Heat is energy transferred between systems or between parts of a system because of a temperature difference. This definition is important because heat is not a substance contained inside an object and is not synonymous with the total thermal energy of an object. Heat describes a process of energy transfer. If two objects at different temperatures are placed in thermal contact, energy tends to flow from the hotter object to the colder one, and the transferred energy is called heat.
In thermodynamics, heat is usually represented by the symbol Q and measured in joules. Heat can cross a system boundary through mechanisms such as thermal conduction, convection, and thermal radiation. The amount of energy transferred as heat depends on the physical process and the temperatures, materials, geometry, and time involved. Once the transfer has occurred, the energy becomes part of the internal energy or other energy forms of the receiving system rather than remaining stored as a substance called heat.
Conduction is one major mechanism of heat transfer and occurs through microscopic interactions within matter. In solids, especially metals, energy can be transferred through interactions among atoms, lattice vibrations, and mobile electrons. In fluids, molecular collisions and microscopic motion contribute to conduction. The rate of conductive heat transfer depends on factors such as thermal conductivity, temperature gradient, area, and distance. Fourier's law provides a mathematical description for heat conduction in many ordinary circumstances.
Materials with high thermal conductivity, such as many metals, transfer heat relatively efficiently, whereas materials such as mineral wool, expanded polymers, and trapped gases are commonly used as thermal insulators because they inhibit heat transfer. Conduction does not require bulk movement of the material, distinguishing it from convection.
Convection is heat transfer associated with the movement of a fluid. When a fluid is heated, changes in density can cause warmer material to rise and cooler material to descend under gravity, producing natural convection. Forced convection occurs when fans, pumps, or other mechanisms deliberately move the fluid. Atmospheric circulation, boiling liquids, heating systems, and many cooling systems involve convection. The microscopic transfer of energy still occurs through interactions between particles, but bulk fluid motion transports energy through space.
Convection can be described using fluid dynamics and heat transfer equations, and its rate depends on properties such as viscosity, thermal conductivity, density, flow velocity, geometry, and temperature difference. Engineers use convection principles when designing radiators, heat exchangers, cooling systems, ventilation equipment, and industrial thermal processes.
Thermal radiation is heat transfer through electromagnetic radiation. Unlike conduction and convection, radiation does not require a material medium and can transfer energy through vacuum. All objects above absolute zero emit electromagnetic radiation because of their thermal state, although the spectrum and intensity depend strongly on temperature. Blackbody radiation provides an idealised model of thermal emission, with the distribution of emitted radiation determined by temperature.
Hotter objects emit more total thermal radiation and shift their peak emission toward shorter wavelengths according to Wien's displacement law. The Stefan Boltzmann law relates total radiated power from an ideal blackbody to the fourth power of absolute temperature. Radiation allows the Sun to transfer energy across the vacuum of space to Earth. It is also important in infrared cameras, thermal insulation, astronomy, climate science, and spacecraft engineering.
Heat transfer is governed by the second law of thermodynamics. In ordinary passive conditions, net heat transfer occurs spontaneously from higher temperature toward lower temperature. A refrigerator can transfer heat from a colder interior to a warmer environment, but it requires external work to accomplish this. Heat engines operate by receiving heat from a high temperature source, converting part of the transferred energy into work, and rejecting some heat to a lower temperature sink.
The second law places limits on the efficiency of these processes. No cyclic heat engine operating between two finite temperature reservoirs can convert all absorbed heat into work. The maximum ideal efficiency of a reversible engine is determined by the temperatures of its hot and cold reservoirs. These principles show that heat transfer is intimately connected to entropy and the directionality of thermodynamic processes.
Heat is often confused with temperature and thermal energy. Temperature is a state variable indicating the thermal state of a system, whereas heat is energy transferred because of a temperature difference. Thermal or internal energy is energy associated with the microscopic state of the system. A cup of boiling water can have a high temperature but contain much less total internal energy than a large tank of warm water because the tank contains much more matter.
When heat enters an object, its temperature may increase, but this is not always the case. During melting or boiling, energy can be transferred as heat while the temperature remains approximately constant because the energy changes the microscopic arrangement or phase of the substance. Thus heat, temperature, and internal energy are related but physically distinct concepts.
The amount of heat required to change the temperature of a substance depends on its mass, specific heat capacity, and temperature change. Under suitable conditions, the relationship can be written as Q equals m c delta T, where m is mass, c is specific heat capacity, and delta T is the temperature change. Phase changes require latent heat rather than a straightforward temperature increase.
Melting, vaporisation, condensation, and freezing involve changes in molecular arrangement and intermolecular energy, and energy transfer can occur without a corresponding temperature change while the phase transition proceeds. Heat capacity and specific heat vary between substances and can depend on temperature and physical conditions. Water has a relatively high specific heat capacity, which contributes to its importance in climate regulation, thermal storage, cooling systems, and biological processes.
Heat transfer is central to natural processes and technology. Earth's climate involves incoming solar radiation, absorption, emission of infrared radiation, atmospheric processes, conduction, and convection. Buildings are designed with insulation, glazing, ventilation, and heating systems to control heat transfer. Engines and power stations convert energy through cycles involving heat transfer and work. Refrigerators and heat pumps use mechanical or electrical work to move heat against a temperature gradient.
Industrial processes use furnaces, boilers, condensers, evaporators, and heat exchangers to control thermal energy flows. Medicine uses controlled heating and cooling in various treatments and preservation processes. Understanding heat therefore requires more than recognising that something becomes warm. It requires identifying the direction, mechanism, magnitude, and thermodynamic consequences of energy transfer between systems.
