Category: Physics Notes PPT

Conservation laws are the backbone of physics. They determine what can occur or what cannot occur. Some of them are considered to be universal in the sense that it is believed that any possible process has to fulfill them without exception. From the fundamental point of view, these conservation laws are either motional (the conservation of momentum, energy, linear momentum, etc..) or material (Conservation of charges). No one has observed any process violating any of these rules. 

However, we remark that the conservation laws are only necessary conditions and not at all sufficient conditions. A hypothetical process that obeys only these conditions is thus not necessarily a process that can really take place in nature. So in classical physics, a process that occurs in nature has to obey an additional condition: the conservation of matter in the form of the conservation of mass. The conservation of mass plays an important role in nature, in physics the conservation of mass is depicted with the help of consideration of conservation of electric charges. The concept of conservation of mass is widely used in the field of classical mechanics, chemistry, and fluid mechanics.

Law of Conservation of Mass:

The law of conservation of mass is formulated and systematized by 18th-century French chemist Antoine Lavoisier. According to the conservation of mass, all the reactions and interactions within a closed system will leave the mass of the system unchanged. In other words, the conservation of mass refers to the fact that the mass of an isolated system or closed system is neither increased nor decreased by the reaction between the parts. The total mass of the system before the reaction will be equal to the mass of the system after the reaction. The matter is always conserved.

What is The Law of Conservation of Mass:

The law of conservation of mass states that the matter can neither be created nor be destroyed, but it can be changed. It is also known as the conservation of matter. From the statement of the law of conservation of mass, it is understood that matter is conserved. In a closed system, the mass of all the substances before the chemical reaction will be the same as the mass of the substances after the chemical reactions.

According to the law of conservation of mass during any chemical reaction, no atoms will be created nor be destroyed, therefore the mass of reactants must be equal to the mass of the products in any low energy thermodynamic process. Initially, it is believed that the law of conservation of mass or the law of conservation of matter originated from classical mechanics, after some time it was modified according to quantum mechanics with the help of the mass-energy relation. In 1789, Antoine Laurent Lavoisier was an 18th-century French chemist who proposed the law of conservation of mass and discovered the conservation of mass. 

Derivation:

Though the conservation of mass mainly focused on the chemical reactions it’s considered to be equally important from the physics point of view as well. According to the conservation of mass, it says that the object or the collection of the object will remain the same no matter how many times we rearrange, it parts it away. With the help of the concept of the theory of relativity, the concept of mass underwent a radical revision. The meaning of mass lost its absoluteness. The mass of an object was seen to be equivalent to energy, and the mass and energy of the system were found to be interconvertible and to increase significantly at increasing high speeds near that of light. 

The meaning of the mass has been always varying according to the concerned subject of discussion. The mass has been viewed mainly in two compatible ways in physics. Sometimes the mass is viewed as a measure in terms of inertia, and sometimes as an opposition that offers forces to the body in motion. Thus, from the perspective of either inertial mass or the gravitational mass, according to the principle of the law of mass conservation, different measurements of the mass of an object considered under various circumstances should always be the same.

Consider a collection of objects located somewhere in space. This quantity of matter or the system of objects with well-defined boundaries is known as a system. The law of conservation of mass then implies that the mass of this given system remains constant it can not be created nor be destroyed, therefore, we get,

[ Rightarrow frac{Dm}{Dt} = 0]

Where, 

Dm is the infinitesimally small part of the considered system of objects. 

The volume occupied by the matter or the system of objects may be changing and the density of the objects within the system may be changing, but the mass of the system remains constant.

Importance of Conservation of Mass:

Physics is essentially an experimental and observational science. Predictions of any theory have to be confronted with experiments and observations. Conservation laws are the backbone of physics. All observed processes should conserve all universal laws and also specific laws associated with the interaction which governs the observed process.

The conservation of mass implies that matter can be neither created nor destroyed—i.e., processes that change the physical or chemical properties of substances within an isolated system or a closed system (such as the conversion of a liquid to a gas) leave the total mass unchanged. Strictly speaking, mass is not a conserved quantity. However, except in nuclear reactions, the conversion of rest mass into other forms of mass-energy is so small that, to a high degree of precision, rest mass may be thought of as conserved.

