Stream lines on this simulation of a supernova show the flow of matter behind
Theoretical astrophysicists use a wide variety of tools which include analytical
models (for example, boltropes to approximate the behaviors of a star) and
computational numerical simulations. Each has some advantages. Analytical models
of a process are generally better for giving insight into the heart of what is going on.
Numerical models can reveal the existence of phenomena and effects that would
otherwise not be seen.
Theorists in astrophysics endeavor to create theoretical models and figure out
the observational consequences of those models. This helps allow observers to look
for data that can refute a model or help in choosing between several alternate or
Theorists also try to generate or modify models to take into account new data.
In the case of an inconsistency, the general tendency is to try to make minimal
modifications to the model to fit the data. In some cases, a large amount of
inconsistent data over time may lead to total abandonment of a model.
Topics studied by theoretical astrophysicists include: stellar dynamics and
evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale
structure of matter in the universe; origin of cosmic rays; general relativity and
physical cosmology, including string cosmology and astroparticle physics.
Astrophysical relativity serves as a tool to gauge the properties of large scale
structures for which gravitation plays a significant role in physical phenomena
investigated and as the basis for black hole (astro)physics and the study of
Some widely accepted and studied theories and models in astrophysics, now
included in the Lambda-CDM model, are the Big Bang, cosmic inflation, dark matter,
dark energy and fundamental theories of physics. Wormholes are examples of
hypotheses which are yet to be proven (or disproven).
event, which can be compared with other objects or events.
The scope and
natural sciences and engineering, measurements do not apply to nominal properties of
objects or events, which is consistent with the guidelines of the International
vocabulary of metrology published by the International Bureau of Weights and
However, in other fields such asstatistics as well as the social and
nominal, ordinal, interval, and ratio scales.
Measurement is a cornerstone of trade, science, technology, and quantitative
varied fields of human existence to facilitate comparisons in these fields. Often these
were achieved by local agreements between trading partners or collaborators. Since
the 18th century, developments progressed towards unifying, widely accepted
standards that resulted in the modern International System of Units (SI). This system
reduces all physical measurements to a mathematical combination of seven base
units. The science of measurement is pursued in the field of metrology.
Most physical quantities include a unit, but not all – some are dimensionless.
Neither the name of a physical quantity, nor the symbol used to denote it, implies a
particular choice of unit, though SI units are usually preferred and assumed today due
to their ease of use and all-round applicability. For example, a quantity of mass might
be represented by the symbol m, and could be expressed in the units kilograms
(kg),pounds (lb), or daltons (Da).
The notion of physical dimension of a physical quantity was introduced by
Joseph Fourier in 1822.
dimensional system built upon base quantities, each of which is regarded as having
its own dimension.
points (alternatively "particles"), bodies (objects), and systems of bodies without
consideration of the masses of those objects nor the forces that may have caused the
Kinematics as a field of study is often referred to as the "geometry of
motion" and as such may be seen as a branch of mathematics.
begins with a description of the geometry of the system and the initial conditions of
known values of the position, velocity and or acceleration of various points that are a
part of the system, then from geometrical arguments it can determine the position, the
velocity and the acceleration of any part of the system. The study of the influence of
forces acting on masses falls within the purview of kinetics. For further details,
mechanics) concerned with the study of forces and torques and their effect on motion,
as opposed to kinematics, which studies the motion of objects without reference to its
causes. Isaac Newtondefined the fundamental physical laws which govern dynamics
in physics, especially his second law of motion.
Generally speaking, researchers involved in dynamics study how a physical
system might develop or alter over time and study the causes of those changes. In
addition, Newton established the fundamental physical laws which govern dynamics
in physics. By studying his system of mechanics, dynamics can be understood. In
particular, dynamics is mostly related to Newton's second law of motion. However,
all three laws of motion are taken into account because these are interrelated in any
given observation or experiment
Conservation laws are fundamental to our understanding of the physical world,
in that they describe which processes can or cannot occur in nature. For example, the
conservation law of energy states that the total quantity of energy in an isolated
system does not change, though it may change form. In general, the total quantity of
the property governed by that law remains unchanged during physical processes.
With respect to classical physics, conservation laws include conservation of energy,
mass (or matter), linear momentum, angular momentum, and electric charge. With
respect to particle physics, particles cannot be created or destroyed except in pairs,
where one is ordinary and the other is an antiparticle. With respect to symmetries and
invariance principles, three special conservation laws have been described, associated
with inversion or reversal of space, time, and charge.
Conservation laws are considered to be fundamental laws of nature, with broad
application in physics, as well as in other fields such as chemistry, biology, geology,
Most conservation laws are exact, or absolute, in the sense that they apply to all
possible processes. Some conservation laws are partial, in that they hold for some
processes but not for others.
