Heat (English)

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3-1: What is heat?

When an object

is heated,

its temperature rises.

Everyone knows that,

when an object

of higher temperature

is in contact with

another of lower

temperature,

heat is transmitted

from the higher

to the lower temperature

object.

Here, what is heat?

It is known well

at present that

the heat transfer

(heat conduction) is

a transfer of energy;

namely, heat

is a form of

energy.

How can we explain this?

This problem

was just the clue

to make a relation

between the macroscopic

and the microscopic world.

[The Phlogiston Theory, the Caloric Theory]

Up to the beginning

of the 19th century,

combustion

had been understood

by supposing an invisible

negative-mass

substance named

phlogiston.

According to this theory,

every combustible object

contains phlogiston

and combustion

is explained as

a process to lose

the phlogiston.

When such an object

as mercury burns,

the mass would increase.

Looking at this fact,

people thought

that the mass

of the phlogiston

is negative.

The heat conduction

had been explained

by introducing

an imponderable fluid

called caloric.

It was considered

that

a higher temperature

object contains

more caloric

than a lower one,

and the transfer

of the caloric implies

the heat conduction.

Lavoisier

confirmed by

carrying out

quantitative chemical

experiments precisely

that combustion

of an object

is a process

in which some

substances combine

with others.

Thereby the theory

of phlogiston was denied.

Moreover, looking at

an ultimate supply

of heat

by friction,

people suspected

the caloric theory

and abandoned it

after Joule's experiment

that will be

described below.

[Nature of Heat]

Using the apparatus

shown in the following

figure,

J. P. Joule

(UK, 1818 - 89)

confirmed that

heat is

equivalent to

energy,

and measured

the mechanical equivalent of heat

that represents

the quantitative relation

between heat

and mechanical energy.

The apparatus used by Joule

This is

the apparatus used

by Joule

who confirmed

the equivalence between

heat and

mechanical energy.

When the weights

go down,

the impellers

in a water tank

rotate to stir the water.

The potential energy

of the weights is

converted into

the kinematical energy

of the

rotation of

the impeller,

and the temperature

of the water rises.

[The Mechanical Equivalent of Heat]

The unit to measure heat

is cal (calorie).

The heat required

to raise the temperature

of 1 g of water by

from

at 1 atmospheric pressure

is 1 cal.

The currently accepted value is

[Statistical Mechanics]

As learned in

Part 1: (Prologue: Atomic Nature of Matter and Electricity,

it became clear

that

a matter consists

of a huge number

of molecules and atoms.

At first,

the motions of these

molecules and atoms are

considered to be described

according to the laws of

the classical theory,

i.e. Newtonian mechanics and

Maxwellian electromagnetism.

Although we cannot watch

the motions of molecules

and atoms directly,

we can connect them

with the quantities

in the macroscopic world,

e.g. the pressure

or the temperature,

calculating an appropriate

statistical average

over a great number

of degrees of

freedom of those molecules

or atoms.

For example,

the pressure of a gas is

the average value

of the forces acting

on the wall

of the container

when the gas molecules

colliding with it.

And the temperature

of the gas denotes

the average value

of the energies of

individual molecules.

Thus the attempt

in which the various

laws in the macroscopic world

are derived

from the microscopic

degrees of freedom

of individual molecules

and atoms

is just

statistical mechanics

developed by

J. C. Maxwell

(UK, 1831 - 79)

and

L. Boltzmann

(Germany, 1844 - 1906).

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