Did You Know:

The history associated with the discovery of the conservation of mass is remarkable. An important idea from ancient Greek philosophy was that “Nothing comes from nothing” so that what exists now has always existed: no new matter can come into existence where there was none before. An explicit statement of this, along with the further principle that nothing can pass away into nothing, is found in Empedocles (approx. 490–430 BC): “For it is impossible for anything to come to be from what is not, and it cannot be brought about or heard of that what it should be utterly destroyed.” We knew about the conservation of mass from ages and it took centuries to frame it to a universal law.

Jain philosophy, a non-creationist philosophy based on the teachings of Mahavira (6th century BC), states that the universe and its constituents such as matter cannot be destroyed or created. The Jain text Tattvarthasutra (2nd century AD) states that a substance is permanent, but its modes are characterized by creation and destruction. A principle of the conservation of matter was also stated by Nasīr al-Dīn al-Tūsī (1201–1274). He wrote that “A body of matter cannot disappear completely. It only changes its form, condition, composition, color, and other properties and turns into a different complex or elementary matter”.

In this world, we have both natural mirrors and man-made mirrors. Mirrors are a fascinating world by themselves. There are mirror museums where you have all shapes and kinds of mirrors which will make you appear tall, short, fat, thin and a variety of other things! Cartoons from our childhood would have their characters enter mirror worlds that were exactly the opposite of our real worlds. 

The study of mirrors is one of the fundamental topics in physics. Do you see how closely this science is connected to our daily lives? Each morning and evening we see our reflection in the mirror at home – we tend to see not only the external self but also end up reflecting on what is going on inside of us. It is said smiling to yourself in the mirror, first thing in the morning fills your day with positivity! 

Convex Mirror – Application and Working of Convex Mirror

Convex mirrors are also popularly called the fish eye mirror or diverging mirror. Its reflective surface bulges in the direction of the light source. A convex mirror has the ability to reflect all the light that falls on them in the outward direction. So, the type of image formed by these mirrors is always imaginary/virtual. 

 

Also, due to the above property the size of the object’s image is smaller as compared to the actual object’s size. Therefore, a convex mirror is the most appropriate in those areas where large objects have to be viewed in a tiny size. This makes it easy for a person to view the image. 

 

How does a Convex Mirror Differ from a Concave Mirror?

A convex mirror is also commonly called a curved mirror. Where the reflective surface of a concave mirror bulges towards the inward direction, in the case of a convex mirror it bulges in the direction where the light source is located. 

 

The image of the object formed by a concave mirror is bigger than the actual size of the object. In the case of a convex mirror, it makes the object appear smaller in size. A concave mirror is used to magnify the image of the object but a convex mirror reduces the image size so that it gets accommodated easily in the mirror. 

 

What are the Ways to recognize a Convex Mirror?

If you have to find which one is a convex mirror out of the different types of mirrors like a plane mirror, concave mirror, and convex mirror, then it can be done just by looking at the size of the image. 

 

When you use a plane mirror, you will find that the size of the image generated by it will be the same as the actual size of the image. In the case of a concave lens, you will find that when the object is placed near the mirror, its image gets enlarged. 

 

You will see an inverted form of the image when the object is kept at a far distance from the mirror. It is only a concave mirror that has the ability to generate an actual inverted image. The size of the image produced by a concave mirror is based on the screen placement. In the case of a convex mirror, it always generates a small-sized straight image.

 

Why is a Convex Mirror called a Diverging Mirror?

A convex mirror is given the name of a “diverging mirror”. It is because of the behavior of the mirror that makes light rays diverge upon reflection. By this, it means that when a light ray is directed on this mirror, it enables the originally parallel light rays that form the beam to deviate/diverge after hitting the reflective surface of the convex mirror.

 

Understanding the working of a Convex Mirror

In a convex mirror, there is a point where light rays received from all the different directions meet. This helps in creating a focused beam. This point is referred to as the “focal point” or the focus. There is another term called “focal length” that determines the distance between the front of the convex mirror and the focal point. 

 

Now let us understand the type of image formed by a convex mirror. A curved or convex mirror bends outward in the shape of a dome. It always forms an imaginary image. The reason behind it is that the “center of curvature” and the point of focus are both imaginary points in the mirror. Both of these points are unable to be reached. 

 

It implies that the type of image made by this type of mirror can’t be projected on the screen. The image generated by the convex mirror is an upright virtual image that is relatively smaller in size than the size of the projected object.

