Heat

Heat is a special kind of energy that moves from one place to another because of a difference in temperature. In science, especially in a field called thermodynamics, heat is described as energy moving across the boundary of a system when there is a temperature difference. It’s important to know that a system itself doesn’t “contain” heat — instead, heat is the energy moving in or out of the system.

We often talk about heat as the warmth or energy stored inside an object, which is related to its temperature. For example, when you touch a hot stove, heat moves from the stove to your hand, making it warm. Scientists measure heat using tools and methods called calorimetry. This can involve watching how much ice melts or how much the temperature of a material changes when heat is added or removed.

In the worldwide system used for measuring things called the International System of Units (SI), the unit for measuring heat is called the joule, written as “J.” This helps scientists all over the world talk about and compare amounts of heat in a clear way.

Notation and units

Heat is a type of energy, and we measure it in units called joules (J). This is the standard unit used in science around the world, known as the International System of Units (SI). However, some areas of engineering use different units, like the British thermal unit (BTU) or the calorie.

The rate at which heat moves is measured in watts (W), which means one joule of energy moving each second. The symbol for heat, Q, was introduced by scientists Rudolf Clausius and Macquorn Rankine around the year 1859.

History

Heat is a form of energy that moves between objects when there is a difference in their temperatures. It is not something that objects "contain," but rather something that moves from one place to another.

In the past, scientists thought about heat in different ways. Some believed heat was a special kind of matter, like a fluid called "phlogiston" or "caloric." Others thought heat was related to the movement of tiny particles in objects. Over time, experiments showed that heat is connected to the movement of particles and to energy changes in objects.

Scientists have studied heat for centuries. In the 1600s and 1700s, they began to understand that heat might be linked to how fast particles in an object are moving. By the 1800s, the idea that heat is a form of energy became widely accepted. Today, we know that heat is one way energy can move, and it plays a key role in how we understand temperature and energy changes in the world around us.

Heat transfer

When a warm object touches a cool object, the warm object cools down and the cool object warms up. This happens because heat moves from the warmer to the cooler object.

Heat can also move through space by radiation. For example, the Sun warms the Earth by sending out heat that travels through space.

Another way heat moves is through convection. This happens when a liquid or gas moves, carrying heat with it. For example, when water in a pot heats up, the warm water rises to the top, and cooler water moves to the bottom to be heated.

In science, we study how heat moves in different situations. One important idea is that heat can make things work. For example, engines use heat to create movement. Another idea is that we can use work to move heat from one place to another, like in a refrigerator.

Latent and sensible heat

In 1847, James Prescott Joule explained two kinds of heat: latent heat and sensible heat. Latent heat is the energy a substance releases or absorbs when it changes state, like when ice melts or water boils, but the temperature stays the same. Sensible heat is the energy that makes particles move, which we feel as temperature changes.

Heat capacity

Heat capacity is a way to measure how much heat energy is needed to change the temperature of an object. It tells us how much heat a material can hold. For example, the heat needed to raise the temperature of a small piece of metal will be different from what is needed for the same amount of water.

Different materials have different heat capacities. Some gases, like helium, keep a steady heat capacity, while others, like hydrogen, change a little depending on temperature. Heat can also change how materials look, like turning ice into water when it gets warm.

Hotness

Hotness is a key idea in thermodynamics. It helps us understand how things can make other things warmer or cooler. When something is hotter, it can give heat to something that is colder.

Some things change too fast or are too mixed up to have a temperature we can measure. But even these can still give or receive heat. For things that stay the same long enough, we can measure their temperature with a special tool called a thermometer. This measurement is called an empirical temperature.

The zeroth law of thermodynamics explains how we know when things are in balance with each other in terms of heat. It helps us understand how heat moves between different objects.

Classical thermodynamics

Heat and enthalpy

Further information: Internal energy and Enthalpy

In thermodynamics, heat is a type of energy that moves between a system and its surroundings because of a temperature difference. It is not something that a system contains, but rather something that moves in or out.

When heat moves into a closed system (one that does not gain or lose matter), it changes the system's internal energy. This change can also happen if the system does work on its surroundings or if work is done on the system. The amount of heat added to the system, called Q, minus the work done by the system, called W, equals the change in internal energy, called Δ_U.

For example, if you heat water in a pot, the heat you add increases the water’s internal energy, making it hotter. If the water then evaporates, it does work on the surrounding air by pushing it away. The heat added and the work done both affect the water’s internal energy.

Heat and entropy

Main article: Entropy

When heat moves, it also affects a property called entropy, which is a measure of how disordered or random things are. In 1856, a scientist named Rudolf Clausius said that when heat moves from a hotter place to a cooler one, the total entropy of the system and its surroundings always increases. This is one way of stating the second law of thermodynamics.

For example, imagine a hot cup of coffee. When it cools down, heat moves from the coffee to the surrounding air. The coffee becomes less hot, and the air becomes slightly warmer. Even though the total amount of energy stays the same, the entropy of the universe increases because the energy spreads out more evenly. This spreading out is a natural process and cannot be reversed without adding more energy from somewhere else.