One particularly important result concerning conservation laws is Noether's
theorem, which states that there is a one-to-one correspondence between each one of
them and a differentiable symmetry in the system. For example, the conservation of
energy follows from the time-invariance of physical systems, and the fact that
physical systems behave the same regardless of how they are oriented in space gives
rise to the conservation of angular momentum.
Oscillation and Wave
Oscillation is the repetitive variation, typically in time, of some measure about
a central value (often a point of equilibrium) or between two or more different states.
The term vibration is precisely used to describe mechanical oscillation. Familiar
examples of oscillation include a swingingpendulum and alternating current power.
Oscillations occur not only in mechanical systems but also in dynamic systems
in virtually every area of science: for example the beating human heart, business
cycles in economics, predator–prey population cycles in ecology, geothermal geysers
in geology, vibrating strings in musical instruments, periodic firing of nerve cells in
the brain, and the periodic swelling of Cepheid variable stars in astronomy.
The simplest mechanical oscillating system is a weight attached to a linear
spring subject to only weight and tension. Such a system may be approximated on an
air table or ice surface. The system is in an equilibrium state when the spring is static.
If the system is displaced from the equilibrium, there is a net restoring force on the
mass, tending to bring it back to equilibrium. However, in moving the mass back to
the equilibrium position, it has acquired momentum which keeps it moving beyond
that position, establishing a new restoring force in the opposite sense. If a constant
force such as gravity is added to the system, the point of equilibrium is shifted. The
time taken for an oscillation to occur is often referred to as the oscillatory period.
Systems where the restoring force on a body is directly proportional to its
displacement, such as the dynamics of the spring-mass system, are described
mathematically by thesimple harmonic oscillator and the regular periodic motion is
known as simple harmonic motion. In the spring-mass system, oscillations occur
because, at the static equilibrium displacement, the mass has kinetic energy which is
converted into potential energy stored in the spring at the extremes of its path. The
spring-mass system illustrates some common features of oscillation, namely the
existence of an equilibrium and the presence of a restoring force which grows
stronger the further the system deviates from equilibrium.
In physics, a wave is an oscillation accompanied by a transfer of energy that
travels through medium (space or mass). Frequency refers to the addition of time.
Wave motiontransfers energy from one point to another, which displace particles of
the transmission medium — that is, with little or no associated mass transport. Waves
consist, instead, ofoscillations or vibrations (of a physical quantity), around almost
There are two main types of waves. Mechanical waves propagate through a
medium, and the substance of this medium is deformed. The deformation reverses
itself owing torestoring forces resulting from its deformation. For example, sound
waves propagate via air molecules colliding with their neighbors. When air molecules
collide, they also bounce away from each other (a restoring force). This keeps the
molecules from continuing to travel in the direction of the wave.
The second main type of wave, electromagnetic waves, do not require a
medium. Instead, they consist of periodic oscillations of electrical and magnetic fields
originally generated by charged particles, and can therefore travel through a vacuum.
These types of waves vary in wavelength, and include radio waves, microwaves,
infrared radiation, visible light,ultraviolet radiation, X-rays, and gamma rays.
Waves are described by a wave equation which sets out how the disturbance
proceeds over time. The mathematical form of this equation varies depending on the
type of wave. Further, the behavior of particles in quantum mechanics are described
by waves. In addition, gravitational waves also travel through space, which are a
result of a vibration or movement in gravitational fields.
A wave can be transverse or longitudinal. Transverse waves occur when a
disturbance creates oscillations that are perpendicular to the propagation of energy
transfer. Longitudinal waves occur when the oscillations are parallel to the direction
of energy propagation. While mechanical waves can be both transverse and
longitudinal, all electromagnetic waves are transverse in free space.
Molecular physics is the study of the physical properties of molecules, the
chemical bonds between atoms as well as the molecular dynamics. Its most important
experimental techniques are the various types of spectroscopy; scattering is also used.
The field is closely related to atomic physics and overlaps greatly with theoretical
chemistry, physical chemistry and chemical physics.
Additionally to the electronic excitation states which are known from atoms,
molecules are able to rotate and to vibrate. These rotations and vibrations are
quantized, there are discrete energy levels. The smallest energy differences exist
between different rotational states, therefore pure rotational spectra are in the far
infrared region (about 30 - 150 µmwavelength) of the electromagnetic spectrum.
Vibrational spectra are in the near infrared (about 1 - 5 µm) and spectra resulting
from electronic transitions are mostly in the visible and ultraviolet regions. From
measuring rotational and vibrational spectra properties of molecules like the distance
between the nuclei can be calculated.
One important aspect of molecular physics is that the essential atomic orbital
theory in the field of atomic physics expands to the molecular orbital theory.
