Laws of Thermodynamics

2014

Laws of Thermodynamics1. The first law of thermodynamics is:

“Whenever heat flows into or out of asystem, the gain or loss of thermal energyequals the amount of heat transferred.”

The first law is also known by another name:

Conservation of energy.

The basic rule is straightforward– However much heat is added or removed from an object must equal the change in the total energy content of that object.

Second Law:Entropy Law Entropy = disorder, randomness or chaos

“In an isolated system, entropy tends to increase spontaneously”

Heat cannot by itself pass from a colder to a hotter body.

Third law:

If an object reaches the absolute zero of temperature (0 K = -273.15 °C = −459.67 °F), its atoms will stop moving.

The statement is: At absolute zero, the entropy of a perfectly crystalline substance is zero.

Specific Heat

The heat required to increase the temperature of a substance is given by the specific heat, Cp

Cp= Q/m∆T

Q =mCp∆T

Specific HeatLarge specific heat means it takes large

quantities of heat with little change in temperature.

Example:Specific heat of 1kg water iscwater = 4184 J/(kg*K)

Examples of Specific Heat

Why do different materials have different specific heats?

There are two main reasons:1. One kilogram will have different numbers of molecules for different substances.2. The heat added can go into other forms of energy besides kinetic energy.

Water’s High Specific Heat

Water has a very high specific heat for a few reasons…..

In part because H2O molecules are light and there are a lot of them in one kilogram

And because a lot of the added heat goes into vibrations and rotations of the molecules and not into kinetic energy.

Conductors: Good vs. Bad

Low specific heat = good thermal conductors

High specific heat = bad thermal conductors

Heat Transfer

The change of temperature depends on Heat transfer: Q in joules (j)Mass of the substance: m in kilograms (kg)The specific heat of the substance: Cp (J/kgK)

ΔT = Q/ mCp

Q = mCpΔT

Energy - Q in J

Mass - m in kg

Specific heat - Cp in J/(kg∙K)

Temperature - T in K

ExampleA 1.63-kg cast-iron skillet is heated on the stove

from 295 K to 373 K. How much heat had to be transferred to the iron?

m = 1.63 kg Q = CpΔTm

ΔT = 78 KCp = 450 J/kgK Q = 57,213 J