# What Is Heat In Physics Short Notes?

## What is Heat?

Heat is a form of energy that transfers from one body or system to another as a result of temperature difference. Heat always flows spontaneously from a higher temperature object to a lower temperature object. Unlike internal energy, which is the total energy of molecular motion within a substance, heat is energy in transit solely due to temperature difference.

Heat and temperature, while related, are distinct physical concepts. Temperature measures the average kinetic energy of particles in matter on a microscopic scale. It is an intensive property, meaning it does not depend on the size or amount of matter. Heat is the transfer of thermal energy driven by temperature difference. It is an extensive property, scaling with the amount of matter.

While higher temperature matter contains more microscopic kinetic energy, temperature alone does not cause heat transfer. Heat only flows when there is a temperature gradient. For example, even though a stove burner may be 500°F and a pot of water on the stove 50°F, no heat flows until the pot is placed on the burner, allowing heat transfer from the higher temperature burner to the lower temperature water.

## Heat Transfer

Heat transfer refers to the ways heat energy can be moved between objects or regions that are at different temperatures. There are three main mechanisms of heat transfer:

### Conduction

Conduction is the transfer of heat between substances that are in direct contact with each other. It occurs when heat energy is transferred through collisions between neighboring atoms and molecules. Materials like metals are good conductors of heat.

### Convection

Convection is the transfer of heat by the actual motion or flow of a fluid. Hotter fluids become less dense and rise up, while cooler fluids become more dense and sink down. This sets up circulation patterns that transfer heat through the bulk movement of the fluid.

Radiation is the transfer of heat energy by electromagnetic waves directly across space. No medium is required for radiation to occur. All objects emit thermal radiation related to their temperature.

## Measuring Heat

Heat can be measured in units of calories or joules. A calorie is defined as the amount of heat required to raise one gram of water by 1 degree Celsius. A joule is defined as the amount of work required to produce one watt of power for one second. There are 4.184 joules in one calorie.

Common devices used to measure heat include:

• Thermometer – Measures temperature change to determine heat flow and capacity.
• Calorimeter – Measures temperature change in a known mass of water to calculate heat flow into or out of the system.

By precisely tracking temperature changes, these devices allow for accurate experimental determination of heat transfer and thermal properties.

## Specific Heat Capacity

The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1 kg of the substance by 1°C. It is a property unique to each substance and depends on the molecular structure and mass of its atoms or molecules. Specific heat capacity is measured in J/kg°C or J/g°C.

Some common values of specific heat capacity are:

 Substance Specific Heat Capacity (J/g°C) Water 4.18 Ice 2.05 Iron 0.449 Aluminum 0.897 Copper 0.385 Gold 0.129 Mercury 0.140

The specific heat capacity is used to calculate the amount of heat (Q) required to change the temperature of a substance using the formula:

Q = mcΔT

Where m is the mass, c is the specific heat capacity, and ΔT is the temperature change.

## Latent Heat

Latent heat is the energy absorbed or released by a substance during a phase change between a solid, liquid or gas. Unlike sensible heat, which causes a change in temperature, latent heat causes a change in the state of matter while the temperature remains constant.

There are two main types of latent heat:

Latent heat of fusion is the heat absorbed when a solid melts into a liquid. For example, when ice melts into water, it absorbs latent heat equal to 334 Joules per gram. This energy breaks down the crystal structure of the solid without changing the temperature.

Latent heat of vaporization is the heat absorbed when a liquid vaporizes into a gas. For instance, when water boils into steam, it absorbs latent heat equal to 2257 Joules per gram. This energy gives molecules enough kinetic energy to escape from the liquid surface as a gas.

Latent heat is an important concept in thermodynamics and materials science. It explains heat transfer during phase changes and informs engineering applications like heat management systems.

## Heat and Phase Changes

Heat plays a critical role in phase changes for matter. A phase change is when a substance transitions from one state to another, such as from a solid to a liquid or a liquid to a gas. Common phase changes that involve heat include:

Melting – The process of a solid turning into a liquid by adding heat energy. For example, ice melts into water when enough heat is added to break the crystalline bonds in the solid state.

Freezing – The reverse process of melting, where a liquid turns into a solid by removing heat energy. Water freezes into ice when enough heat is removed for water molecules to form a rigid crystalline structure.