 

Below are the major possibilities of a convex mirror for the object positioning in a convex mirror.

 

When an object is located at infinity

When the object lies at infinity, then a tiny image is produced at a principal focus point. This point is located behind the mirror. You will see that the image produced is virtual, highly diminished and erect.

 

When an object lies between the pole and infinity of the mirror

When an object lies between the pole and infinity of this mirror, then you will see a diminished form of an erect and virtual image. This image is created between focus and pole behind the convex mirror.

 

What are the Areas of Application of a Convex Mirror? 

There are different functions performed by a convex mirror that makes it useful in a variety of applications. Let us see some of the important uses of convex mirrors. 

Automobiles: Safety and efficiency are the two most essential requirements when you drive on the road. By providing a good overview to drivers about the rear and side view of the roads, and surroundings, help drivers to foresee risky situations and avoid accidents. The convex mirror is one of the important things that offer a clearer rear view of the road to ensure maximum safety all the time. 

 

A convex mirror has broader fields of view as compared to reflective surfaces of concave and plane mirrors. Due to this, convex mirrors are used in forming side mirrors off an automobile. These mirrors provide a fish-eye view to the person who drives the automobile. This feature helps them to get a clear view of the rear area of the road easily.

 

The virtual image created of the object has a smaller size than its actual size. It provides a wide field view. Convex mirrors provide heightened safety for motor drivers on roads, alleys, and driveways where very little visibility is there.

 

Automated Teller Machines 

Convex mirrors are kept close to the ATM or automated teller machine to permit the customers to figure out whether someone is standing behind them. This is used to ensure the security of the customers while they use an ATM. It ensures safety from the theft of withdrawals of cash or any other precious item. Also, it keeps the identity of the machine user safe and secure.

 

In-Camera 

Some mobile phones come with a selfie camera built into them that aids users to capture a self-portrait shot.

 

Inside Buildings

Large hospitals, hotels, schools, offices, apartment buildings or stores use convex mirrors to allow people to view what goes around a corner. This helps in avoiding minor or major collisions.

 

In Sunglasses 

Convex mirrors are used in developing a sunglass lens. The overall purpose is to reflect the sunlight away from the pers
on who wears these glasses.

 

In the Magnifying Glass

Two convex mirrors when placed next to each other can form a magnifying glass.

 

In Street Light

A convex mirror is used as a street light reflector due to its ability to spread light rays over a larger region.

 

In Telescope

Convex mirrors are used in telescopes to see far off objects clearly in the form of point-sized images. 

  

Production Process

Another important use of a convex mirror is in the work environment at the time of the production process. A convex mirror placed on the conveyor belt assists in viewing a product from varying angles. 

 

This enhances the quality of the product by getting aware of any kind of production-based faults and improves the efficacy of production processes. This happens by eliminating the requirement to assess the quality of products on the conveyor belt.

Curie point is also named as Curie Temperature. Curie temperature is the temperature above which some changes are made due to its impact on certain magnetic materials. Curie temperature diminishes the magnetic properties of the material. 

If you consider some rocks and minerals, then you will notice that there is remnant magnetism. Also, the remnant magnetism appears below the Curie point.

The temperature is about 570 °C (almost 1,060 °F). This is the result of the general magnetic mineral magnetite. ‘Pierre Curie’ – is the name behind the temperature of Curie point. Find the definition of curie point in brief. 

Do you know the curie point of nickel? 

Nickel Curie point possesses a temperature of 627 K. 

Curie Temperature Definition

Curie point definition is very easy to understand. It is named after the French physicist. He had discovered it in 1895. He also put forward certain laws that were related to some magnetic properties in temperature change.

Let’s find something interesting that is related to the Curie point. Consider an example of iron—atoms with a temperature of 770 °C (1,418 °F). Each iron atom acts as a tiny magnet at this temperature spontaneously. Each of them will align themselves as some kind of magnetic material. 

In the case of pure iron, the atomic magnets are distributed within each microscopic region. Pure iron is considered among the categories of Ferromagnetic materials. The directions of the magnetic fields are the same so that their magnetic fields strengthen each other.

Well, you won’t find the same in antiferromagnetic materials. Their materials possess the atomic magnets that have the alternate property of magnetic fields. They act in opposite directions. This tendency explains that their magnetic fields cancel each other. 

