Topic 3

Thermal physics

Last lesson?

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

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

Kelvin Temperature

• Lord Kelvin

• 1824-1907

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.

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.

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.

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 x 1023?

Imagine the whole of the United states covered in unpopped popcorn to a depth of six miles!

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)

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 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

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

Mole of gas

One mole of any gas occupies 24 dm3 (24000 cm3) at standard temperature and pressure (20°C and 1 atmosphere)

You will be given this in a

question

Moles of gas = volume (cm3)/24000 = volume (dm3)/24

Learn this!

Let’s try some questions:

Pages 161 and 162Questions 4, 5 and 6

Don’t forget to read the relevant pages in your

textbook (Pages 158-162)

Today’s lesson

• Define specific heat capacity and thermal capacity.

• Solve problems involving specific heat capacities and thermal capacities.

Imagine if…………

Imagine if……..

Two beakers of water were heated by identical heaters for an equal amount of time.

The beaker with less water would end up hotter. WHY?

Imagine if………

Imagine if……..

Two beakers, one containing water and one containing kerosene (equal masses) were heated by identical heaters for an equal amount of time.

The beaker containing the kerosene would be twice the temperature! WHY?

Since the amount of heat energy supplied is the same to both substances, it seems that different substances require different amounts of heat energy to cause the same temperature rise.

Heat Capacity

The relationship between the amount of heat energy a substance requires to raise its temperature by a given amount is called its thermal capacity. It is measured in J.°C-1 or J.K-1.

Definition to learn

• Thermal capacity is the amount of energy needed to raise the temperature of a substance by 1K.

Calculations using Thermal capacity

Energy absorbed = Thermal capacity x Temp rise

 

E = QΔT

J J.°C-1 °C

Specific heat capacity

Specific heat capacity is the amount of energy needed to raise the temperature of unit mass of a substance by 1K

Specific heat capacity of water = 4186 J.kg-1.°C-1

Specific heat capacity of kerosene = 2010 J.kg-1.°C-1

Specific heat capacity of mercury = 140 J.kg-1.°C-1

Calculations using S.H.C.

Energy absorbed = Mass x Specific Heat capacity x Temp rise

 

Q = mcΔT

J kg

J.kg-1.°C-1 °C

For example

500 g of olive oil is heated until its temperature rises by 120°C. If the specific heat capacity of olive oil is 1970 J.kg-1.°C-1, how much heat energy was used?

Energy absorbed = Mass x Specific Heat capacity x Temp rise

Energy absorbed = 0.5 x 1970 x 120

Energy absorbed = 118200 J

Let’s try some questions!

Page 171 Qs 2, 3 and 4

Investigation time!

Let’s do an experiment to measure specific heat capacities