Vaporization – The transition from a liquid to a gas, which happens through evaporation or boiling. Applying heat provides enough energy for liquid particles to escape into the gas phase.

Condensation – The reverse of vaporization, where a gas condenses into a liquid by losing heat. Cooling a gas allows the particles to come together in the denser liquid state.

The amount of heat required to change a substance’s phase is called latent heat. Each substance has a specific latent heat capacity tied to its chemical bonds and molecular structure. More heat is required to boil a pot of water into steam than to simply raise its temperature. The addition or removal of heat enables molecules to undergo phase changes.

## Heat Transfer Equations

There are two main equations used to describe heat transfer:

### Fourier’s Law

Fourier’s law describes heat conduction, which is the transfer of heat between substances that are in direct contact with each other. Fourier’s law is:

q = -kA(dT/dx)

Where q is the heat flux (W), k is the thermal conductivity (W/m·K), A is the cross-sectional area (m2), and dT/dx is the temperature gradient (K/m). This equation shows that heat flux is directly proportional to the temperature gradient and the thermal conductivity of a material.

### Heat Equation

The heat equation describes how heat diffuses through a material over time. The one-dimensional heat equation is:

ρcp(∂T/∂t) = k(∂2T/∂x2)

Where ρ is density (kg/m3), cp is specific heat capacity (J/kg·K), ∂T/∂t is the rate of temperature change over time (K/s), and ∂2T/∂x2 is the second spatial derivative of temperature (K/m2). This equation shows that the rate of change in temperature is proportional to the thermal conductivity and spatial temperature distribution.

## Measuring Temperature

Temperature is a measure of how hot or cold an object is. There are three main temperature scales used to measure temperature:

Celsius – The Celsius scale, also known as the centigrade scale, uses degrees Celsius (°C). On the Celsius scale, water freezes at 0°C and boils at 100°C. Normal human body temperature is 37°C.

Fahrenheit – The Fahrenheit scale uses degrees Fahrenheit (°F). On the Fahrenheit scale, water freezes at 32°F and boils at 212°F. Normal human body temperature is 98.6°F.

Kelvin – The Kelvin scale uses the kelvin (K) unit. The Kelvin scale does not use degrees. On the Kelvin scale, absolute zero is 0 K and water freezes at 273.15 K. Normal human body temperature is 310.15 K.

To convert between Celsius and Fahrenheit:

°F = (°C × 9/5) + 32

°C = (°F − 32) × 5/9

To convert between Celsius and Kelvin:

K = °C + 273.15

°C = K − 273.15

## Thermal Expansion

Thermal expansion refers to the phenomenon where matter expands in volume when heated. Almost all matter expands upon heating and contracts upon cooling. This occurs because heating increases the kinetic energy and vibrational motion of the atoms and molecules in the material, causing the atomic bonds to stretch and material to take up more space.

There are three types of thermal expansion:

• Linear expansion – when an object expands along one dimension or axis
• Area expansion – when a two dimensional surface expands
• Volume expansion – when an object expands in all directions, increasing its volume

Thermal expansion has many practical applications and implications. For example:

• Allowance gaps are left between rail road tracks to account for expansion on hot days
• Bridges have expansion joints to allow for expansion and contraction
• Thermal expansion must be accounted for when fitting machined parts together
• Liquids in thermometers expand predictably and are used to measure temperature

## Heat Engines

A heat engine is a device that converts heat energy into mechanical energy. Heat engines operate on the basis of thermodynamic cycles, where a working fluid is alternately heated and cooled in a cyclic process. The heat source supplies energy to the working fluid at high temperature. The working fluid then converts some of this heat energy into mechanical work. After the mechanical work is done, the working fluid releases waste heat at a lower temperature. This cycle is then repeated continuously.

The efficiency of a heat engine depends on the difference between the high and low temperatures. Higher efficiency is achieved with larger temperature differences. Examples of heat engines include:

Steam engine – Uses steam as the working fluid in a piston cylinder arrangement. Fuel is burned to boil water into steam at high pressure. This steam pushes the piston to produce rotating mechanical power. Steam engines powered early forms of transportation and manufacturing during the Industrial Revolution.

Internal combustion engine – Uses gases as the working fluid in piston cylinders. Fuel is ignited inside the cylinder, heating and expanding the gases to push the piston. Gasoline and diesel engines in vehicles are common examples. The combustion of the fuel occurs inside the engine.