You may notice different types of spontaneous arrangements in ferrimagnetic materials. This is a mixture of both patterns. They are generally involved in two types of magnetic atoms. This feature enables them to yield the property of partial reinforcement of magnetic fields.

Miscellaneous Facts on Currie Point

Three classes are there that involve the raising of temperature to the Curie point. They apply to any type of material. Numerous spontaneous arrangements are found in these types of disrupts. 

Among them, only a few weak magnetic behaviours exist. All of these processes are kind of more general. We call them paramagnetism. For information, you should know about the highest Curie point as the value for cobalt is 1,121 °C (2,050 °F). 

The rise of temperature above the Curie point can lead to the production of roughly similar patterns of decreasing paramagnetism. This behavior is constant in all three classes of materials. 

When you try to cool down the temperature of these materials below their Curie points, the magnetic atoms will start to realign spontaneously. This is the behaviour that initiates the effect of ferromagnetism or antiferromagnetism among the metals.

Néel temperature is the behavior that is pointing towards the antiferromagnetic Curie point. The name is given to the term in honour of the French physicist Louis Néel. He had explained antiferromagnetism successfully in 1936.

Application of Curie Point

Steel wire has atoms that tend to behave as a magnet when they are subjected to the electric field.  At this moment, they act like tiny magnets. Each end of the steel rod turns into a north and south pole. 

Generally, these atoms do not possess any significant direction of the magnetic field. They all are in different directions. So, you can say that the steel does not exhibit any type of net magnetic field. 

At a moment when you try to bring a magnet close to the wire, it makes the steel atoms close to each other and stays in a lineup format. The queued atomic magnets help the steel wire to convert into a magnet. The steel does not have any magnetic behavior in nature, but it turns into something that attracts the original magnet.

The process of magnetization can be disturbed by high temperatures. Thermal energy is very much responsible for the steel atoms to wobble back and forth. The energy leads to the disturbing tendency of the magnetic alignment. 

When you notice the maximum vibration among the atoms, the behaviour of being the atomic magnets do not remain as usual. At this moment, the steel gives up its magnetism. Curie point is the reason for which this occurs.

Do You Know?

You must have heard of a core of molten iron inside the earth. This iron ore cannot be magnetized, which has a temperature above the Curie point.

How the earth behaves as a magnet and possesses a magnetic field. Due to its magnetism, it has a North and a South magnetic pole. An electromagnet is a reason for the generation of the magnetic field of the earth.  

Do you know why? This is due to the passage of electrical currents flowing through the liquid metal core deep inside the earth.

Being a science enthusiast, you may have come across the word dark matter, or maybe you already have a theory of your own. Well, let’s take a deeper dive into the dark matter, and let’s find out what is dark about it. Dark matter may be defined as a form of matter considered to be approximately 95 percent of the matter of the universe. After several scientific studies, it is assumed that dark matters are about 27% of its total mass-energy density or about 2.241×10⁻²⁷ kg/m³. In this article, we are going to discuss dark matter theory, the meaning, and its discovery in detail. 

The Discovery

The existence of the ‘missing mass’ or ‘dark matter’ was first proposed by Swiss American astronomer Fritz Zwicky. In 1933, he discovered that only 1 percent of the mass of all the stars in the Coma cluster is needed to keep the galaxies from escaping the gravitational pull of the cluster. In the 1970s, this theory was approved upon observations by American astronomers, Vera Rubin and W. Kent Ford. They observed, within a galaxy, the mass of the visible stars is only about 10 percent that is required to keep the stars orbiting the center of the galaxy. To account for the slightly increasing (or constant) orbital velocity of the stars, the mass of the galaxy within the orbit of the star must be increasing linearly with the distance of the center of the galaxy from the stars.

Dark Matter Meaning

There are several observations to imply the presence of dark matter such as gravitational effects. Until and unless more matter is present beyond our observations, they are not explained by accepted theories of gravity. Dark matters are substances that do not absorb, reflect or emit light and hence not visible. Dark matter is a non-interacting substance, and this is what makes it dark and mysterious. It is considered that about 5 percent of the universe is known to us. The rest 95 percent includes about 27 percent of dark matter and 68 percent of dark energy. We are pretty sure of the existence of dark matter, but still to find out what dark matter is and how it exactly works.

