THERMAL PHYSICS

 Matter is most commonly found in solid, liquid or gas form. We will discuss the properties of these different 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 fit 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

 Pressure  Symbol – P  Unit – Pascal  Unit Symbol – Pa  Volume  Symbol – V  Unit – meter cubed or centimeter cubed  Unit Symbol – m 3 or cm 3 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: P 1 V 1 = P 2 V 2 BOYLE’S LAW

 A fixed mass of gas has an initial volume of 15cm 3. When the volume is increased to 45cm 3 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