Thermodynamics: Understanding Heat, Work, and Energy, Slides of Physics

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introduction

What is Thermodynamics

Thermodynamics is the study of effects of

work , heat and energy on a system.

Thermodynamics basics

Zeroth Law A B^ C

If A and B and B and C are in

thermal equilibrium, then A and C are

in thermal equilibrium. [i.e. At same T]

First Law of Thermodynamics

• Conservation of energy:

q = Δe + w

• The amount of heat ( q ) added to a

system is equal to the change in internal

energy ( Δe ) of the system plus any

work ( w ) done by the system

Entropy

T

dq

s rev or

∆ = ∫ T

dq

s sys

Third law of thermodynamics

The Entropy of a pure crystalline substance is zero at zero Kelvin. So unlike internal energy and other state functions it has absolute value.

Types of Heat Transfer

• Heat can be transferred by:
  • Conduction
  • Convection
  • Radiation

Conduction

• Conduction is the transfer of heat from molecule to molecule within a substance
• Molecules must be in direct contact with each other
• If you put one end of a metal rod over a fire, that end will absorb the energy from the flame.
• Molecules at this end of the road will gain energy and begin to vibrate faster
• As they do, their temperature increases.
• The heat is being transferred from the warmer end to the colder end, and eventually to your finger.

Convection

• Convection is the transfer of heat by the mass movement of a fluid (such as water and air) in the vertical direction (up and down)
• Convection occurs naturally in the atmosphere
• On a sunny day, the Earth’s surface is heated by radiation from the Sun.
• The warmed air expands and becomes less dense than the surrounding cold air.
• Because the warmed air is less dense (weighs less) than cold air, the heated air rises.

Convection

• As the warm air rises, the heavier cold air flows toward the surface to replace the rising air.
• This cooler air becomes heated in turn and rises.
• The cycle is repeated.
• This vertical exchange of heat is called convection and the rising air parcels are known as thermals

Radiation

Emitted radiation can be:

• Absorbed Increasing the internal energy of the gas molecules.
• Reflected Radiation is not absorbed or emitted from an object but it reaches the object and is reflected back. The Albedo represents the reflectivity of an object and describes the percentage of light that is sent back.
• Scattered Scattered light is deflected in all directions, forward, backward, sideways. It is also called diffused light.
• Transmitted Radiation not absorbed, reflected, or scattered by a gas. The radiation passes through the gas unchanged.