# Thermodynamics and Engines

Thermodynamics is all about how we produce and use energy (heat), and it's super useful for understanding engines. By using thermodynamics, we can simplify the engine and just focus on what's going in and coming out. The laws of thermodynamics help us figure out how efficient an engine can be and how energy and work are connected.

Okay, now let's talk about systems. In physics, a system can be anything that takes inputs and gives outputs. It could be a machine, a living thing, or even just an ice cube melting in water. Systems can be big or small, living or non-living, and can range from a bunch of gas particles mixing in a room to an entire planet and all its energy processes. Understanding systems is key to understanding thermodynamics and engines.

## Thermodynamic systems

A thermodynamic system is any system that has a boundary and where energy is exchanged. This means anything can be a thermodynamic system, from our own bodies to the sun! By using thermodynamic systems, we can study how energy and mass are exchanged without getting bogged down in all the nitty-gritty details of individual processes. It's a helpful way to look at the big picture and understand how things work.

### Thermodynamics and Engines: Heat

Heat is the energy that gets transferred to a system or object. This energy transfer is often called thermal change. When the thermal energy changes, it affects the kinetic energy of the particles that make up a substance. This change in kinetic energy is especially noticeable in liquids and gases. So when we talk about heat, we're really talking about the energy that's being transferred and how it affects the particles that make up the substance.

### Thermal energy and the internal energy of a gas

Thermal energy is the energy of an object's molecules or atoms, which is related to the kinetic energy of its particles moving randomly. The higher the kinetic energy of the particles, the higher the thermal energy of the object.

Potential energy refers to the energy stored in gas molecules and is composed of the kinetic energy of individual molecules and their disordered movement, as well as the potential between the molecules that make up. "U". It's important to note that the kinetic and potential energy of the gas as a whole should not be confused with the kinetic and potential energy of its individual molecules.

Thermodynamics is the branch of science that studies how systems behave when work, heat, and entropy change. There are four laws of thermodynamics that every object in the universe follows. The zeroth law is the law of thermal equilibrium, the first law describes the internal energy of a substance, the second law is the entropy law of irreversibility, and the third law states that at absolute zero (zero kelvin), entropy reaches a constant value. These laws are universal and are crucial for understanding how various systems and engines work.

### The zeroth law of thermodynamics

According to the Zeroth Law of Thermodynamics, two objects or systems are in thermal equilibrium (at the same temperature) if both are in equilibrium with a third object or system. This means that if two objects are both separately in thermal equilibrium with a third object, then they are in thermal equilibrium with each other.

For example, when two cupcakes are removed from the oven, they will have different temperatures. However, after some time, they will reach the same temperature as the kitchen air, meaning they have reached thermal equilibrium.

### The first law of thermodynamics

The first law of thermodynamics states the energy of an isolated object or system remains constant, and that energy can be transformed but not destroyed. This can be expressed mathematically as ΔU = Q - W, where ΔU is the change in the internal energy of the object, and W and Q are the work and heat absorbed or released by the object.

This law is essentially an energy conservation law, as the energy of an object will only change if it either receives or produces work or if it absorbs or releases heat. For example, when you heat water in an oven, the internal energy of the water increases as the molecules begin to move faster. This increase is caused by the heat from the oven, and in this case, the water does not produce any work.

### The second law of thermodynamics

The second law of thermodynamics dictates the direction in which energy flows. Rudolf Clausius, a German scientist stated that from a colder object to a object intervention of another process.

We are all familiar with the phenomenon of heat escaping when we open a window on a cold day or an oven door after baking. However, less intuitively or logically, machines that produce work are unable to convert 100% of the fuel into work.

The implications of the second law of thermodynamics are that a machine or system always experiences a non-recoverable heat loss. This loss of heat or energy is part of an irreversible process that cannot be reversed without using more energy. The degree of irreversibility is measured by a variable called entropy, symbolized by 'S'.

