Definitions and functions of temperature and heat. Science summary for middle school.The difference between a cold body and a hot body is that the atoms of a hot body move faster than those of a cold body. If the body is a solid its atoms cannot move freely but vibrate around their fixed positions. The higher the temperature of a body, the faster the motion of its molecules.
Heat is a form of energy called thermal energy . The increase in temperature of a body depends on the amount of heat absorbed. The greater the amount of heat absorbed, the higher the temperature reached.
Heat is the total thermal energy possessed by a body , while temperature expresses the speed of the individual atoms of that same body.
TEMPERATURE: MEASUREMENT
The thermometer is the instrument used to measure temperature . A common type of thermometer is the column thermometer. It is made up of three elements: bulb, stem and graduated scale. The bulb is the sensitive part of the thermometer, filled with liquid that can be mercury or colored alcohol . To measure the temperature of a body, the bulb must be placed in close contact with it.
The stem is a thin glass tube in which the liquid rises and falls according to the temperature variation. The graduated scale is used to translate the level of the liquid column into a numerical measurement of the temperature. The unit of measurement of temperature is the degree centigrade and its symbol is °C. Temperatures below 0 are written preceded by the minus symbol (-). There are different types of thermometers:
- The clinical thermometer ;
- The weather thermometer ;
- The digital thermometer .
To build a thermometer scale, two reference points are needed, two fixed and constant temperatures :
- The 0 °C point which corresponds to the temperature of melting ice, that is, the ice that is transforming into water.
- The 100°C point which corresponds to the temperature of boiling water.
The range 0-100 is divided into 100 equal parts, each corresponding to one degree centigrade. This scale is called centigrade because the interval between the two fixed points is divided into 100 degrees. This scale is also called the Celsius scale in honor of the Swedish astronomer who first proposed it in 1742.
In certain situations it is important to know how the temperature varies in a given interval of time. In these cases, measurements should be taken at regular intervals of time and the data recorded in a table. It is also useful to draw a line graph with the collected data.
THERMAL EXPANSION
The expansion of bodies caused by an increase in temperature is called thermal expansion .
Thermal expansion depends on two main factors:
- The variation of temperature
- The type of materials
The volume changes of objects we use every day are so small that we cannot see them with the naked eye. We can demonstrate the thermal expansion of solids with the Gravesande ring experiment , named after its inventor.
The device consists of an iron ring and a sphere of the same metal suspended from a chain. At room temperature, the sphere passes exactly through the ring. We heat the sphere over an alcohol burner and try to pass it through the ring again.
We notice that it no longer passes through because it has expanded. After a few minutes, when it has cooled down, the sphere passes through the ring again. Small objects undergo thermal expansions that are invisible to the naked eye , while in a very large object the expansion becomes visible and its effects can be very important.
To demonstrate the thermal expansion of liquids, a thermoscope is used . It is a simple device consisting of a small glass bottle topped by a thin glass tube. The bottle is full of liquid. If we warm it between our hands, the level of the liquid in the tube rises. This means that the volume of the water has increased. If we cool the bottle, the water in the tube drops, demonstrating that the volume of the water has decreased. Liquids undergo thermal expansion to a different extent depending on the type of liquid.
Water is the only liquid that does not follow the law of thermal expansion in the range between 0 °C and 4 °C . If we freeze a glass of water we notice that the level of ice formed is higher than the initial one, by melting it we notice that its volume decreases. Below 0 °C and above 4 °C water contracts and expands like all other liquids.
To demonstrate the thermal expansion of gases, you need to do a simple experiment with a plastic bottle and a balloon. First, you need to cool the air in a bottle by placing an empty plastic bottle without a cap in the freezer. After an hour, you take the bottle and attach a balloon to the neck of the bottle. As the bottle heats up, the balloon inflates. This means that the volume of air in the bottle has decreased. Therefore, gases, like solids and liquids, follow the law of thermal expansion .
HEAT PROPAGATION
If two bodies in contact have different temperatures , heat passes spontaneously from the hotter body to the colder one. The passage of heat continues until the two bodies reach the same temperature. In this case, the two bodies are said to be in thermal equilibrium. Heat propagates in three different ways: by conduction, by convection, by radiation .
Conduction is the transmission of heat that occurs by contact, without the movement of matter. It is characteristic of solid bodies. Some solid materials such as silver, copper and aluminum conduct heat very quickly and are called good thermal conductors. Other materials such as wood, glass, polystyrene conduct heat very slowly and are called poor conductors or thermal insulators.
When we turn on the radiators, the house heats up in a short time. While the hot air rises, other cold air is attracted towards the radiator, from below. The cold air in turn heats up and starts to rise again. In this way, a cycle of air currents is created that transport heat to all areas of the room, called convective motion. Convection is the transmission of heat that occurs through the movement of matter between areas at different temperatures. It is characteristic of liquids and gases.
Radiation is the transmission of heat in the form of thermal rays . Thermal rays or infrared rays are a particular type of electromagnetic waves. The heat of the Sun must overcome 150 million kilometers of empty space to reach us, so it uses the method of radiation.
STATE CHANGES
When a substance changes from one state of aggregation to another, we say that it has undergone a change of state .
There are six possible state changes:
- From solid to liquid and vice versa (melting and solidification)
- From liquid to gas and vice versa (vaporization and condensation)
- From solid to gas and vice versa (sublimation and frosting)
To cause a change of state to occur, it is necessary to add or subtract heat from a body.
Melting is the transition from a solid to a liquid state. Solidification is the transition from a liquid to a solid state. The melting point of a substance is the temperature at which it melts, that is, it changes from a solid to a liquid state.
Vaporization is the passage from the liquid state to the gaseous state. Condensation is the passage from the gaseous state to the liquid state. The passage from the liquid to the gaseous state can occur in two ways: evaporation, during which the molecules of the liquid detach from its surface and disperse in the form of gas and boiling, the vapor bubbles form inside the liquid and are expelled outside, and boiling occurs at a very specific temperature, and together they constitute vaporization .
The direct passage from the solid state to the gaseous state, without passing through the liquid state, is called sublimation. The direct passage from the gaseous state to the solid state is called frostbite .
