What Is Heat In Natural Science?

Heat is a form of energy transfer between objects or systems that are at different temperatures. It is a process that increases the internal energy of a system, which causes an increase in its temperature. Heat does not represent a property of a system, but rather a process of energy transfer due to temperature differences.

Some key concepts related to heat include:

  • Thermal energy – The total internal kinetic and potential energy of molecules within a substance. Heat transfer causes changes in thermal energy.
  • Temperature – A measure of the average kinetic energy of molecules. Higher temperatures mean faster molecular motion.
  • Heat capacity – The amount of energy required to raise a substance’s temperature by 1°C or 1° Kelvin.
  • Thermal conductivity – The rate at which a material conducts heat.

Heat plays a vital role in many areas of science, engineering and everyday life. Some applications and effects of heat transfer include:

  • Heating and cooling systems such as furnaces, air conditioners, refrigerators.
  • Engines, power plants, and energy generation processes.
  • Chemical reactions, phase changes, material properties.
  • Biological processes such as metabolism, fever, adaptation to environments.
  • Weather and climate patterns.

Thermal Energy Transfer

Heat can transfer between objects or locations through three main mechanisms: conduction, convection, and radiation. These processes allow thermal energy to spread out and reach equilibrium.


Conduction is the transfer of heat between objects or substances that are in direct physical contact with each other. It occurs when electrons of atoms and molecules vibrate and bump into each other, transferring the kinetic energy of their thermal motion. Materials like metals are good conductors as their free electrons can easily transport thermal energy.


Convection is the transfer of heat by the motion of fluids and gases. It occurs between a surface and the adjacent fluid or gas in motion. The fluid circulates within the material, moving the heated particles away from the source and transferring heat through the substance. Convection depends on the circulation and currents in the fluid.


Radiation is the transfer of heat through electromagnetic waves or photons. All objects emit infrared radiation depending on their temperature. Thermal radiation does not require direct contact between the heat source and the receiver. Instead, energy radiates through space or any transparent medium. The hotter the body, the greater the intensity and frequency of radiation emitted.

Measuring Temperature

Temperature is a measure of the average kinetic energy of molecules and atoms. It is commonly measured using thermometers that rely on the expansion of substances in response to heat. There are several temperature scales used today.

The most commonly used scale is Celsius, where water freezes at 0°C and boils at 100°C. The Fahrenheit scale is also widely used, particularly in the United States, where water freezes at 32°F and boils at 212°F. Absolute temperature scales like Kelvin are used in scientific contexts, with 0 K representing absolute zero, the lowest theoretically possible temperature where molecules have minimal motion.

Thermometers work by correlating a measurable physical change with temperature. Liquid-in-glass thermometers rely on the expansion of liquid in a sealed glass tube. As temperature increases, the liquid expands and rises in the tube. Digital thermometers use thermistors or thermocouples to measure electric signals that vary with temperature. Infrared thermometers measure radiant heat being emitted from an object’s surface.

When heated, most materials undergo thermal expansion, slightly increasing in size. This property allows bimetallic strips and liquid thermometers to work. But expansion can also cause problems, like railroad tracks buckling on hot days. Engineers must account for thermal expansion in designs for infrastructure and devices.

Heat Capacity

Heat capacity refers to the amount of heat required to raise the temperature of a substance by one degree. It is an intensive property, meaning it does not depend on the amount of material present. Heat capacity is measured in joules per kelvin (J/K).

The specific heat capacity of a substance is the amount of heat required to raise one gram of the substance by one degree Celsius. This property varies for different materials. Substances like water have a high specific heat capacity, meaning they can absorb a lot of heat energy before changing temperature. Metals generally have lower specific heat capacities.

When heat is added to a substance, the energy goes into increasing the kinetic energy of its molecules. The molecules vibrate and rotate faster. Before the temperature can rise, enough energy must be added to increase the molecular motions throughout the material. Substances with higher specific heats require more energy to speed up their molecules to change their temperature.

The amount of heat required depends on the substance’s mass, specific heat capacity, and temperature change. This relationship is described by the equation: Q = mcΔT, where Q is heat energy, m is mass, c is specific heat capacity, and ΔT is temperature change.

Understanding heat capacity helps engineers design materials that can effectively absorb and release heat for applications like heat sinks in computers or insulation in buildings. It also allows scientists to model the flow of energy through systems and processes that involve temperature changes.

Thermodynamics Laws

Thermodynamics is the study of heat and energy. There are four fundamental laws of thermodynamics that describe the relationship between thermal energy, heat, and work.

Zeroth Law of Thermodynamics

The zeroth law of thermodynamics states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law helps define the concept of temperature.

First Law of Thermodynamics

The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed in an isolated system. The change in internal energy of a closed system equals the amount of heat supplied to the system minus the amount of work done by the system.

Second Law of Thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. Entropy is a measure of disorder, so this law expresses that natural processes tend to evolve toward states of higher disorder.

Third Law of Thermodynamics

The third law of thermodynamics states that the entropy of a pure crystalline substance approaches zero as its temperature approaches absolute zero. This law helps define absolute zero on the thermodynamic temperature scale.

Phase Changes

When matter changes state from a solid to a liquid, a liquid to a gas, or a solid directly to a gas, it undergoes a phase change. The most common phase changes are melting, freezing, evaporation, condensation, and sublimation.

