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In physics, the study of heat is a fundamental branch of thermodynamics that deals with the transfer of thermal energy. Understanding heat is essential for explaining how energy moves through matter and why objects change temperature. A common point of confusion is the distinction between heat and temperature, but they are two separate concepts:
- Heat is the transfer of thermal energy between objects or systems due to a temperature difference. It is a process of energy transfer and is measured in units of energy, such as Joules (J). For example, when you touch a hot stove, the energy that moves from the stove to your hand is heat.
- Temperature is a measure of the average kinetic energy of the particles within a substance. It is a property of an object, indicating how hot or cold it is, and is measured in degrees Celsius (°C), Fahrenheit (°F), or Kelvin (K). A large swimming pool at 20°C has more heat energy than a small cup of boiling water at 100°C because it contains a much greater number of molecules, even though the cup of water has a higher temperature.
The transfer of heat occurs in three primary modes:
- Conduction: This is the transfer of heat through direct contact, primarily in solids. In this process, particles in the hotter part of an object vibrate more rapidly and collide with neighboring, slower-moving particles, transferring energy. A classic example is a metal spoon getting hot when left in a cup of hot coffee.
- Convection: This is the transfer of heat through the movement of fluids (liquids or gases). When a fluid is heated, it becomes less dense and rises, while the cooler, denser fluid sinks. This creates a circular current, or convection current, that distributes heat. This is how a space heater warms a room or how water boils in a pot.
- Radiation: This is the transfer of heat through electromagnetic waves, which requires no medium. All objects with a temperature above absolute zero emit thermal radiation. This is how the sun’s energy travels through the vacuum of space to warm the Earth, and it’s also why you can feel the heat from a fire without touching it.
These concepts are governed by the Laws of Thermodynamics, which are foundational to all of physics. The Zeroth Law defines thermal equilibrium, stating that if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. The First Law is the law of conservation of energy, stating that energy cannot be created or destroyed, only transferred or changed in form. The Second Law introduces the concept of entropy and dictates that heat will always spontaneously flow from a hotter object to a colder one, and that no energy transfer is ever 100% efficient.
