Basic Physics Tutorials

Thermometers and temperature scales

Article about Thermometers and temperature scales

1. Thermometers

The tool designed to measure temperature is a thermometer. There are many types of thermometers, but the working principle is the same. Usually, we use thermometric objects, the nature of matter that changes with temperature. If the temperature of the object changes, the shape, and size of the object also changes. Most thermometers use objects that can expand or shrink when the temperature changes. Thermometers that are often used consist of glass tubes, where there is alcohol or mercury in the center of the tube. When the temperature increases, alcohol or mercury in the container expands so that the length of the alcohol or mercury column increases. Conversely, when the temperature decreases, the length of the alcohol or mercury column decreases. On the outside of the glass tube, there are numbers which are the scale of the thermometer. The number shown by the upper end of the alcohol or mercury column states the value of the temperature of the object being measured.

The Zeroth Law of Thermodynamics

Article about The Zeroth Law of Thermodynamics

So far, we have only observed the thermal equilibrium experienced by two objects in contact.

To understand the concept of thermal equilibrium in more depth, let’s review three objects (say objects A, object B, and objects C). For example, object B and object C do not touch each other, but object A in contact with object B and object A in contact with object C. Observe the image below. Because in contact with each apart from object A and object B are in thermal equilibrium, so also object A and object C are in thermal equilibrium. Are objects B and C that do not touch each other also in thermal equilibrium? If we only use logic, we can say that object B and object C are also in thermal equilibrium, even though they are not in contact. Object A and object B are in thermal equilibrium, meaning the temperature of the object A = the temperature of object B.

Definition of temperature and thermal equilibrium

Article about Definition of temperature and thermal equilibrium

Definition of temperature

Ever touched ice? What do you feel when your hands touch ice? What if what you touch is fire? When touching ice, your hands feel cold, when you touch the fire, your hands feel hot. Hot, warm, cool, cold states what?

The concept of temperature starts from the heat and cold experienced by our sense of touch. Based on what is felt by the sense of touch, we say an object is hotter than another object or an object is cooler than another. Hot objects have higher temperatures, while cold objects have lower temperatures. The cooler an object, the lower the temperature. Conversely, the hotter an object, the higher the temperature. The size of the heat or cold of an object is called temperature. In the subject of gas kinetic theory, you will understand more deeply the definition of temperature; what happens to the molecules from an object so that it can feel hot, warm, cool or cold.

Phases of matter (based on microscopic properties)

Article about Phases of matter (based on microscopic properties)

In everyday life, we often encounter three different phases of matter. There are solid substances (e.g., stones, iron, etc.), liquids (water, gasoline, etc.) and gas substances (air, etc.). The three-phase of these substances can be distinguished based on their ability to maintain their shape and size.

Solids usually maintain a fixed shape and volume. The liquid does not keep an attached form, but adjusts its way to the container that is occupied. For example, if we put water in a glass, the shape changes like a glass. If water is put into the bathtub, the shape changes like a bathtub. The volume of liquid is typically always fixed. A glass of water if it is placed in a bath, the amount of water remains in a glass. The shape of the water can change, but the size never varies. Keep in mind that the number of solids and liquids can change if given a considerable force.

Atomic theory and kinetic theory

Article about Atomic theory and kinetic theory

Atomic theory

For thousands of years, the ancient Greeks believed that every pure substance (such as gold, iron, etc.) consisted of atoms. According to them, if a pure substance is cut into small pieces, then the tiny parts are cut again, then cut back… and so on, then there will be the smallest pieces that cannot be cut again. The smallest pieces that cannot be cut again are called atoms. Atom means “cannot be divided” (Greek language)

At that time, the atom is considered to be no longer divided. But later some scientists discovered electrons and atomic nuclei (protons and neutrons) so that the assumption that atoms could not be subdivided was wrong. So, atoms consist of electrons (negatively charged) and atomic nuclei. Electrons circle the nucleus. Inside the nucleus, there are protons (positively charged) and neutrons (neutral or not charged).

Changes of phase Critical temperature Triple point

Article about Changes of phase Critical temperature Triple point

In the discussion of the ideal gas law, it has been explained that the ideal gas law describes the behavior of real gas accurately only when the pressure and density of real gas are not too large. If the pressure and real gas density are large enough, the ideal gas law provides inaccurate results, likewise, when the temperature of real gas approaches the boiling point. This is related to the interactions that occur between real gas molecules. Gas pressure is inversely proportional to gas volume. When the gas pressure is large enough, the gas volume becomes smaller. Because the gas volume is low, the distance between the gas molecules becomes closer. When the distance between molecules becomes closer, the molecules attract each other. It’s like when you put a piece of iron on a magnet. If the distance between the magnet and iron is far enough, the magnet cannot pull iron. But if the distance between magnet and iron is close, iron is drawn closer.

Van der Waals Equation of State

Van der Walls is the name of a Dutch physicist, J. D. van der Waals (1837-1923). The Van der Waals Equation of State is an equation of state of a gas, similar to The equation of state of an ideal gas. The difference is, The equation of state of an ideal gas cannot provide accurate results if the pressure and density of real gas are large enough. Whereas The Van der Waals Equation of State can produce more accurate results.

The existence of this equation originated from Van der Waals, who realized the limitations of the equation of state of an ideal. Waals, modify the equation of state of the ideal gas, by adding several factors that also influence the real gas condition, when the pressure and density of the real gas are large.

Evaporation

The evaporation process can be explained using kinetic theory. Like gas molecules, water molecules also move. The difference is that water molecules cannot be scattered because the attraction between molecules can still hold them together. Conversely, the attraction between the gas molecules is fragile, so that the gas molecules cannot fuse. When moving, water molecules have velocity. There are water molecules that have high speeds; there are also water molecules that have a small velocity. The distribution of the velocity of the water molecule resembles the Maxwell distribution.

Evaporation occurs when the speed of the water molecule is large enough so that the attraction between the water molecules is unable to hold it together. Similar to rockets moving into space, the speed of a rocket is large enough so that the gravitational force of the earth cannot hold it to stay on earth. Note that only molecules with large velocities can escape from the attraction between molecules. The molecules with small speeds remain together like water.

Boiling

Boiling is a process of changing liquid into gas. Boiling occurs when the saturated vapor pressure is equal to the air pressure (air pressure = atmospheric pressure). We only discuss boiling water. The saturated vapor pressure of water is directly proportional to the temperature of the water, the higher the water temperature, the higher the pressure of saturated vapor. When we heat water, small bubbles usually appear on the bottom of the container. The existence of bubbles indicates the change in a liquid into a gas. If the saturated vapor pressure in the bubble is smaller than the outside air pressure, the bubble will shrink and disintegrate before arriving at the surface. Bubbles are destroyed because the thrust force of the outer air is higher than the thrust force of the steam inside the bubble. The external air pressure is higher than the vapor pressure in the bubble so that the outside air has a more significant force (P = F / A).

Humidity

Humidity states the amount of water vapor in the air. When it rains, the atmosphere is very humid because there is a lot of water vapor in the air. Conversely, if the water vapor in the air is very little, the air is arid. The amount of water vapor in the air is expressed by the relative humidity.

The relative humidity is the ratio of the partial pressure of steam to the pressure of saturated vapor of water at a certain temperature (steam is water vapor). Relative humidity is expressed in percent, mathematically formulated: