# Physics/Basics

Welcome to Physics/Basics an educational article part of the School of Physics and Astronomy at Wikiversity. This article is meant to be a beginners introduction into the study of Physics.

## Introduction
### What is Physics?

    Physics is a branch of science that studies the nature and properties of matter and energy.

### If you want to contribute

If you want to contribute, or discuss something, you can take a look here, in the discussion area.

## Scientific Method

The study of natural phenomena may be done through the Scientific method, established by Galileo Galilei. This involves four main steps:
* Characterization, which includes definitions, observations, and creating records of measurements and fact relating to the subject of inquiry.
* Hypotheses, that is, possible explanations of prior observations and measurements.
* Prediction, that is, making predictions based on prior observation combined with hypotheses.
* Experiment, designed to test the predictions through new observations.
The observations of the new experiment add to the body of knowledge in the characterization, and then the steps may repeat. In some cases, hypotheses have been well enough established that they change little or not at all with new observations, they become established theory, but human knowledge may never be complete, and new observations and experiments may show something missing from existing theories.

### Measurement

Characterization is of two types: qualitative and quantitative. The first is based on our senses. The second gathers information that has a numeric value, through measurement.
Measurement consists of comparing information about an object and comparing it with a unit of measure, such as comparing the length of an object to a yardstick. There are many kinds of units.
The International System Of Units define units of wide scientific usage, and their respective names, and symbols.
Measurement can be direct or indirect, and scientific instruments have range, precision or sensitivity, accuracy, and speed, or how rapidly or how often measurements may be taken and recorded.

### Vector quantities

As seen above, a quantity has a simple unit. This one has a number value and a unit of measure.
Now, let's look at a quantity a little more complex: a vector. A vector has, other than a number value and a unit of measure, also a direction, and the number value is known as magnitude, because it's also a way. That is, we are saying that a vector has two features: direction and magnitude. Now, consider magnitude has two sub-features: way and a unit (this one again has a number value and unit of measure).

## Matter

Matter is made of atoms, particles originally considered indivisible. When atoms are close enough they repel each other; a little further apart, they are attracted to each other.
The size of atoms is extremely small. Approximately, we consider an atom is to an apple as an apple is to the Earth.

## Temperature

What is known as temperature, which we experience as hot or cold, is motion of atoms, either loosely in a plasma or gas, or vibrationally, jiggling in place. If this motion increases, the temperature increases. If the motion decreases, matter is colder. When the motion has decreased to the minimum possible, the temperature is called absolute zero.

## States of Matter

The four main states of matter are: solid, liquid, gas and plasma. For any given substance, the state of matter is related to the temperature. In order of increasing temperature, the states will be considered

### Solid

Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other, either in a regular geometric lattice (crystalline solids, which include metals and ordinary ice) or irregularly (an amorphous solid such as common window glass).
The branch of physics that deals with solids is called solid-state physics, and is the main branch of condensed matter physics (which also includes liquids). Materials science is primarily concerned with the physical and chemical properties of solids. Solid-state chemistry is especially concerned with the synthesis of novel materials, as well as the science of identification and chemical composition.

### Liquid

A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, and plasma), and is the only state with a definite volume but no fixed shape. A liquid is made up of tiny vibrating particles of matter, such as atoms, held together by intermolecular bonds. Water is, by far, the most common liquid on Earth. Like a gas, a liquid is able to flow and take the shape of a container. Most liquids resist compression, although others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, leading to wetting phenomena.
The density of a liquid is usually close to that of a solid, and much higher than in a gas. Therefore, liquid and solid are both termed condensed matter. On the other hand, as liquids and gases share the ability to flow, they are both called fluids. Although liquid water is abundant on Earth, this state of matter is actually the least common in the known universe, because liquids require a relatively narrow temperature/pressure range to exist. Most known matter in the universe is in gaseous form (with traces of detectable solid matter) as interstellar clouds or in plasma form within stars.

### Gas

Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide). A gas mixture would contain a variety of pure gases much like the air. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image. One type of commonly known gas is steam.
The gaseous state of matter is found between the liquid and plasma states,[1] the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases[2] which are gaining increasing attention.[3] High-density atomic gases super cooled to incredibly low temperatures are classified by their statistical behavior as either a Bose gas or a Fermi gas. For a comprehensive listing of these exotic states of matter see list of states of matter.
At higher temperatures, the average motion of the atoms (or molecules, more-tightly bound combinations of atoms) becomes faster, and strong enough to break up attracted clumps, so the atoms or molecules are free, not bound to each other.

#### Pressure

Enclosed in a container, gas atoms are constantly striking the walls, which receive an average push, called pressure.
If we halve or double the amount of gas in the container (the number of atoms or molecules of gas), the pressure will halve or double. If we halve or double the temperature (measured in degrees above absolute zero), the same.

