Heat, Internal Energy, and Work

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The studies of heat, energy and work is called thermodynamics. It is essential to know how energy moves between systems and affects the state of matter. Heat, internal energy and work are the three core ideas of thermodynamics.

Heat

A difference in temperature causes heat, a type of energy, to move from one body to another. Until thermal equilibrium is attained, it always moves from a hotter object to a cooler object. Heat is generally calculated in calories (cal) and the SI unit is the joule (J).  
Where 1 cal = 4.186 J.

Modes of Heat Transfer

Heat can be transported in three primary ways:
Conduction: This is the process by which heat moves from one solid particle to another solid particle without the particles themselves moving. Example: By placing one end of a metal rod in a flame causes it to heat up.
Heat, Internal Energy, and Work-Metal rod
Metal rod
Convection: This process, which takes place in gases and liquids, transfers heat through fluid motion. Boiling water, for example, causes hot water to rise and cooled water to sink.
Heat, Internal Energy, and Work-Boiling water
Boiling water
Radiation: This process uses electromagnetic waves to transmit heat without the need for a medium. For example, the Earth is heated by the Sun.

Internal Energy

The overall energy that a system has as a result of its molecules’ random motion and interactions with one another is known as its internal energy. It is made up of two primary parts:
Kinetic Energy (KE): This is caused by particle motion, and happens with vibrational, rotational, and translational motion.
Potential Energy (PE): This is happen because of the substance’s bonds and intermolecular forces.

Factors Affecting Internal Energy

  • Temperature: As kinetic energy rises with temperature, internal energy also rises.
  • State of Matter: Because molecules move more freely in gases than in solids or liquids, gases often have larger internal energies.
  • Amount of Substance: A system with more molecules has greater internal energy.
As a state function, internal energy is only dependent on the system’s current state and not on the route taken to get there.

Work in Thermodynamics

The energy transfer that takes place when a force is applied across a distance is known as work in thermodynamics. Work is done in the case of gases when they compress or expand inside a system.

Expression for Work Done by a Gas

Mathematically, work done by a gas is given by: W = PΔV
Where:
  • W is the work done (Joules)
  • P is the pressure (Pascal)
  • ΔV is the change in volume (cubic meters)

Types of Work Done

Positive Work: When a gas expands (ΔV > 0), it exerts positive work on its surroundings.
Negative Work: When a gas is compressed (ΔV < 0), the gas is subjected to negative work from its surroundings.


First Law of Thermodynamics

According to the first law of thermodynamics, a system’s change in internal energy is equal to the heat it receives less the work it does.
Mathematically, ΔU = Q – W
Where:
  • ΔU = Change in internal energy
  • Q = Heat added to the system
  • W = Work done by the system

Special Cases of the First Law of Thermodynamics

Isothermal Process (ΔU = 0): Heat input is entirely transformed into work because the temperature remains constant.
Adiabatic Process (Q = 0): Since there is no heat exchange, the system’s own energy is used to do work.
Isochoric Process (ΔV = 0, W = 0): All heat given increases internal energy because the volume stays constant.
Isobaric Process (P = constant): The system’s volume and internal energy are altered when heat is introduced.


Summary

We may better understand how energy is transmitted and transformed in many physical and chemical processes by having a solid understanding of heat, internal energy, and work. In addition to physics, engineering, meteorology, and even biological systems like human metabolism depend on these ideas.
Temperature is a measurement of the average kinetic energy of the particles in a substance, whereas heat is the energy that is transferred between two substances as a result of a temperature differential. Temperature is expressed in degrees Celsius (°C) or kelvin (K), while heat is expressed in joules (J).
The sum of the kinetic and potential energy of the molecules in a system is known as internal energy. According to the first rule of thermodynamics, it varies as heat is added or removed or when the system undergoes work.
ΔU = Q − W
Where ΔU is the change in internal energy, Q is the heat added, and W is the work done by the system.
When a gas compresses or expands under pressure, work is produced. In a gas system, the formula for work performed is:
W = PΔV
Where ΔV is the volume change and P is the pressure. When a gas expands, it produces positive work; when it compresses, it produces negative work.
Heat transmission occurs in three primary ways:
Conduction: Direct contact transfer, such as heating metal rods.
Convection: Transfer by the movement of a fluid, like boiling water. 
Radiation: Heat from the Sun is one example of a transfer via electromagnetic waves.
There is no heat exchange with the environment during an adiabatic process (Q = 0). Work done on or by the system is the cause of any change in internal energy. Example: A gas’s temperature rises when it is compressed quickly.

According to the first law of thermodynamics, energy can only be moved or transformed; it cannot be created or destroyed. It serves as the foundation for all thermodynamic processes by explaining the relationship between heat, work, and internal energy in a system.
A system’s internal energy is increased when heat is introduced, which can cause the system to change states (solid to liquid, liquid to gas) or raise its temperature. Internal energy drops as heat is removed, which lowers the temperature or results in phase changes in the opposite direction.

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