### The third law of thermodynamics

The third law of thermodynamics establishes a connection between the temperature of a system, the atomic arrangement of the system, and its energy. It states that 'the entropy of a system at absolute zero is constant'. In this, the entropy a of water at 120 degrees at sea level. At this temperature, the water has turned into a gas, and its entropy is high since its molecules move freely with high kinetic energy. As the temperature decreases below 100 degrees, the disorder of the water molecules reduces, and the water returns to its liquid state. Its molecules are linked together by stronger forces, and the water has less kinetic energy compared to its gaseous state. When the temperature drops below 0 degrees, the molecules have even less kinetic energy, and their disorder decreases further, as the particles arrange themselves in a crystal form. In this state, the molecules cannot move freely. When the water reaches the lowest temperature possible, zero kelvin (or absolute zero, which is -273.1459 Celsius), its molecules cannot move at all, and the crystals have reached the state of least possible disorder.

During this entire process, the disorder of the particles decreases as energy is extracted from the water. As the disorder decreases, the entropy also decreases until it reaches a state where it cannot decrease any further, becoming constant.

## What are engines?

In the field of thermal engineering, engines are machines that either use thermal energy to produce work or use work to modify the system's thermal energy.

A cooling or heating machine is a type of machine that utilizes work, denoted as 'W', to change the thermal energy of a system. An air conditioning system is an example of a cooling machine, which uses electrical energy to pass air from the room through a series of tubes containing a liquid or gas that absorbs the heat of the air, releasing it outside the room.

On the other hand, a thermal engine or heat engine is a machine that uses heat, denoted as 'Q', to produce work. An example of this is a combustion engine in a car, which uses gas and an electrical spark to produce a controlled explosion that generates energy and heat. The energy from the explosion is transformed into movement by the car's mechanical components, which we refer to as 'work'.

See also the following diagrams of a cooling machine (left) and a heat engine (right):

Efficiency is an important factor to consider when it comes to engines. It is the measure of the amount of work done with the energy used. The efficiency of an engine can be expressed as the ratio of work done (W) to energy used (Q). It has no units and is dimensionless. The minimum efficiency for any work done is 0, and it cannot be greater than 100%.

Another important concept related to engine efficiency is the thermal efficiency, which is the difference between heat input and heat output. The work done by the engine is equal to the difference between heat input and heat output. This formula is useful in calculating the efficiency of engines, as it helps to determine how effectively the energy is being used.

Thermodynamic cycles thermal engine that involve heat and work. These cycles are systems in which four variables change: volume, pressure, entropy, and temperature. The changes are the result of changes in the internal energy, the work produced or received, or the heat produced or received. Examples of thermodynamic cycles include the Carnot cycle, Otto cycle, Stirling cycle, Ericsson cycle, Rankine cycle, Diesel cycle, and Brayton cycle. Each cycle is composed of four steps: compression of a fluid, heat addition to the fluid, expansion of the fluid, and heat rejection by the fluid.

In summary, thermodynamics is an area of physics that studies energy exchanges involving heat and work. Engines in thermal engineering are machines that use heat to produce work, and they can alter the energy of a system using mechanical work. The four laws of thermodynamics specify how energy exchanges occur in every system. The efficiency of engines is an important consideration, and it can be calculated using the ratio of work done to energy used. Thermodynamic cycles involve a series of processes performed by a thermal engine that involve heat and work.

## Thermodynamics and Engines

How is thermodynamics used in engines?

It is used to model engines, as thermodynamics deals with how work and energy are consumed and produced.

How do the laws of thermodynamics affect heat engines?

The laws of thermodynamics determine the maximum efficiency of a heat engine and how work and energy are related.

What is the thermodynamic efficiency of the engine?

Efficiency is the amount of work done with the energy used.

Is a car engine a thermodynamic system?

Yes, a car engine is a machine that produces mechanical work using fuel in a process that also produces heat.