Molecular modelling encompasses all theoretical methods and computational
techniques used tomodel or mimic the behaviour of molecules. The techniques are
used in the fields of computational chemistry, drug design, computational biology
and materials science for studying molecular systems ranging from small chemical
systems to large biological molecules and material assemblies. The simplest
calculations can be performed by hand, but inevitably computers are required to
perform molecular modelling of any reasonably sized system. The common feature of
molecular modelling techniques is the atomistic level description of the molecular
systems. This may include treating atoms as the smallest individual unit (the
Molecular mechanics approach), or explicitly modeling electrons of each atom
(thequantum chemistry approach).
Thermodynamics is the branch of science concerned with heat and
temperature and their relation to energy and work. It states that the behavior of these
quantities is governed by the four laws of thermodynamics, irrespective of the
composition or specific properties of the material or system in question. The laws of
thermodynamics are explained in terms of microscopic constituents by statistical
mechanics. Thermodynamics applies to a wide variety of topics in science and
engineering, especially physical chemistry, chemical engineering and mechanical
Historically, thermodynamics developed out of a desire to increase the
efficiency of early steam engines, particularly through the work of French physicist
Nicolas Léonard Sadi Carnot (1824) who believed that engine efficiency was the key
that could help France win the Napoleonic Wars.
Scottish physicist Lord Kelvin
Thermo-dynamics is the subject of the relation of heat to forces acting between
contiguous parts of bodies, and the relation of heat to electrical agency.
The initial application of thermodynamics to mechanical heat engines was
extended early on to the study of chemical systems. Chemical thermodynamics
studies the nature of the role ofentropy in the process of chemical reactions and
provided the bulk of expansion and knowledge of the field.
formulations of thermodynamics emerged in the following decades. Statistical
of the collective motion of particles from their microscopic behavior. In 1909,
Constantin Carathéodory presented a purely mathematical approach to the field in his
axiomatic formulation of thermodynamics, a description often referred to as
4. Electromagnetic oscillations
The electromagnetic wave equation is a second-order partial differential
equation that describes the propagation of electromagnetic waves through a medium
or in a vacuum. It is a three-dimensional form of the wave equation.
In his 1865 paper titled A Dynamical Theory of the Electromagnetic Field,
Maxwell utilized the correction to Ampère's circuital law that he had made in part III
of his 1861 paper On Physical Lines of Force. In Part VI of his 1864 paper titled
Electromagnetic Theory of Light,
some of the other equations of electromagnetism and he obtained a wave equation
with a speed equal to the speed of light. He commented:
The agreement of the results seems to show that light and magnetism are
affections of the same substance, and that light is an electromagnetic disturbance
propagated through the field according to electromagnetic laws.
Maxwell's derivation of the electromagnetic wave equation has been replaced
in modern physics education by a much less cumbersome method involving
combining the corrected version of Ampère's circuital law with Faraday's law of
charge periodically reverses direction, whereas in direct current (DC, also dc), the
flow of electric charge is only in one direction. The abbreviations AC and DC are
often used to mean simplyalternating and direct, as when they modify current or
AC is the form in which electric power is delivered to businesses and
residences. The usual waveform of alternating current in most electric power circuits
is a sine wave. In certain applications, different waveforms are used, such as
triangular or square waves.
Audio and radio signals carried on electrical wires are also examples of
alternating current. These types of alternating current carry information encoded (or
modulated) onto the AC signal, such as sound (audio) or images (video). These
currents typically alternate at higher frequencies than those used in power
Electric power is distributed as alternating current because AC voltage may be
increased or decreased with a transformer. This allows the power to be transmitted
through power lines efficiently at high voltage, which reduces the power lost as heat
due to resistance of the wire, and transformed to a lower, safer, voltage for use. Use
of a higher voltageleads to significantly more efficient transmission of power. The
power losses (
) in a conductor are a product of the square of the current (I) and
the resistance (R) of the conductor, described by the formula
This means that when transmitting a fixed power on a given wire, if the current
The power transmitted is equal to the product of the current and the voltage
(assuming no phase difference); that is,
Consequently, power transmitted at a higher voltage requires less loss-
transmitted at hundreds of kilovolts, and transformed to 100–240 volts for domestic
High voltage transmission lines deliver power from electric generationplants
High voltages have disadvantages, the main one being the increased insulation
required, and generally increased difficulty in their safe handling. In a power plant,
power is generated at a convenient voltage for the design of a generator, and then
stepped up to a high voltage for transmission. Near the loads, the transmission
voltage is stepped down to the voltages used by equipment. Consumer voltages vary
depending on the country and size of load, but generally motors and lighting are built
to use up to a few hundred volts between phases.