Thermal physics

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Topic 3

• Thermal physics

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What is Temperature?
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Temperature
TEMPERATURE determines the direction of flow of
thermal energy between two bodies in thermal
contact
This is another way of saying that if an object
is hotter than another, heat energy will flow
from the hotter object to the colder!
Obvious, but important to remember!
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Temperature

• Temperature is also a measure of the average
  random kinetic energy of the particles in a
  substance.

Note that they are not all travelling at the same
speed.
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Temperature

• The hotter the temperature, the faster the
  average speed of the particles

Note that they are not all travelling at the same
speed.
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Thermal equilibrium
Two bodies in thermal contact will eventually
reach the same temperature. The two bodies are
now said to be in thermal equilibrium.
WARM
WARM
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Celcius Scale

• In the Celcius scale, a value of zero degrees is
  assigned to the freezing point of water, and a
  value of one hundred degrees to the boiling point
  of water.

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Making a Celcius thermometer

• Place a glass tube containing mercury in a
  mixture of ice and water and labelling the
  position of the mercury as zero.

0C
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Making a Celcius thermometer

• Then place the tube in boiling water and label
  the new length as 100C.

100C
0C
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Making a Celcius thermometer

• Finally the range from 0 to 100 is subdivided
  into equal intervals.

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Pressure and Temperature

• http//phet.colorado.edu/sims/ideal-gas/gas-proper
  ties_en.jnlp

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Kelvin Temperature

• Lord Kelvin
• 1824-1907

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Kelvin Temperature

• Kelvin temperature is proportional to the
  average random kinetic energy of the particles in
  a substance.

Note that they are not all travelling at the same
speed.
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Absolute/Kelvin temperature and Celsius

• T (in Kelvin) T (in degrees Celcius) 273
• Just to mention for now that zero Kelvin is the
  lowest possible temperature.

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Measuring temperature

• The thermometer has to be placed in thermal
  contact with whatever is being measured until the
  thermometer and object are in thermal equilibrium.

100C
0C
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So what is heat?

• Heat is the amount of thermal energy. For
  example, the sparks from a sparkler are at around
  800C but do not burn your skin. However, a hot
  cup of tea at around 100C will burn your hand
  badly. This is because the tea contains more heat
  energy, even though it is cooler. Thermal energy
  is transferred by a temperature difference.

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Internal Energy

• This is the sum of the kinetic energies and
  potential energies of the particles in a substance

If you imagine the forces between particles as a
spring, you can see if the particles are pulled
apart or squashed together that energy is stored
in the spring. Similarly there is potential
energy between the particles in a substance.
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Internal Energy

• The internal energy of a substance can be
  changed by heating it (or taking heat from it) or
  by doing work on the object (or having the object
  do work).

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Moles!
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Moles!

• Equal masses of different elements will contain
  different numbers of atoms (as atoms of different
  elements have different masses)

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Moles!

• It is sometimes useful for physicists and
  chemists (but we dont care about them) to
  compare the number of atoms or molecules in an
  amount of substance.
• To do this we use the idea of moles.

A chemist
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Moles!
You need to learn this definition.

• One mole of a substance contains the same number
  of molecules/atoms as in 12 grams of carbon-12.
• This number (of atoms or molecules) is known as
  the Avogadro constant (NA) which is equal to 6.02
  x 1023

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How big is 6.02 x 1023?

• Imagine the floor of this classroom covered in
  unpopped popcorn

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How big is 6.02 x 1023?

• Imagine the all the floors in the school covered
  in unpopped popcorn

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How big is 6.02 x 1023?

• Imagine the whole of Busan covered in unpopped
  popcorn

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How big is 6.02 x 1023?

• Imagine the whole of Korea covered in unpopped
  popcorn

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How big is 6.02 x 1023?

• Imagine the whole of Europe covered in unpopped
  popcorn

You are here!
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How big is 6.02 x 1023?

• Imagine the whole of Europe covered in unpopped
  popcorn to a depth of 6 miles!

You are here!
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How big is 6.02 x 1023?

• Imagine the whole of Europe covered in unpopped
  popcorn to a depth of 6 miles! Count the grains
  and that is 6 x 1023!
• 600000000000000000000000
• Thats how big 6.02 x 1023 is!

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Moles!
You need to learn this definition.

• One mole of a substance contains the same number
  of molecules/atoms as in 12 grams of carbon-12.
• This number (of atoms or molecules) is known as
  the Avogadro constant (NA) which is equal to 6.02
  x 1023

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Moles!

• For example, Hydrogen (H2) has a relative
  molecular mass of 2, so 2 grams of hydrogen (one
  mole) contains the same number of molecules as
  atoms in 12g of carbon-12 (6.02 x 1023)

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Moles!

• It follows therefore that 7g of lithium (atomic
  mass 7), 20g neon (atomic mass 20) or 39 g
  potassium (atomic mass 39) all contain the same
  number of atoms (1 mole or 6.02 x 1023 atoms)

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Moles!

• The number of moles of a substance can thus be
  found by dividing the mass of substance by its
  relative atomic or molecular mass
• n mass/RAM

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Example

• How many moles of sulphur atoms are there in
  80g of sulphur? How many grams of carbon would
  have the same number of atoms?

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Example

• How many moles of sulphur atoms are there in
  80g of sulphur? How many grams of carbon would
  have the same number of atoms?
• N mass/RAM 80/32 2.5 moles

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Example

• How many moles of sulphur atoms are there in
  80g of sulphur? How many grams of carbon would
  have the same number of atoms?
• N mass/RAM 80/32 2.5 moles
• Mass of carbon RAM x n 12 x 2.5 30 g

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Relative formula mass

• We can use the idea of moles and apply it to
  molecules using relative formula mass.
• C2H5OH
• RFM (2 x 12) (6 x 1) (1 x 16) 46
• 46g of ethanol I mole of ethanol molecules

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Equal volumes

• Luckily, equal volumes of gas contain the same
  number of particles (at the same temperature and
  pressure)

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Equal volumes

• For example, 1 litre of nitrogen contains the
  same number of molecules as there are atoms in
  one litre of Argon.
• A litre is 1000 cm3 or 1 dm3

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Mole of gas

• One mole of any gas occupies 22.4 dm3 (22.400
  cm3 or 22.4 litres) at standard temperature and
  pressure (25C and 1 atmosphere)