Melting occurs when a solid turns into a liquid. For example, an ice cube melts into liquid water when heat is added. The melting point is the temperature at which a substance changes from a solid to a liquid state.

Freezing occurs when a liquid turns into a solid. For example, liquid water freezes into ice when heat is removed. The freezing point is the temperature at which a substance changes from a liquid to a solid state.

Evaporation occurs when a liquid turns into a gas or vapor. For example, liquid water evaporates and becomes water vapor. Evaporation happens at the surface level as molecules gain enough energy to break free from the liquid and enter the gas phase.

Condensation is the reverse of evaporation, where a gas or vapor turns into a liquid. For example, water vapor condenses into liquid water on cold surfaces. This occurs when the vapor loses enough energy for the molecules to come together again as a liquid.

Sublimation is when a solid turns directly into a gas, without going through the liquid phase. A common example is dry ice (solid carbon dioxide) sublimating and turning into carbon dioxide gas.

Heat Transfer Rates

a pot of water boiling on a stove, demonstrating heat transfer changing the water's state from liquid to gas.
There are three main mechanisms of heat transfer: conduction, convection, and radiation. Each occurs at different rates depending on various factors.

Conduction Rate

The rate of conductive heat transfer is determined by the thermal conductivity of a material (k), the cross-sectional area (A), and the temperature difference (ΔT) over a distance (L). It is calculated using the formula:


Materials like metals have high thermal conductivity and transfer heat quickly by conduction. Insulators like wood have low conductivity and slow the conduction rate. Increasing the temperature difference or cross-sectional area increases conductive heat transfer.

Convection Rate

Convection rates depend on the velocity of the fluid motion transporting heat. Faster velocities result in faster heat transfer. Convection rates also increase with greater temperature differences. Fluid properties like low viscosity and high thermal conductivity boost the convection rate. Other factors like the surface area and geometry exposed to fluid flow play a role as well.

Radiation Rate

The rate of radiative heat transfer relies on the emissivity of the surface, its area, and the difference between its own temperature versus the surroundings. It increases with higher emissivity values, larger surface areas, and greater temperature gradients. The orientation of surfaces also impacts radiation rates based on view factors.

Real-World Applications

Heat plays an important role in many real-world applications and technologies. Here are some key examples where heat principles are leveraged:


In industrial processes, heat is used for smelting metals, refining oil, drying materials, and generating steam for electricity production. Boilers, furnaces, and kilns rely on the careful application of heat to drive key industrial processes.


Internal combustion engines in cars, trucks, planes, and other vehicles operate based on small controlled explosions that generate heat to drive pistons and provide power. The principles of thermodynamics govern how efficiently this process converts heat into work.


Heating food is essential for cooking and prepares food for safe human consumption. Different cooking methods like baking, frying, sautéing all utilize heat to induce physical and chemical changes in food.

Heating and Cooling Systems

HVAC (heating, ventilation, and air conditioning) systems use heat pumps and furnaces to control temperatures in buildings. Heat transfer principles help determine the efficiency and design of these essential climate control systems.


Heat drives natural processes like weather patterns, ocean currents, the water cycle, and much more. Understanding thermal dynamics helps scientists model and predict complex natural phenomena and climate trends.

Effects on Matter

Heat, or thermal energy, impacts matter in a variety of ways. Three major effects heat has on the properties of matter are: thermal expansion, change of state, and exceeding temperature limits.

Thermal expansion refers to the increase in volume or decrease in density that occurs when matter is heated. As temperature increases, the kinetic energy of molecules and atoms rises. This causes bonds to vibrate more and molecules to move farther apart, taking up more space. Solids typically expand in all directions, liquids expand mainly vertically, and gases expand to fill their container.

Heat can also cause phase changes or changes of state in matter. The addition of thermal energy can cause a solid to melt into a liquid, a liquid to vaporize into a gas, a gas to condense into a liquid, a liquid to freeze into a solid, or a solid to sublimate directly into a gas. Each phase change occurs at a specific temperature range for that substance.

Finally, exceeding certain temperature limits can cause irreversible changes or damage to matter. Plastics and other polymers can melt, deform, or burn when heated excessively. Foods and biological materials will cook and eventually char or burn. Metals and alloys will melt, oxidize or corrode at high temperatures. Exceeding upper or lower temperature thresholds causes physical and chemical changes that may be permanent.


In summary, heat is a form of energy that is transferred between objects or systems due to temperature differences. The study of heat and heat transfer, known as thermodynamics, is crucial for understanding many natural phenomena and technological systems.

Some key concepts covered in this overview of heat include thermal energy, temperature, heat capacity, phases changes, heat transfer mechanisms like conduction, convection and radiation, and real-world applications like heating and cooling systems, heat engines, and weather patterns.

Understanding how heat behaves and transfers between objects enables scientists and engineers to properly design technologies, predict system performance, and model complex interactions. Careful study of thermodynamics has led to major advances that impact our everyday lives.

While this was just a brief introduction, it should provide a foundation for appreciating the significance of heat transfer across many domains. Whether describing the flow of energy in Earth’s atmosphere, analyzing the thermodynamics of a chemical reaction, or optimizing a manufacturing process, the physics of heat transfer play a crucial role.

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