Heat & Temperature | Lucent for GK - UPSC PDF Download

Introduction

Heat is a form of energy that is transferred between objects or systems due to a temperature difference. It is the energy in transit from a hotter object to a colder object. Heat can be transferred by three main mechanisms: conduction, convection, and radiation.

Units of heat

Heat is measured in terms of energy and is typically expressed in joules (J) or calories (cal). The SI unit, joule, is commonly used for scientific calculations. One calorie is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.

Scales of temperature measurement

Temperature is a measure of the average kinetic energy of the particles in a substance. There are three commonly used scales of temperature measurement:

• Celsius (°C): The Celsius scale is based on the freezing and boiling points of water. The freezing point of water is assigned 0°C, and the boiling point is assigned 100°C at standard atmospheric pressure.
• Fahrenheit (°F): The Fahrenheit scale is commonly used in the United States. It is based on the freezing and boiling points of water, with 32°F as the freezing point and 212°F as the boiling point.
• Kelvin (K): The Kelvin scale is the SI scale of temperature. It is based on the absolute zero point, which is the lowest possible temperature where all molecular motion ceases. The Kelvin scale is the most commonly used scale in scientific calculations.

Range of different thermometers

Different types of thermometers are used to measure temperature over various ranges. Here are a few examples:

• Mercury-in-glass thermometer: This type of thermometer consists of a glass tube containing mercury. It is used for measuring temperatures ranging from -38°C to 356°C.
• Alcohol-in-glass thermometer: Similar to the mercury-in-glass thermometer, this thermometer uses alcohol instead of mercury. It is used for measuring temperatures below -38°C.
• Thermocouples: Thermocouples are made of two different metals joined together. They can measure a wide range of temperatures, from very low temperatures to extremely high temperatures, depending on the type of thermocouple used.
• Infrared thermometers: These thermometers measure temperature without physical contact. They can be used for measuring the temperature of objects at a distance, making them useful in industrial applications.

Total radiation pyrometer

A total radiation pyrometer is an instrument used to measure high temperatures without physical contact. It operates based on the principle of total radiation emitted by a hot object. It consists of a lens or mirror system that focuses the radiation onto a detector, such as a thermopile. The output of the detector is then used to determine the temperature of the object being measured.

Change of state

In physics, a change of state refers to the transition of a substance from one physical state to another. The three main states of matter are solid, liquid, and gas. The change of state can occur through heating or cooling, and it involves the absorption or release of heat energy.

• Solid to liquid: This change is called melting or fusion. Heat energy is added to the solid, increasing its temperature until it reaches the melting point. At this point, the solid begins to change into a liquid while maintaining a constant temperature.
• Liquid to gas: This change is called vaporization or evaporation. Heat energy is added to the liquid, increasing its temperature until it reaches the boiling point. At the boiling point, the liquid changes into a gas, and the temperature remains constant during the process.
• Solid to gas: This change is called sublimation. Some substances can directly change from the solid state to the gas state without passing through the liquid state. Dry ice (solid carbon dioxide) is an example of a substance that undergoes sublimation.

Latent heat

Latent heat is the heat energy absorbed or released during a change of state without a change in temperature. It is associated with the breaking or formation of intermolecular bonds between particles. There are two types of latent heat:

• Latent heat of fusion: This is the heat energy absorbed or released during the process of melting or freezing. It is the amount of heat required to change one unit mass of a substance from solid to liquid or vice versa at a constant temperature.
• Latent heat of vaporization: This is the heat energy absorbed or released during the process of vaporization or condensation. It is the amount of heat required to change one unit mass of a substance from liquid to gas or vice versa at a constant temperature.