P5 Heat and Particles

Matters of state

• Solids: tightly packed, in a regular pattern, strong intermolecular forces, fixed volume, fixed mass, fixed shape, cannot flow, cannot be compressed.
• Liquid: close together with no regular arrangement, molecules touching, medium forces of attraction, has fixed volume and no determinate shape, take the shape of their container, cannot be compressed.
• Gas: Moving in all directions very quickly, no longer bond by intermolecular forces, occupies all the space, has the most kinetic energy, can be compressed.
• When heat is added, the energy is used to overcome the attractive forces between molecules of solids, liquids and gases to change their state. There is no change in overall kinetic energy and hence no temperature increase. The heat used in this process is called latent heat of vaporization or latent heat of fusion.
• 热胀冷缩。
• Everyday applications include leaving gaps between railroads so in heat expansion won’t cause it to break, pipes may be destroyed when cooled excessively, etc.

Density

• Density equation: Mass / volume
• Irregularly shaped solid calculations can be done using water, measuring cynlinder, etc.

Collision Theory

• • Temperature is just how fast the molecules are vibrating, how much kinetic energy they have.
  • The motion of the molecules will be faster when temperature is higher. exert more frequent collisions / collide at greater speed, with tyre wall, more force exerted on tyre walls.

 

• Pressure and volume: P1V1 = P2V2

 

Evaporation

• Skin/body gives the molecules energy.
• Evaporation is the escape of the most energetic molecules from the surface of a liquid.
• Evaporation leads to consequent cooling as it takes the energy away from the body that it escaped from.
• Higher temperature, higher surface area and higher air flow means more evaporation.

Heat transfer

• • Put rods of different materials above a bunsen burner. Apply wax to the ends of the rods, whichever rod with the wax melting faster is the best conductor.
  • Heat transfer in solids by conduction happens when adjacent atoms vibrate against one another and pass on energy.

 

• Conduction is  microscopic collisions of particles and movement of electrons within a body.

• Metals are the best conductors

 

• • Applications: cooking, heating a tin, etc.

 

• Convection is the main method of heat transfer in fluids(liquids and gases)

• Hot air rises, cold air falls

 

• • Heat makes the air less dense
  • Experiment: put a lighted candle wisk and see the smoke go with the hot air rising
  • Applications: boiling water, air con

 

• Radiation transfers heat and does not require a medium to travel through

 

• • Measure temperatures to show good emitters and good absorbers of infrared radiation like black, dull, matt, and bad emitters and bad absorbers are white, shiny.

 

• Infra-red radiation is the part of the EM spectrum that gets involved in radiation

 

• Application: Radiators in cars and stuff are painted black so produce cooling effect.

Thermal capacity

• • Thermal capacity is amount of heat taken to change the temperature of an object.
  • Temperature is a measure of how hot something is in degrees
  • Heat is a measure of the thermal energy contained in an object in Joules

 

• Specific heat capacity is amount of energy needed to change the temperature of 1 kg of the substance by 1°C.

• Equation: Energy (joules) = mass(kg) * specific heat capacity (c) * △Temperature (°)

 

Melting and boiling

• • Melting is the process by which heat supplied to a solid turns it into a liquid.
  • Melting point/boiling point is the temperature at which it changes state.
  • Boiling is the process by which heat supplied to a liquid turns it into a gas.
  • Melting and boiling happens without a change in temperature.
  • Boiling happens at the boiling point, happens throughout the object, is fast, and usually needs a source of energy.
  • Evaporation happens at all temperatures, is slow, only at the surface, and takes energy from the surroundings.

 

• Condensation is the reverse of vaporisation and the conversion of a vapour or gas to a liquid.

 

Solidification/freezing is the reverse of melting and the conversion of a liquid to a solid.