Topic 3: Thermal Physics

What is Temperature?

TemperatureTEMPERATURE determines the direction of flow of thermal energy between two bodies in thermal contact

HOT COLD

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!

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.

Temperature

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

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

Thermal equilibriumTwo bodies in thermal contact will eventually reach the same temperature. The two bodies are now said to be in thermal equilibrium.

WARM WARM

Measuring Temperature

In 1742, Andreas Celsius created the temperature scale that is known by his name.

Celsius Scale

In the Celsius 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.

Making a Celsius thermometerPlace a glass tube containing mercury in a mixture of ice and water and labelling the position of the mercury as zero.

0°C

Making a Celsius thermometerThen place the tube in boiling water and label the new length as 100°C.

0°C

100°C

Making a Celsius thermometerFinally the range from 0 to 100 is subdivided into equal intervals.

0°C

100°C

Kelvin Temperature

• Lord Kelvin

(William Thomson)

• 1824-1907

Absolute/Kelvin temperature and Celsius

K= C + 273

Zero Kelvin is the lowest possible temperature.

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.

So what is heat?

So what is heat?Heat is thermal energy.

It is energy transferred from a higher temperature object to a lower temperature object.

So what is heat?

For example, the sparks from a sparkler are at around 800°C but do not burn your skin. However, a hot cup of tea at around 100°C will burn your hand badly.

This is because the tea contains more heat, even though it has a lower temperature.

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.

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).

Moles!

Boo!

Moles!

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

Moles!

It is sometimes useful to compare the number of atoms or molecules in an amount of substance.

To do this we use the idea of moles.

Moles!

• 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

You need to learn this definition.

How big is 6.02 x 1023?

Imagine the floor of this classroom covered in unpopped popcorn

How big is 6.02 x 1023?

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

How big is 6.02 x 1023?

Imagine the whole of Chicago covered in unpopped popcorn

How big is 6.02 x 1023?

Imagine the whole of Illinois covered in unpopped popcorn

How big is 6.02 x 1023?

Imagine the whole of the United States covered in unpopped popcorn

How big is 6.02 x 1023?Imagine the whole of the United States covered in unpopped popcorn to a depth of 6 miles!

How big is 6.02 x 1023?

Imagine the whole of the United States covered in unpopped popcorn to a depth of 6 miles! Count the grains and that is 6.02 x 1023!

602,000,000,000,000,000,000,000

That’s how big 6.02 x 1023 is!

Moles!

For example, Hydrogen (H2) has a 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)

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)

Moles!

The molar mass is defined as the mass of one mole of particles.

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?

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

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

Pressure and Temperature

• http://phet.colorado.edu/sims/ideal-gas/gas-properties_en.jnlp

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

What about moles and

gases?

Equal volumes

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

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

Mole of gas

One mole of any gas occupies 24 dm3 (24000 cm3) at standard temperature and pressure (25°C and 1 atmosphere)

You will be given this in a

question