Dark Matter Theory

The existence of dark matter can be confirmed upon the discoveries of Swiss American astronomer Fritz Zwicky and Jan Oort. According to them, the motion of the galaxies and nearby stars of the Coma clusters violates the expected motions according to Newton’s law of gravity. But still, any direct evidence of the presence of the dark matter is not encountered.

According to NASA, we know more about what a dark matter isn’t than what it is. Dark matter cannot be seen in the form of our known cosmic objects such as stars and planets. It is not in the form of baryons as we know we would be able to detect baryonic clouds by their absorption of radiation passing through them. Again, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter collides with another matter. Based on how many gravitational lenses we see, we can rule out large galaxy-sized black holes. High concentrations of matter cause the bending of light, which passes near them from objects further away. But we cannot see enough lensing events so that it can suggest that such objects have the required 25% contribution of dark matter. There must be several theories about the existence of dark matter, but no direct evidence is acquired till now. 

Did You Know?

Dark matter is a subject that gives us goosebumps, thinking about the fact that it is yet to be discovered. The Discovery of dark matter and dark energy would give us a better picture of the universe and how it exactly works. Here are some facts about dark matter. The substance of dark matter doesn’t interact with light and that is why dark matter is invisible. Though dark matter cannot be seen, it can be felt for its powerful cosmic impact. It is predicted that the powerful impact of dark matter binds the galaxies together. According to some research, dark matter might be cold in nature. Also, dark matter might not have any existence.

Heat Equation

Heat is a form of energy that transfers from one medium to the other mediums and it usually travels from the hotter region to the colder region of the conductor. There are ways to transfer the heat according to the medium of the conductor. Heat is transferred in solids by the process of conduction, in liquids and gases by the process of convection, and electromagnetic waves in the form of the radiation of heat. The heat equation in one dimension is a partial differential equation that describes how the distribution of heat evolves over the period of time in a solid medium, as it spontaneously flows from higher temperature to the lower temperature that will be the special case of the diffusion.

Heat Equation Derivation

Derivation of the heat equation in one dimension can be explained by considering a rod of infinite length. The heat equation for the given rod will be a parabolic partial differential equation, which describes the distribution of heat in a rod over the period of time. As the heat energy is transferred from the hooter region of the conductor to the lower region of the conductor.

The form of the equation is given as:

[frac{∂u}{∂t}] = α[[frac{∂^{2}u}{∂x^{2}}] + [frac{∂^{2}u}{∂y^{2}}] + [frac{∂^{2}u}{∂z^{2}}]]

where, α is a real coefficient of the equation which represents the diffusivity of the given medium.

 

Derivation of the Heat Equation in One Dimension

The amount of heat energy required to raise the temperature of  the given rod by ∂T degrees is 

CM. ∂T, which is  known as the specific heat of the conductor,

Where,

C –  positive physical constant of heat determined by the conductor
M – the mass of the conductor

The rate at which heat energy transferred in the surface of the conductor is directly proportional to the surface area and the temperature gradient at the surface of the conductor and this constant of proportionality is known as the thermal conductivity of heat which is denoted by K

Consider a rod of finite length with cross-sectional area A and mass density ρ.

The temperature gradient of the function is given as

[frac{∂T}{∂x}](x + dx,t)

The rate at which the heat energy transferred from the right end of the given rod is given as

KA[frac{∂T}{∂x}](x + dx,t)

The rate at which the heat energy transferred from the left end is given as

KA[frac{∂T}{∂x}](x , t)

As the temperature gradients are positive from both ends then the temperature of the conductor must increase.

As the heat flows from the hot region to a cold region of the given rod, heat energy should enter from the right end of the rod and transferred to the left end of the rod.

So the equation as per the condition is given as

KA[frac{∂T}{∂x}](x + dx,t) – KA[frac{∂T}{∂x}](x , t)dt where it is the time period.

Now the temperature change in the given rod is can be written as

[frac{∂T}{∂x}](x , t)dt

The mass of the rod will be

Density = mass/volume

  ρ = M/A.dx

M =  ρA.dx

Now, the heat equation can be written as

CρAdx[frac{∂T}{∂x}](x , t)dt = KA[[frac{∂T}{∂x}](x + dx,t) – [frac{∂T}{∂x}](x , t)]dt

Dividing both sides of the above equation by dx and dt and taking limits of it dx and it ->0, then CρA[frac{∂T}{∂x}](x , t) = KA[frac{∂^{2}T}{∂x^{2}}](x , t)  

The equation will be,

[frac{∂T}{∂x}](x , t) = α[^{2}][frac{∂^{2}T}{∂x^{2}}](x , t)

Where,

α[^{2}] = [frac{K}{Cρ}]

is the thermal diffusivity of the given rod. 

