P2 | Thermal Physics

• Solids

  • Solids have a definite shape and a definite volume
  • Solids cannot flow and are not compressible
• Liquids

  • Liquids have no definite shape but do have a definite volume
  • Liquids are able to flow to take the shape of a container but they are not compressible
• Gases

  • Gases have no definite shape and no fixed volume
  • Gases can flow to take the shape of their container and are highly compressible
• Arrangement & Motion of Particles

  A) High

  B) Medium

  C) Low

  D) Randomly

  E) Randomly

  F) Regular

  G) fixed

  H) position

  I) Vibrate

  J) around

  K) quickly

  L) directions

  M) Low

  N) Greater

  O) Highest

  
• • The forces between particles affect the state of matter
  • This is because the magnitude of the forces affects the relative distances and motion of the particles
  • This affects the ability of the substance to
  • Change shape
  • Change volume
  • Flow
  • The particles that make up matter include
  • Atoms
  • Molecules
  • Ions
  • Electron
• Solids

  • The molecules in a solid are held in place by strong intermolecular forces
  • They only vibrate in position
  • The distance between them is fixed
  • This gives the solid its rigid shape and fixed volume
• Liquids

  • The molecules in a liquid have enough energy to overcome the forces between them
  • They are still held close together
  • The volume of the liquid is the same as the volume of the solid
  • Molecules can move around (by sliding past each other)
  • This allows the liquid to change shape and flow
• Gases

  • The molecules in a gas have more energy and move randomly at high speeds
  • The molecules have overcome the forces holding them close together
  • Because of the large spaces between the molecules
  • The gas can easily be compressed and is also able to expand
  • Gases flow freely
• • The amount of pressure that a gas exerts on its container is dependent on the temperature of the gas
  • This is because particles gain kinetic energy as their temperature increases
  • As the temperature of the gas decreases, the pressure on the container also decreases
  • In 1848, Mathematician and Physicist, Lord Kelvin, recognised that there must be a temperature at which the particles in a gas exert no pressure
  • At this temperature they must no longer be moving, and hence not colliding with their container
  • This temperature is called absolute zero and is equal to -273 °C
• Motion of Particles in a Gas

  • Molecules in a gas are in constant random motion at high speeds
  • Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:
  • With the walls of its container
  • With other molecules
  • Pressure in a gas is caused by the collisions with the surface (walls) of the container

  
• Gas molecules hit the sides of the container and exert a force, which creates pressure.

  
• Pressure = Force / Area
• The kinetic theory of matter states:

  1. All matter is made of particles which can be individual atom molecules or ions
  2. These particles are always in motion
  3. Solids vibrate about a fixed position
  4. Liquids move freely around each other
  5. Gases randomly move quickly
• Brownian Motion

  the random motion of microscopic particles when observed through a microscope
• • Brownian motion is the random movement of particles in a liquid or a gas produced by large numbers of collisions with smaller particles which are often too small to see
  • When small particles (such as pollen or smoke) are suspended in a liquid or gas, they can be observed through a microscope moving around in a random, erratic fashion
• Light, fast-moving molecules collide with larger particles, giving them a little nudge

  
• Thermal Expansion

  • Temperature increases
  • Motion increases
  • Spacing between particles increases
  • Volume increases
  • Density decreases
• Thermal Contraction

  • Temperature decreases
  • Motion decreases
  • Spacing between particles decreases
  • Volume decreases
  • Density increases
• Pressure & Volume (Constant Temperature)

  • If the temperature of a gas remains constant, the pressure of the gas changes when it is:
  • Compressed – decreases the volume which increases the pressure
  • Expanded – increases the volume which decreases the pressure

  
• • Evaporation is a change in state of a liquid to a gas
  • It happens; 
  • At any temperature
  • Only from the surface of a liquid
• • The molecules in a liquid have a range of energies
  • Some have lots of energy, others have very little
  • Their average energy relates to the temperature of the liquid
• • Evaporation occurs when more energetic molecules moving near the surface of the liquid have enough energy to escape
  • The average energy of the liquid is reduced
  • Therefore liquids are cooled down by evaporation
• When materials are heated, they expand because the molecules start to move faster which makes them knock into each other and push each other apart

  
• Molecules don't expand during thermal expansion, but the space in between them does

  A) slightly

  B) more

  C) cannot

  D) overcome

  E) partially

  F) overcome

  G) significantly

  H) completely

  I) overcome

  
• • A rise in the temperature of an object increases its internal energy
  • This can be thought of as due to an increase in the average speed of the particles
  • Increasing speed increases kinetic energy
• Internal energy

  The total energy stored inside a system by the particles that make up the system due to their motion and positions
• • Positions of the particles relative to each other affects their potential energy
  • Together, these two make up the internal energy of the system

  A) kinetic

  B) potential

  
• • How much the temperature of a system increases depends on:
  • The mass of the substance heated
  • The type of material
  • The amount of thermal energy transferred in to the system
• The specific heat capacity, c, of a substance is defined as

  The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C
• • Different substances have different specific heat capacities
  • If a substance has a low specific heat capacity, it heats up and cools down quickly (ie. it takes less energy to change its temperature)
  • If a substance has a high specific heat capacity, it heats up and cools down slowly (ie. it takes more energy to change its temperature)

  
• • Ice melts at 0 °C
• • Pure water boils at 100 °C
• When a substance is changing state, the substance does not change temperature

  A) solid

  B) melting

  C) liquid

  D) boiling

  E) gas

  
• • Boiling is also a change in state from liquid to gas
  • Boiling happens only at the boiling point of the liquid
  • The change of state happens all through the liquid (seen as bubbles in boiling water, for example)

  A) any temperature

  B) only at boiling point

  C) surface

  D) throughout

  
• Good thermal conductors are solids which easily transfer heat
• Good thermal conductors

  • Metal pan
  • Ceramic tea cup
• Bad thermal conductors (also called insulators) are solids which do not transfer heat well
• Bad thermal conductors (insulators)

  • Woolen blanket
  • Layers of cardboard or paper
• Metal

  Good conductor of heat
• Wood

  Good insulator, meaning it is a poor conductor of heat