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Unit three -106- Chapter eight

Low temperature physics

Ideal Gas:

It is the gas at which each of the following are neglected:

1.

...

2.

...

Van Der Waals effect:

The real gas (differs than / similar to) ideal gas.

Where the increase in its density leads to . between the

molecules due to force.

At high (pressure / temperature) Van Der Waals interaction takes over, where

the molecules (attracted / repeal) each other, and eventually attract more and

more of molecules, until the gas (condensed / evaporated).

This mechanism helps to achieve ....

Mechanism of achieving low temperatures:

The mechanism for achieving very low temperature depends on

. by ...

The latent heat for any matter is (greater / smaller) than its specific heat

energy.

Therefore, by contact pre-cooled material such as ice or dry ice to any

substance, that substance will (lose / gain) energy which used in ..,

while the ice will (lose / gain) energy as (latent heat / specific heat) and low

temperature can be reached.

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Unit three -107- Chapter eight

By contact Nitrogen to ice, It (lose / gain) energy, and ice (lose / gain) energy,

then temperature reach 77K (liquid Nitrogen), then by using of liquid

Nitrogen we reach 4.2K (liquid Helium).

Super fluidity:

Defilation: ..

Helium liquid has the property of super fluidity where:

1. It loses its ., and it suffers .. with the

walls of the container. Therefore It can . uninterruptedly

against gravity.

2. It has .., so it consider as one of the best

..

Dewars Flask:

Heat energy transfer by: , .. and ....

Therefore Dewars flask consists of:

walls metallic or glass (Pyrex) flask

The spacing between the walls is . to prevent .

and ..

The walls are plated by to minimize heat transfer by

...

It is used to store liquid Nitrogen (boiling point 77K), liquid Oxygen

(boiling point 90K), while for liquid Helium (boiling point 4.2K)

.. are used with .. filling

the space between the two flask, to prevent ... because

Helium has very low and low . point.

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Unit three -108- Chapter eight

How does a refrigerator work?

Heat energy Qth gained by any gas is used in:

1. Increase the . energy U of the gas, which manifested by

increase its .

2. by gas molecules W

There are two types of thermal exchange:

Isothermal Process:

Qth = . + .

Qth = constant + .

Where . =zero

Qth = .

Both of . and .. of the gas are constant.

Thermal energy Qth gained by the gas convert into ....

Such as (heating / condensing / evaporating) of matter at (constant /

variable) temperature.

Adiabatic Process:

Qth = . + .

Constant = + .

Where . =zero

. = .

The gas is thermally (isolation / contact) with the surrounding medium.

When work done by the gas molecules (+ve / -ve) W it will (decrease /

increase) its internal energy (-ve / +ve) U.

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Unit three -109- Chapter eight

Such as (liquefied / evaporated/ cooling) gas.

When an external work done on the gas molecules (+ve / -ve) W, it will

(increase / decrease) its internal energy (+ve / -ve) U.

Such as (compress / rarefaction) the gas.

Operation of a refrigerator:

The base idea of refrigerator is .. pump that transfers

.. from the refrigerator to the outside by using of circulation of refrigerant liquid

(Freon), and the internal energy of the refrigerant liquid during complete cycle will

(change / not change) U (greater than / equal to / smaller than) zero

Therefore the electric energy E required for the pump (greater than / equal

to / smaller than) the work done by the pump (greater than / equal to / smaller than)

the heat energy transfer to the outside.

In the refrigerator cabin:

The Freon is liquid at low temperature and high pressure.

Freon (gains / lose) heat energy form the cabin, to (increase / decrease) its

temperature up to the boiling point (-30C). where the heat energy Qth

converted into (internal energy U / work W)

Freon gains heat energy from the cabin, to convert into gas at constant

temperature (isothermal / adiabatic), process where Qth (greater than /

equal to / smaller than) W.

In the thermally isolated compressor:

The compressor does work (on / by) the gas to increase its pressure,

therefore the work of the gas molecules will be (+ve / -ve).

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R

TTc

Unit three -110- Chapter eight

Since the compressor is isolated therefore the process is (isothermal /

adiabatic) process and (W+U) is (greater than / equal to / smaller than)

zero

Since the work W of gas molecules is ve therefore the change in the

internal energy of the gas U will be (+ve / -ve) and the temperature of

the gas will (increase / decrease).

In the condenser (outside the refrigerator):

The compressed gas loss its heat energy to the surrounded to convert into

liquid at constant temperature (isothermal / adiabatic) process

In the expansion valve:

Before Freon inter the refrigerator, it allow to expand in an adiabatic

process, where work done by the gas molecules to expand (+ve / -ve)

W, which lead to (increase / decrease) its temperature (-ve / +ve) U

Superconductivity:

Definition: ...

By decrease the temperature, the resistance of the matter (increase /

decrease), until it reaches critical (transitional) temperature, therefore the

electric resistance drop to zero (approximately).

Critical temperature:

....

The electric resistance of the

superconductor materials approximately

(greater than / equal to / less than) zero,

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Unit three -111- Chapter eight

therefore no heat energy companied the flow of the electric current and

no electric energy consumed.

It means that the electric current will not stop flowing even if the

potential difference is removed. (G.R)

....

Superconductive materials:

Platinum, aluminum, zinc, lead, mercury and some metallic compounds

(paramagnetic / diamagnetic / ferromagnetic) materials.

Application of superconductivity:

1. Superconductors are used in electric circuit of satellites, because

..

2. It is used in electric power plants and in transmission line, because

.

3. It is used in high speed (magnetically levitated) train, where the train

raised above the rails for a few centimeters, (eliminates / increase)

friction, which lead to (decrease / increase) its velocity (meissner effect).

Meissner effect:

1. Superconductors are (paramagnetic / ferromagnetic / diamagnetic)

materials, in which the total magnetic field inside the matter (greater than

/ equals to / less than) zero.

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Unit three -112- Chapter eight

2. When a permanent magnet is placed over a disk of superconducting

material, the magnetic field due to the permanent magnet produce

. in the superconducting material.

3. The electric current in the supper conductor produce ..

in (same / opposite) direction to the external field, in order to

..

4. Due to the magnetic field of the permanent magnet and the magnetic field

produced in the supper conductor which are in

magnitude and .. in direction, the permanent magnet

remains hanging in the air.

Magnetic Resonance:

Magnetic Resonance imaging (MRI):

1. Superconductors are used to produce ..

2. The alternating magnetic field affects the hydrogen nucleus in the body,

to stimulate it to produce .

3. The waves produced by the hydrogen nucleus will be indicative for

in the body.

4. It is used to form an image to the internal parts of the body, also it is used

to indicate the . and ..

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