Lecture 7 Temperature and Heat. Buoyancy.

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Lecture 7Temperature and Heat. Buoyancy.
Chapter 4.1 ? 4.6
Outline

• Temperature and Heat
• Density and Pressure
• Buoyancy

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Temperature
All material objects in the Universe consist of
small particles called atoms.

• Atoms are constantly moving ? have energy

The amount of energy they have depends on their
mass Kinetic energy KEmv2/2 Rest energy E0m0c2
Temperature is a physical quantity that is
related to sensations of hot and cold. It is
related to energy of an object that we feel.
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Temperature and Heat
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Thermometers
Thermometer is a device that measures
temperature. It uses the property of substances
to expand when heated and shrink when cooled.
The most common thermometers use mercury or
alcohol.
Thermometers for high temperatures use pairs of
metal strips, which disconnect above a certain
temperature.
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Temperature scales
Any temperature measurement makes sense if it is
done on a scale.
There are 3 major temperature scales currently
used in the world Celsius (Centigrade), Kelvin,
and Fahrenheit.
The Celsius scale set its zero-point (0o C) at
the water freezing point and 100o C at the water
boiling point. The Kelvin scale has a zero-point
at the so-called absolute zero (?273o.16 C) and 1
K 1oC. The Fahrenheit scale zero-point is
attained by mixing equal parts of water, ice, and
salt (32oC). The water boiling point is 212oF.
1oC1.8oF.
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Relations between the Temperature Scales
Celsius to Kelvin K C 273 Celsius to
Fahrenheit F 1.8(C) 32 Fahrenheit to
Celsius C 5/9 (F ? 32o)
The normal body temperature is 37oC 98o.6 F
310 K The coldest temperature ever registered on
Earth ?89oC ?128oF 245 K
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Heat
The heat in a material body is a sum of the
kinetic energies of all its separate particles.

• The more heat the body contains, the higher its
  temperature.
• Joule is the unit for heat

Substances differ by the amount of heat needed to
raise their temperature by 1o.
It takes 4.2 kJ to raise the temperature of 1 kg
of water by 1oC.
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Heat Transfer
Conduction (transfer through a solid
body) Convection (transfer through a fluid or
gas) Radiation (by electromagnetic waves)
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Metabolic Energy
The biochemical processes that extract energy
from food are called metabolism.
Only 10?20 of metabolic energy can be converted
into mechanical work. Measured in kilocalories
1kcal 4.2 kJ.

• The rest of the energy is converted into heat.

The maximum power output of an animal depends on
the surface area and mass. Metabolic rate of a
person varies from 80 to 1200 W.
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Fluids
Particles vibrate around fixed positions in a
solid, move around them in a liquid, and move
freely in a gas.
Liquids and gases are called fluids (because they
flow).
Some properties of materials
Density is mass per unit volume d density
m mass V volume d
m/V
Density
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Pressure
Pressure is a force per unit area p pressure
F force A
area p F/A
Unit of pressure 1 pascal (Pa) 1 N/m2 1 kPa
1000 Pa
Barometers
Pressure
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Buoyancy
Buoyancy is an ability of a solid body to float
in a fluid
If an object is immersed in a fluid, an upward
force acts upon it.
The force on the bottom is higher than that on
the top.
The difference between the 2 forces is the
buoyant force. It enables balloons to float in
the air and ships to float in the sea.
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Archimedes Principle
Buoyant force on an object in a fluid is equal to
the weight of the fluid displaced by the object.
Fd dVg Fd buoyant force, d density, V
volume, g 9.8 m/s2 acceleration of gravity
Demonstration2
Demonstration1
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Summary

• Temperature is a measure of the average kinetic
  energy of all particles in an object

• Density describes mass of a unit volume and is
  different for different materials

• Pressure is a force applied to a unit area

• Buoyancy is a property of objects to float in a
  fluid.
• Weight of an object has to be less than the
  buoyant force.