Hence the above-derived equation is the Heat equation in one dimension.

There are so many other ways to derive the heat equation. However, here it is the easiest approach.  In detail, we can divide the condition of the constant in three cases post which we will check the condition in which, the temperature decreases, as time increases. It is the phenomena of the heat or any form of energy that they will lose energy while traveling from one medium to the other.  Ultimately after the integration, we will get the same equation of the heat in one dimension.

 

Application of the Heat Equation

• The heat equation is used to modify the automobile engines, as it tells you about the specific heat of the conductor which gives you the idea about the rate of heat absorption by the engine and capacity to hold the heat.

• The most common use in the medical field is when the patient gets relief from pain with the help of the hot water bag. In that case, the heat gets transferred from the hotter region to the colder region.

You might have seen on hilly areas, tiny drops rest on the cool surfaces at night. Also, you might have seen beautiful shiny drops on leaves during winters. Do you know what these two statements pertain to?

Well! We find dew on the cold surfaces and dewdrops form on the grass in winter that’s why we find a lustrous view on leaves.

So, do you know the following things?

1. How dew is formed? 

2. How to dew?

3. Use of dew gauge to know about dew.

4. Difference between dew and fog

If not, this article will discuss how dew drops are formed with a dew factor and how to distinguish between dew and fog.

How are Dew Drops Formed?

We all are surrounded by air. This air contains moisture during the monsoon. At his moment, we call this air the humid air. 

Now, when there is a temperature rise, this temperature rise leads to the evaporation of water, and the evaporations result in the formation of water vapours.

So, we understood that around us contains water vapours which we call moisture or humidity. Hot air contains more moisture as compared to cool air. During the chilled night when the hot air comes in contact with the cold surface, water vapour present in it condenses and takes the form of droplets, which is the exact dew definition.

In simple words, the hot air condenses and form tiny droplets that make a coating on the cold surfaces and the leaves of the grassland. A beautiful scenic view of the dew is shown below:

Now, let’s understand the science behind the formation of dew:

Formation of Dew

Water vapour condenses into droplets relying on the temperature. The temperature at which droplet formation occurs is called the dew point. When the surface temperature drops, gradually reaching the dew point, atmospheric water vapour condenses to form small droplets on the surface, and that is how dew drops are formed. 

From the above text, we understood how dew formed. Now, we will distinguish between dew and frost.

Difference Between Dew and Fog

So far we understood what is dew, but we are still left with the concept of fog. 

Fog is a mist that we encounter while driving on hilly roads. We also hear in the news that due to the fall of an iceberg, the temperature may lower and that is the time when the days are unclear and mist tightly hugs the mountains. The unclear vision we find in the below image is the fog:

The process of the formation of dew distinguishes it from hydrometeors, i.e., meteorological occurrences of water that form directly in the air that cools to its dew points (around condensation nuclei), and this is called the fog or frost. The thermodynamic principles of formation for both are the same. However, dew is usually formed at night.

Now, let’s discuss the difference between dew and fog in a tabular form:

Distinguish Between Dew and Fog

Important Facts

The occurrence of these tiny droplets on the ground results in difficulty among the players to have a grip on the ball. This difficulty is called the dew factor.

A Dew gauge is a device that measures the amount of dew on the surface.

Conclusion

Dew

• Dewdrops are formed when water vapour condenses and it rests on the cold surfaces, metals, leaves, grasslands, and so on.

• If we were to distinguish between dew and frost, dew is a tiny droplet that we are already knowledgeable about; however, when these condensed drops freeze further in a cold environment like Ladakh, Yukon, Verkhoyansk, Vostok; these droplets turn to frost.

Fog

• Fog is something that appears to be close to the earth’s surface and we observe this beauty while driving in hill stations. However, the fog never rests on the surface, it remains in the air. 

• Fog is more often considered the mist that is a thick cloud of tiny water droplets covering the mountains.