In this lesson, we will cover several important concepts related to temperature and heat. First, we will define temperature as the quantity that determines the direction in which thermal energy flows. We will also define heat as the energy transferred due to a temperature difference between two objects.

Next, we will list the basic properties needed to construct a thermometer and learn how to convert temperatures between different scales, including Fahrenheit, Celsius, and Kelvin. We will discuss how the rise in temperature of a body is related to an increase in its internal energy.

We will define heat capacity and specific heat capacity, and describe the concepts of heat of fusion and heat of vaporization, which involve energy transfer without a change in temperature during phase changes.

Additionally, we will conduct experiments to determine the heat of fusion and heat of vaporization for ice and water, and we will sketch temperature-time graphs to illustrate these processes.

Finally, we will explain evaporation, highlighting the differences between boiling and evaporation, and discuss how evaporation leads to cooling. By the end of this lesson, you will have a clearer understanding of these fundamental concepts in thermodynamics.

TEMPERATURE AND HEAT

When we touch an object, we can sense whether it feels hot or cold. This sensation relates to the temperature of the object, which indicates how hot or cold it is. For example, a candle flame is hot and has a high temperature, while ice is cold and has a low temperature. Although our sense of touch can give us a rough idea of temperature, it is not always reliable or safe, especially when dealing with hot objects. Therefore, we need a more accurate method to measure temperature.

To understand temperature better, we can look at the concepts of thermal contact and thermal equilibrium. For instance, when storing ice in summer, people often wrap it in cloth or keep it in a thermos flask to minimize its contact with warmer surroundings. This helps prevent the ice from melting quickly. Similarly, when you set a cup of hot tea in a room, it gradually cools down. However, it stops cooling once it reaches the same temperature as the room. This shows that heat flows from hot objects to cold ones until they reach thermal equilibrium.

When two objects at different temperatures touch, the hotter object loses energy, causing its temperature to drop, while the colder object gains energy and its temperature rises. This transfer of energy between the two objects is referred to as heat.

In summary, heat is the energy transferred from one body to another due to a temperature difference. Once heat enters an object, it becomes part of that object’s internal energy and is no longer considered heat energy.

Internal energy is the total kinetic and potential energy associated with the atoms and molecules in a substance. It depends on several factors, including the mass of the object and the kinetic and potential energies of its molecules. The kinetic energy of these molecules is related to their motion and temperature, while potential energy arises from the forces between them.

THERMOMETER

A thermometer is a device used to measure temperature. Certain substances change in response to temperature variations, making them suitable for use as thermometric materials. For instance, some materials expand when heated, change color, or alter their electrical resistance. Most liquids expand when heated, which is why they are commonly used in thermometers.

A good thermometric liquid should have several properties: it needs to be visible, expand uniformly with temperature changes, have a low freezing point and a high boiling point, not wet glass, conduct heat well, and have a low specific heat capacity.

A liquid-in-glass thermometer consists of a bulb connected to a thin capillary tube. When the bulb comes into contact with a hot object, the liquid inside expands and rises in the tube. Mercury is commonly used in thermometers because it remains liquid over a wide temperature range, freezing at -39 °C and boiling at 357 °C. Mercury thermometers are often used in laboratories and homes to measure temperatures between -10 °C and 150 °C.

Thermometers have a scale marked on their stem, which includes two fixed points: the lower fixed point is marked when the thermometer is placed in ice, and the upper fixed point is marked when it is placed in steam above boiling water.

There are three common temperature scales:

1. Celsius Scale: The interval between the lower and upper fixed points is divided into 100 parts, with 0 °C for the lower point and 100 °C for the upper point.
2. Fahrenheit Scale: This scale divides the interval into 180 parts, with the lower fixed point at 32 °F and the upper at 212 °F.
3. Kelvin Scale: In the SI system, temperature is measured in kelvins (K). The interval between the fixed points is divided into 100 parts as well. The lower fixed point is 273 K and the upper is 373 K. Absolute zero, the lowest possible temperature, is 0 K, which corresponds to -273 °C.

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