Thermodynamic Nature of Energy

P M V SubbaraoProfessor

Mechanical Engineering Department

The Root Cause behind All Thermodynamic Actions…..

Change in Energy During A Process : Control Mass

Q -W depends only on the initial and final states and not on the path followed between the two states.

• Therefore it is the differential of a property of the system.

• This property is the energy of the mass and is given the symbol E.

• ThusWQdE 212112 WQEE

E = Micro Kinetic energy + Micro potential energy +Macro kinetic energy + Macro potential energy + …..

E = Internal energy +Macro kinetic energy + Macro potential energy + …..

........ PEKEUE

........ dPEdKEdUdE

The first law of thermodynamics for a CM during an infinitesimal process

WQdPEdKEdU

A Simple Process to Increase Macroscopic Kinetic Energy

The process of drilling connected with system Motor – Pulley drive – Drilling fixture-tool-workpiece (M-PD-DF-T-WP).

WQdPEdKEdU

First Law Analysis of Acceleration of Drilling Tool

Work done on tool leads to increase in rotational speed of the tool.No scope for heat transfer.No chance of change in other forms of energy.

WdKE

ddKE

drFdKE

dIdKE

d

dt

dIdKE

ddt

dIdKE

For acceleration drill tool

dIdKE

0

22

1

drmdKE

Can we use Heat Action to Increase Kinetic Energy of A System?

Should it be a control mass or Control volume or either of the two?

SUN

The Sun provides 175 billion joules of to the Earth’s atmosphere each hour.

Of this, approximately 1-2% is converted to wind energy.

Global Wind Patterns

A Process to Utilize Macroscopic Potential Energy

Hydro Electric Plant : The Work Done by A Falling Water Ligament patm

H

psur = patm

Microscopic Energy

• This energy is defined as the energy associated with the random, disordered motion of molecules and due to intermolecular forces.

• It is separated in scale from the macroscopic ordered energy associated with moving or stationary objects;

• It refers to the invisible form of energy at atomic and molecular scales.

• Popularly known as Internal Energy, U.

Internal (Microscopic) Energy : Ideal Gas

• Internal energy involves energy at the microscopic scale.

• Potential and Kinetic energies of individual molecules/atoms.

• But the potential energy is associated with intermolecular forces which are presumed to be zero in an ideal gas.

• Therefore the internal energy of an ideal gas is entirely kinetic energy.

Internal (Microscopic) Energy : Monatomic Ideal Gas

• For an ideal monatomic gas, this is just the translational kinetic energy of the linear motion of the "hard sphere" type atoms.

TRnU ~

2

3~

TmRU 2

3

For a monatomic ideal gas this change in internal energy is given by :

Internal (Microscopic) Energy : Diatomic Ideal Gas

• For polyatomic gases there is rotational and vibrational kinetic energy as well.

Internal (Microscopic) Energy : Polyatomic Gas

Internal (Microscopic) Energy : Other Substances

• For real gases, liquids and solids the internal energy is the sum of kientic and potential energies associated with the intermolecular attractive forces.

Increase of Internal Energy

Supply enough heat to each of these systems till the there is 1C increase in temperature.

Internal Energy and Temperature