Thermal equilibrium and heat exchange

Understanding the concepts of thermal equilibrium and heat exchange is important to gain a deeper understanding of how temperature and heat interact in our daily lives. In this explanation, we will go over these fundamental ideas in a clear and straightforward way. We will include examples, diagrams, and analogies to help you understand these concepts.

What is heat?

Heat is a form of energy. It is what we feel when we touch something warm or hot. Heat moves from warmer areas to cooler areas. This transfer of energy can occur in different ways: by conduction, convection, or radiation.

The unit of heat energy is joule (J). Sometimes heat energy is also measured in calories.

Temperature vs heat

Although we often use the terms heat and temperature interchangeably in daily conversation, they have different meanings in physics.

• Temperature: Temperature tells us how hot or cold something is. It is measured with a thermometer, usually in degrees Celsius (°C) or Fahrenheit (°F). Temperature is an indicator of the average kinetic energy of the particles in a substance. More kinetic energy means a higher temperature.
• Heat: Heat is energy that flows because of a difference in temperature. When heat is transferred to a substance, the particles of the substance move faster, and its temperature rises.

Let us understand this with an example:

Imagine you have a pot of water on the stove. As you heat it, the temperature of the water rises. As the water particles gain energy, they move more quickly. Heat is being transferred from the stove to the pot and the water, causing the water's temperature to rise.
  

What is thermal equilibrium?

Thermal equilibrium is a state where two or more objects reach the same temperature and no heat flows between them. This means they are balanced in terms of thermal energy.

Think about when you hold a cup of hot cocoa. Initially, the cup feels warm to your hand because the heat from the hot cocoa is transferring to your cold hand. After a while, both the cup and your hand will be at the same temperature, and no more heat will flow between them. They are now in thermal equilibrium.

• Example 1: If you put a piece of ice in a glass of hot water, the ice begins to melt as heat flows from the hot water to the piece of ice. Over time, as energy is exchanged, the whole system (water and melted ice) will reach the same temperature.
• Example 2: When you walk into a warm room on a cold day, you may feel hot at first. However, as you sit in the room, your body exchanges heat with the surrounding air until you feel comfortable. Your body and the room eventually reach thermal equilibrium.

Heat exchange system

Heat can be exchanged between objects or systems by conduction, convection, and radiation.

Conductivity

Conduction is the transfer of heat through a substance without moving it. Heat flows by conduction when particles of matter collide with neighboring particles, thereby transferring energy.

Example: When you touch the handle of a hot metal pan, the heat travels through the solid metal to your hand by conduction.
  

Here's a simplified diagram to show how conduction works:

In this diagram, heat starts at the left side (hot side) of the metal rod and slowly moves toward the right side (cool side) through conduction.

Convection

Convection is the transfer of heat by the movement of a fluid (liquid or gas). This happens because the hot fluid is less dense than the cold fluid, so it rises. As it rises, the cold fluid takes its place, creating a cycle.

Example: Water is boiling in a pot. The hot water at the bottom rises to the top, while the cold water at the top sinks to the bottom, creating a circulating cycle.
  

Here's a basic diagram that shows how convection works:

In this diagram, the arrows show how hot liquid rises, and cold liquid falls, creating a convection current.

Radiation

Radiation is the transfer of heat via electromagnetic waves. Unlike conduction and convection, radiation does not require a medium; it can occur in a vacuum.

Example: Heat reaching the Earth from the Sun. The sun's rays travel through the vacuum of space to warm our planet.
  

The following simplified diagram shows radiation:

In this view the Sun emits energy in the form of waves, which travel through space to reach the Earth.

Calculation of heat exchange

The amount of heat exchange can be calculated using the formula:

Q = mcΔt
  

• Q is the heat exchanged (in joules).
• m is the mass of the substance (in kilograms).
• c is the specific heat capacity (in joules per kilogram per degree Celsius).
• ΔT is the change in temperature (in degrees Celsius).

This formula helps us determine how much heat energy will be needed to change the temperature of a substance by a certain amount.

Conclusion

Understanding thermal equilibrium and heat exchange is important to understand how energy moves and is balanced in our universe. Whether it's the heat of the sun, the comfort of a warm room, or the cooling of a hot drink with ice, these basic principles are at work. Through conduction, convection, and radiation, heat energy is always in motion, trying to reach a state of equilibrium.

By understanding these concepts, we are able to better understand many natural phenomena and energy systems in science, engineering, and everyday life.

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