be at least partially understood by humans. It is also commonly believed that those laws could be used to predict everything about the universe’s future if complete information was available about the present state of all light and matter.

Matter is generally considered to be anything that has mass and volume. Many concepts integral to the study of classical physics involve theories and laws that explain matter and its motion. The law of conservation of mass, for example, states that mass cannot be created or destroyed. Further experiments and calculations in physics, therefore, take this law into account when formulating hypotheses to try to explain natural phenomena.

Physics aims to describe the function of everything around us, from the movement of tiny charged particles to the motion of people, cars, and spaceships. In fact, almost everything around you can be described quite accurately by the laws of physics. Consider a smart phone; physics describes how electricity interacts with the various circuits inside the device. This knowledge helps engineers select the appropriate materials and circuit layout when building the smart phone. Next, consider a GPS system; physics describes the relationship between the speed of an object, the distance over which it travels, and the time it takes to travel that distance. When you use a GPS device in a vehicle, it utilizes these physics equations to determine the travel time from one location to another. The study of physics is capable of making significant contributions through advances in new technologies that arise from theoretical breakthroughs.


Global Positioning System: GPS calculates the speed of an object, the distance over which it travels, and the time it takes to travel that distance using equations based on the laws of physics.

1. Basic Guide

The following table illustrates the major inventions and their inventors in physics uses −

Centigrade scaleAnders Celsius
WatchPeter Henlein
RadioGuglielmo Marconi
TelephoneAlexander Graham Bell
ElectricityBenjamin Franklin
Electric Light BulbThomas Edison
ThermometerGalileo Galilei
TelescopeHans Lippershey and Zacharias Janssen; later Galileo
TelegraphSamuel Morse
Cosmic RaysVictor Hess (but the term ‘cosmic rays’ first used by Robert Millikan
AutomobileKarl Benz
Magnetic TapeFritz Pfleumer
TransformerMichael Faraday (later Ottó Titusz Bláthy)
Electromagnetic InductionMichael Faraday
Quantum mechanicsWerner Heisenberg, Max Born, and Pascual Jordan
Wave mechanicsErwin Schrödinger
Nuclear ReactorEnrico Fermi
Fuel CellWilliam Grove
AirplaneWright Brothers
BarometerEvangelista Torricelli
CameraNicéphore Niépce
Diesel EngineRudolf Diesel
HelicopterIgor Sikorsky
DynamiteAlfred Nobel
LiftElisha Otis
Laser PrinterGary Starkweather
Mobile PhoneMartin Cooper
Printing PressJohannes Gutenberg
Video GamesRalph Baer
Steam engineThomas Newcomen
Railway EngineGeorge Stephenson
Jet EngineFrank Whittle
SeismographJohn Milne
Electric GeneratorMichael Faraday
TelevisionJohn Logie Baird
RefrigeratorWilliam Cullen (later Oliver Evans)
CarburetorLuigi De Cristoforis & Enrico Bernardi
Air BrakeGeorge Westinghouse
Atomic bombRobert Oppenheimer, Edward Teller et al
Air conditionerWillis Carrier
Machine GunSir Hiram Maxim
RadarSir Robert Alexander Watson-Watt
SubmarineCornelius Drebbel (later) David Bushnell
First military submarineYefim Nikonov
TransistorJohn Bardeen, Walter Brattain, and William Shockley
GalvanometerJohann Schweigger
LaserTheodore H. Maiman (first demonstrated)
Neon lampGeorges Claude
Rocket EngineRobert Goddard
TypewriterChristopher Latham Sholes
1. Basic Guide

Major Instruments and Their Uses

The following table illustrates the major scientific instruments and their uses −

AccelerometerMeasures acceleration
AltimeterMeasures altitude of an aircraft
AmmeterMeasures electric current in ampere
AnemometerMeasures wind speed
BarometerMeasures atmospheric pressure
BolometerMeasures radiant energy
CaliperMeasures distance
CalorimeterMeasures heat (in chemical reaction)
CrescographMeasures growth in plant
DynamometerMeasures torque
ElectrometerMeasures electric charge
EllipsometerMeasures optical refractive indices
FathometerMeasures depth (in sea)
GravimeterMeasures the local gravitational field of the Earth
GalvanometerMeasures electric current
HydrometerMeasures specific gravity of liquid
HydrophonesMeasures sound wave under water
HygrometerMeasures atmospheric humidity
InclinometerMeasures angel of slope
InterferometerInfrared light spectra
LactometerMeasures purity of milk
MagnetographMeasures magnetic field
ManometerMeasures pressure of gas
OhmmeterMeasures electric resistance
OdometerMeasures distance travelled by a wheeled vehicle
PhotometerMeasures intensity of light
PyrometerMeasures temperature of a surface
RadiometerMeasures intensity or force radiation
RadarDetects distance object, e.g. aircraft, etc.
SextantMeasures angle between two visible objects
SeismometerMeasures motion of the ground (earthquake/seismic waves)
SpectrometerMeasures spectra (light spectrum)
TheodoliteMeasures horizontal and vertical angles
ThermopileMeasures small quantities of radiant heat
ThermometerMeasures temperature
UdometerMeasures amount of rainfall
ViscometerMeasures the viscosity of fluid
VoltmeterMeasures volt
Venturi meterMeasures flow of liquid
1. Basic Guide

