The first law of thermodynamics states that:

“Energy can be neither created nor destroyed but one form of energy can be converted to another form.”

For example, consider a ball is placed on the top of a table initially. It will have certain potential energy ( Energy possessed by virtue of its height ) as it is at a height from the ground. When it is allowed to fall from the table this potential energy will be converted into kinetic energy ( Energy possessed by virtue of its motion ). This kinetic energy will be converted to heat, sound, etc. when it touches the ground.
In the application of the first law to a given process, the sphere of influence of the process is divided into two parts namely system and surroundings, which is illustrated in the image below.

Figure 1: Diagram to show the difference between system, surroundings and boundary.

The region in which the process occurs is the System and everything which the system interacts is the surroundings. First law of thermodynamics applies to both system and surroundings. In general,

Δ Energy of system  +  Δ Energy of surrounding  = 0

For the above example if you consider ball as a system  initial energy is potential and final energy is kinetic,but the energy is gained by surroundings as heat and sound.

Systems are of two types.

• Open = System which exchange both mass and energy with surroundings.
• Closed = System which exchange only energy with surroundings.

For simplification here we are considering closed systems only. In general system contains some internal energy ( in the form of attractions and vibrations ) and this tend to change when the heat is added or removed, when work is done on the system or delivered by the system.For closed systems energy transfer between system and surroundings takes place in the form of work and heat. ( where as in open systems internal energy will be associated in transit also i.e., at entry and exit of the system ). For closed systems energy changes mostly occur in internal energy. So,

Δ Energy of system = Change in internal energy  = ± Q ± W

Only change in internal energies can be found as it is hard to know the energy associated with  attractions and vibrations. Q is heat and W is work.

Sign Convention For Heat And Work

• Heat given by the system, Heat produced by the system = -Q
• Heat given to the system, Heat supplied to the system = +Q
• Work done by the system, work produced by the system = -W
• Work done on the system, work given to the system = +W

Example:

Δ Internal energy  =  Q – W

Heat is given to the system and work is done by the system.

Adrian Michaels

Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).

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Temperature is an objective measure of hot or cold and is measured using a thermometer. Temperature is used to determine if 2 system would be in thermal equilibrium in the event, they came into contact with each other through a diathermic wall and establish the energy flow.

There are two types of boundary:

• Diathermic: Thermally conduction, heat transfer occurs passing through freely.
• Adiabatic: Thermally insulting, no heat transfer to the surroundings.

An example of diathermic is when a hot object in thermal contact with a cold(er) object, resulting in heat flowing through the hot object to the cold object until thermal equilibrium is achieved where the heat flow stops and both objects are at the same temperature. For an adiabatic system, the heat in the hot object will not transfer to the cold object, as the heat in the hot object won’t go leave to it surroundings which would be the cold object (figure 1).

Figure 1: Adiabatic and diathermic wall heat transfer ( CIET, 2012).

What Is Pressure?

The pressure is afore exerted by a fluid per unit area, and the standard pressure is 1 bar which is measured with equipment that includes; barometers, manometers, fibre optic and piezoelectric. Most pressuring-measuring devices are calibrated to read zero when in the atmosphere, this is called gauge pressure.

In most equations absolute pressure is used, where p = 0 corresponds to a perfect vacuum and in a lot of equation this is called p_abs. Also, pressures below atmospheric pressures are called vacuum pressures.

There is a relationship that relates these types of pressures (figure 2):

p_abs = p_atm + p_{g     &}   p_abs = p_atm – p_vac

Figure 2: Relationship between gauge, absolute and vacuum pressure (Insta Tools, 2020).

Equilibrium

A system is in equilibrium with its surrounds when it does not tend to undergo spontaneous change. Whilst a system is in equilibrium, it will experience no changes when it is isolated from its surrounding.

Types of equilibrium:

• Mechanical equilibrium: No change in pressure within the system and doesn’t change with time.
• Thermal equilibrium: Temperature is constant throughout the system.
• Chemical equilibrium: Chemical composition is the same in the system and doesn’t change with time.
• Phase equilibrium: A system involving two phases and the mass of each system reaches an equilibrium level and stays within the system.

Describing A System

When describing a system, the state variables (p, n, T, V…) are independent of the system’s history and the number of properties required to fix the state of the system is given by the state postulate:

“The state of a simple compressible system is specified completely by two independent intrusive properties.”

Simple compressible system:  If the system involves no electrical, magnetic, gravitational, motion and surface tension acting on it.

For a one-component system, all that is required is n (moles) and 2 variables, all other properties then follow:

V = f(n, p, t)

Example for notation:

A cylinder containing 2 moles of nitrogen gas at 1 bar pressure and a temperature of 25 ^oC (figure 3).

Notation: 2 N₂ (g, 1 bar, 25 ^oC)

Figure 3: Cylinder containing nitrogen gas for notation example.

References

CIET. (2012). Physics-02. Retrieved from CIET: https://ciet.nic.in/moocspdf/Physics02/Unit08/keph_201201_eContent2019.pdf

Insta Tools. (2020). Difference Between Absolute and Gauge Pressure. Retrieved from Insta Tools: https://instrumentationtools.com/difference-between-absolute-and-gauge-pressure/

Dr. Adam Zaidi

Dr. Adam Zaidi, PhD, is a researcher at The University of Manchester (UK). His doctoral research focuses on reducing carbon dioxide emissions in hydrogen production processes. Adam’s expertise includes process scale-up and material development.’

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Thermodynamics is a branch of physics that involves the relationship between heat, work and temperature and other forms of energy. The word thermodynamics can be split up in two: Thermo which refers to heat and dynamics which refers to motion and is the study of:

• The movement of heat.
• Heat and work.

Thermodynamics is expressed in terms of four laws that are universally valid and cannot ever be broken:

• 0^th law – Defines temperature (T)
• 1^st law – Defines energy (U)
• 2^nd law – Defines entropy (S)
• 3^rd law – The numerical value of entropy

Basic Thermodynamic Definitions

These are a list of some of the most important phrases used in thermodynamics and need to be remembered, they are straightforward and self-explanatory so won’t require a lot of effort to remember.

• System: Quantity of matter of fixed identity of the universe that we are studying.
• Surroundings: Rest of the universe.
• Boundary: Surface splitting the system from the surroundings.
• Open system: Mass and energy can move between the system and the surroundings.
• Closed system: Only energy can transfer between the system and the surrounding and not mass.
• Isolated system: No mass or energy can be transferred between the system and surroundings.

Dr. Adam Zaidi

Dr. Adam Zaidi, PhD, is a researcher at The University of Manchester (UK). His doctoral research focuses on reducing carbon dioxide emissions in hydrogen production processes. Adam’s expertise includes process scale-up and material development.’

The post Basic Thermodynamic Concepts And Definitions appeared first on Engineeringness.