Thermal Physics

THERMAL PHYSICS

Matter is most commonly found in solid, liquid or gas form. We will discuss the properties of these diff erent states of matter.

STATES OF MATTER

Solids have a fixed shape and a fixed volume.The molecules in a solid have a rigid

structure.The force of attraction between molecules is

strong.

SOLIDS

Liquids have a fixed volume but change shape to fi t the container.

The molecules in a liquid stay in contact but move around freely.

The force of attraction is strong enough to keep the molecules from completely moving away.

LIQUIDS

Gases do not have a fixed volume or shape.The molecules in a gas are far apart and

move quickly.The forces of attraction between molecules

is negligible.

GASES

The temperature of a gas is the average kinetic energy of its molecules.

Objects with a high temperature have molecules moving at a high speed. If the temperature is decreased the speed of the molecules decreases.

TEMPERATURE IN GASES

The pressure of a gas on a surface is due to the impacts of gas molecules with the surface.

When a molecule impacts the surface it exerts a tiny force. Billions of these impacts occur every second creating a steady pressure on the surface.

PRESSURE IN A GAS

The following diagram shows the names of the different changes of state.

CHANGES OF STATE

The changes of state can be described by the movement of molecules.

Melting – when a solid is heated the molecules begin vibrating to the point at which they break free of the rigid structure.

Freezing – When a liquid is cooled the molecules slow down and form a rigid structure.

CHANGES OF STATE

The changes of state can be described by the movement of molecules.

Boiling – When a liquid is heated the molecules move quickly and break free from each other.

Condensing – When a gas is cooled the molecules move more slowly and the force of attraction increases.

CHANGES OF STATE

Molecules in a solid are in a fixed structure.Molecules in a liquid move in contact with

each other.Molecules in a gas are far apart and move at

high speed.Increasing the temperature of a gas increases

the average speed of its molecules.The pressure of the gas on a surface is

caused by its molecules repeatedly hitting the surface.

Practice: pg 71 #1,2 and pg 73 #1,2

SUMMARY

PRESSURE, TEMPERATURE AND VOLUME IN GASES

In 1785, Robert Brown observed pollen grains floating on water. He observed that the pollen grains moved randomly.

Using molecular theory, it has been explained that the small water molecules were constantly colliding with the large pollen grain. This caused it to move randomly.

This motion is called Brownian Motion.

RANDOM MOTION OF PARTICLES

If we had a container of sealed gas, what would happen to the pressure if we increase the temperature? (Note: we are keeping the same volume.)

The pressure would increase because the molecules would be moving faster and there would be more collision with the walls of the container.

PRESSURE AND TEMPERATURE

Imagine a piston filled with a gas.

The temperature and mass of the gas are constant.

If we force the piston down, what will happen to the pressure?

GAS PRESSURE AND VOLUME

If the piston is forced down, the volume decreases.

The pressure in the tube will increase because the molecules will impact the surface more often.

GAS PRESSURE AND VOLUME

What would happen to the piston if we lift the piston upwards?

GAS PRESSURE AND VOLUME

If the piston is forced up, the volume increases.

The pressure in the tube will decrease because the molecules will impact the surface less often.

GAS PRESSURE AND VOLUME

We see that when volume decreases the pressure increases. When the volume increases the pressure decreases.

This means that volume and pressure are “inversely proportional”.

GAS PRESSURE AND VOLUME

PressureSymbol – PUnit – PascalUnit Symbol – Pa

VolumeSymbol – VUnit – meter cubed or centimeter cubedUnit Symbol – m3 or cm3

VOLUME AND PRESSURE

For a fixed mass of gas at a constant temperature:

pressure x volume = constant

When you are comparing two situations you can use the following equation:

P1V1 = P2V2

BOYLE’S LAW

A fixed mass of gas has an initial volume of 15cm3. When the volume is increased to 45cm3 the pressure is measured at 60kPa. What was the original pressure?

BOYLE’S LAW EXAMPLE

Brownian motion is the random motion or small particles due to the impacts of gas molecules on each particle.

The pressure of a gas in a sealed container increases if the gas temperature is increased.

For a fixed mass of gas at constant temperature, the pressure x volume = constant.

SUMMARY