Physics – Measurement Units

The following table illustrates the major measuring units in physics −

Mass And Related Quantities
ForceFNewton (N)
PressurePPascal (Pa)
Dynamic viscosityηPa.s
Acoustic pressurepPascal (pa)
Dynamic volumevm3
Electricity and Magnetism
PowerPwatt (W = J/s)
EnergyWjoule (J = N.m)
Magnetic field strengthHampère per metre (A/m)
Electric fieldEvolt per metre (V/m)
quantity of electricityQcoulomb (C = A.s)
Electrical resistanceRohm (Ω = V/A)
electrical capacitanceCfarad (F = C/V)
Potential differenceUvolt (V = W/A)
International System of Units
ampereAElectric Current
kelvinKThermodynamic temperature
molemolAmount of substance
candelacdLuminous intensity
steradiansrSolid Angle
newtonNForce, weight
pascalPapressure, stress
jouleJenergy, work, heat
wattWPower, radiant, flux
coulombCElectric charge
voltVVoltage, electromotive force
faradFElectric capacitance
ohmΩElectric resistance
teslaTMagnetic flux density
degree Celsius0CTemperature
henryHMagnetic induction
AngstromÅWave length

Conversion of Units

Unit IValue in another unit
1 Inch2.54 centimeter
1 Foot0.3048 meter
1 Foot30.48 centimeter
1 Yard0.9144 meter
1 Mile1609.34 meter
1 Chain20.1168 meter
1 Nautical mile1.852 kilometer
1 Angstrom10-10 meter
1 Square inch6.4516 square centimeter
1 Acre4046.86 square meter
1 grain64.8 milligram
1 dram1.77 gm
1 ounce28.35 gm
1 pound453.592 gram
1 horse power735.499 Watt
1. Basic Guide

Physics – Branches

The following table illustrates the major branches and their sub-branches) of physics −

Classical mechanics
Newtonian mechanics
Analytical mechanics
Celestial mechanics
Applied mechanics
Analytical mechanics
Dynamics (mechanics)
Elasticity (physics)
Fluid mechanics
Thermodynamics and statistical mechanicsHeat
Condensed matter physics
Solid state physics
High pressure physics
Surface Physics
Polymer physics
Atomic and molecular physics
Atomic physics
Molecular physics
Chemical physics
Gravitation physics
High-energy astrophysics
Planetary astrophysics
Plasma physics
Solar physics
Space physics
Stellar astrophysics
Nuclear and particle physics
Nuclear physics
Nuclear astrophysics
Particle physics
Particle astrophysics
Applied Physics
Chemical Physics
Communication Physics
Engineering physics
Laser Physics
Medical physics
Physical chemistry
Plasma physics
Quantum electronics
1. Basic Guide

Development of Physics

  • During the ancient period, the development of physics took place with the development of astronomy.
  • However, during the medieval period, a notable work of the Arab writer and scientist Ibn Al-Haitham revolutionized the concept of physics.
  • Ibn Al-Haitham had written a book in seven volumes namely “Kitāb al-Manāẓir “also known as “The Book of Optics.”
  • In this book, Ibn Al-Haitham disprove the ancient Greek concept of vision and introduced a new theory.
  • Ibn Al-Haitham had also introduced the concept of the pinhole camera.
  • During the late medieval period, Physics became a separate discipline of the natural science.
  • In making physics as a separate discipline, the major contributions were given by the European scientists.
  • These modern European scientists had been introduced different concepts of physics and discovered and invented many new technologies.
  • For example, Copernicus replaced the ancient view of geocentric model and introduced the heliocentric concept; Galileo invented the telescopes, Newton discovered the laws of motion and universal gravitation, etc.
  • The era of modern physics came with the discovery of quantum theory by Max Planck and theory of relativity by Albert Einstein.
Physics Scientists
  • After development of modern physics, the ear of applied physics commenced where emphasis is given on ‘research’ on a particular use.
  • The particle physicists have been consistently designing and developing the high energy accelerators, detectors, and computer programs.
  • Nuclear physics is another branch of modern physics that studies the constituents and interactions of the atomic nuclei.
  • The most widely known inventions and applications of nuclear physics are the generation of nuclear power and the development of nuclear weapons technology.
  • At present, the physic scientists are working on the concept of high-temperature superconductivity.
1. Basic Guide


  • Physics is one of the most significant disciplines of natural science, which describe the nature and properties of matters.
  • The term ‘physics’ is derived from the Ancient Greek word i.e. ‘phusikḗ’ meaning ‘knowledge of nature’.
Physics Introduction


  • Physics is the branch of natural science that studies the nature and properties of matter and energy.
  • The significant subject matter of physics includes mechanics, heat & thermodynamics, optics, sound, electricity, magnetism, etc.
  • Development of Physics also makes significant contributions in the field of technologies. For example, inventions of new technology such as television, computers, cell phone, advanced home appliances, nuclear weapons, etc.