The terms “heat” and “temperature” are often used interchangeably, but in physics, they are distinct concepts. Temperature is a property describing an object’s thermal state, while heat is the energy that moves between objects. Understanding this difference is fundamental to comprehending how energy interacts with matter, influencing everything from weather patterns to cooking food.
What Temperature Actually Measures
Temperature quantifies the average kinetic energy of a substance’s constituent particles. All matter is composed of atoms and molecules in constant motion, and the faster these particles move, the higher the substance’s temperature. When energy is added, it increases this internal motion, which a thermometer registers as a rise in temperature.
Temperature is typically measured using scales like Celsius and Fahrenheit. The Kelvin scale is the standard in scientific work because it is directly proportional to the average kinetic energy. On the Kelvin scale, zero represents absolute zero, the theoretical point at which all particle motion ceases.
Heat as Energy in Motion
Heat, or thermal energy, is defined as the transfer of energy between a system and its surroundings solely due to a temperature difference. Heat is not contained within an object; it is the process of energy moving across a boundary. This energy transfer always proceeds spontaneously from the region of higher temperature to the region of lower temperature.
The transfer continues until both objects reach the same temperature, a state known as thermal equilibrium. Because heat is a form of energy transfer, it is measured in standard energy units, such as Joules (J) or calories.
How Energy Moves Between Objects
The transfer of heat from a warmer substance to a cooler one occurs through three distinct mechanisms.
Conduction
Conduction involves the transfer of energy through direct contact, primarily in solids. This occurs as faster-vibrating particles in the hotter section physically bump into and transfer energy to slower-moving particles nearby. A metal spoon quickly getting hot when placed in soup is a common example.
Convection
Convection is the transfer of heat through the macroscopic movement of fluids, such as liquids or gases. When a fluid is heated, it expands and becomes less dense, causing it to rise, while cooler, denser fluid sinks. This creates a continuous circulating current that distributes the thermal energy throughout the fluid, such as the rolling boil in a pot of water.
Radiation
Radiation involves the transfer of energy via electromagnetic waves, such as infrared radiation. Unlike the other two methods, radiation does not require any medium or physical contact and can travel through a vacuum. The warmth felt from the sun or near a glowing fire is thermal energy transferred through radiation.
Why Added Heat Does Not Always Raise Temperature
Adding heat to a substance does not always result in a temperature increase, a phenomenon explained by specific heat capacity and latent heat.
Specific Heat Capacity
Specific heat capacity measures the heat energy required to raise the temperature of a specific mass of a substance. Water has a high specific heat capacity, meaning it takes a large amount of energy to change its temperature, which helps keep oceans relatively stable.
Latent Heat and Phase Change
Thermal energy may instead be used for a phase change, such as melting or boiling, a process governed by latent heat. During a phase transition, the added heat energy breaks the intermolecular bonds holding the particles together. This energy increases the potential energy of the molecules rather than their kinetic energy, causing the temperature to remain constant until the phase change is complete.