### Plasma

Plasma (from Greek πλάσμα, "anything formed"[1]) is one of the four fundamental states of matter, the others being solid, liquid, and gas. A plasma has properties unlike those of the other states.
A plasma can be created by heating a gas or subjecting it to a strong electromagnetic field applied with a laser or microwave generator. This decreases or increases the number of electrons, creating positive or negative charged particles called ions,[2] and is accompanied by the dissociation of molecular bonds, if present.[3]
The presence of a significant number of charge carriers makes plasma electrically conductive so that it responds strongly to electromagnetic fields. Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container. Unlike gas, under the influence of a magnetic field, it may form structures such as filaments, beams and double layers.
Plasma is the most abundant form of ordinary matter in the Universe, most of which is in the rarefied intergalactic regions, particularly the intracluster medium, and in stars, including the Sun.[4][5] A common form of plasmas on Earth is seen in neon signs.
Much of the understanding of plasmas has come from the pursuit of controlled nuclear fusion and fusion power, for which plasma physics provides the scientific basis.

### Crystal

A crystal or crystalline solid is a solid material whose constituents, such as atoms, molecules or ions, are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations.
The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification. The word crystal is derived from the Ancient Greek word κρύσταλλος (krustallos), meaning both “ice” and “rock crystal”,[1] from κρύος (kruos), "icy cold, frost".[2][3]
Examples of large crystals include snowflakes, diamonds, and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks, ceramics, and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics.

## Features of Matter
### Mass

Mass is a measure of the amount of matter in an object. It is related to weight, because the weight of an object on the Earth is the force (of gravity) by which the object is attracted to the Earth, which depends on its mass. However, mass also shows as inertia, how hard it would be to push or throw an object, which is independent of gravity.

### Density

Density is the thickness of a solid, liquid or gas measured by its mass per unit of volume.

## Motion

Motion is the change of position of an object with respect to time and a reference frame.
The positions through which the object passes during motion form a line, called a trajectory.
Motion has two main features: velocity, the distance traveled by the object per unit of time, together with the direction of travel (magnitude and direction define a vector), and acceleration, the rate of change of the velocity with respect to time. Acceleration is also a vector. Speed is the absolute value of the velocity without regard to direction.

### Uniform circular motion
## Forces

A force is a natural phenomenon that changes the motion of an object. It is often said that a force can trigger other many phenomena, such as thrust, drag, torque, deformation, etc. These effects can be get up into really one: motion changing. Yes! Because, for example, deformation of an object is indeed a change of motion, or position, of the particles composing it.
Force is a vector quantity, so has both direction and magnitude.

### Newton mechanical laws
#### First law (law of inertia)

    if an object is motionless, or its motion is the type of uniform-linear, there are two possible reasons: it isn't subject to forces or the result of forces acting upon it are null.

#### Second law (acceleration)

    If an object has a change in motion, its change is equal to the sum of the forces on the object.

#### Third law (action-reaction law)

    All forces between two objects exist in equal magnitude and opposite direction.

### Electromagnetic force
## Atomic Theory

We have seen above that matter is made of atoms, but now we go in a little deeper.
Substances (or matter) can be of two types: chemical compounds (molecules) or chemical elements.
* A molecule is a combination of two or more atoms attached to each other
* An element is a single atom, nowadays are known more than a hundred present in nature

### Nomenclature and Symbols

Each element is unique and identified by a Chemical symbol, formed by one or two letters, the first need to be capitalised. For example:
All known elements and their respective symbols are in the periodic table.
A chemical compound (Molecule) is expressed with a chemical formula. This one contains chemical elements and numbers. Each element has a number subscripted at the right side indicating number of atoms of its type in the molecule. For example:
H$_2$O = two atoms of hydrogen and one of oxygen, this is water CaCl$_2$ = a salt, one atom of calcium and two of chlorine. C$_6$H$_{12}$O$_6$ = glucose O$_2$ = a molecule of two atoms of oxygen

### Atomic structure

A little in deep. What is an atom? How it's made?
An atom is made of a nucleus composed of protons and neutrons, around which orbit some electrons. Protons and Neutrons are bound to each other by nuclear force. Electrons are bound to nucleus by electromagnetic force. Protons have positive electric charge, Electrons have negative, and Neutrons have no electric charge.
* Each atom has a fixed number of protons, called Atomic number. This number is unique for each type of element and therefore identify itself in the periodic table.
* Each atom has with the atomic number also the Mass number, which represents the number of neutrons contained in the nucleus. This, instead of the atomic number, isn't fixed, that is each atom can have a different number of neutrons. Atoms of the same element, so with the same atomic number, but with a different number of neutrons are called Isotopes.
* Electrons of each atom are disposed in more levels following a scheme, a.k.a. Electron Configuration of the Atom. Electrons staying in the outest level are Valence Electrons; these are weakly bounded relatively to the nucleus, so get in relation with other atoms establishing the chemical behavior of their owner.