2.3 Thermal processes

Anotações:

• Conduction is the main method of thermal energy transfer in solids Metals are extremely good at conducting heat Non-metals are poor at conducting heat whilst liquids and gases are extremely poor Poor conductors are called insulators Materials containing small pockets of trapped air are especially good at insulating, as air is a gas and hence a poor conductor

1. describe experiments to demonstrate the properties of good and bad thermal conductors

   Anotações:

   • Take long thin strips of different metals (eg brass, aluminium, copper etc) and fix a drawing pin to the end of each strip using drops of wax. Position the other end of the metal strip into a Bunsen flame. Record the time taken for the wax to melt an the drawing pin to fall off. Fastest time shows best conductor of heat (variables to keep the same are distance from the flame, and thickness of the metal)
2. give a simple molecular account of conduction in solids including lattice vibration and transfer by electrons

   Anotações:

   • When a substance is heated, the atoms start to move around (vibrate) more As they do so they bump into each other, transferring energy from atom to atom. Metals are especially good at conducting heat as the electrons can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now positively charged metal ions. The ions are packed closely together and vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to the cooler parts by the free electrons. These move through the structure, colliding with ions as they go.

Anotações:

• Convection is the main way that heat travels through liquids and gases as the particles are free to move around. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less energy. (Convection cannot happen in solids)

1. recognise convection as an important method of thermal transfer in fluids
2. relate convection in fluids to density changes

   Anotações:

   • Liquids and gases expand when heated, because the particles move faster.  As a result the particles take up more volume, with more gaps between particles. This makes the liquid or gas in hot areas less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas sinks into the warm areas. This causes convection currents.
   • Convection currents can be seen in lava lamps. As the wax inside the lamp warms up, it becomes less dense than the liquid and so rises. Convection also explains why hot air balloons rise.  Convection is seen on a much bigger scale in our weather and ocean currents
   1. describe experiments to illustrate convection

      Anotações:

      • Place a beaker with cold water on a tripod over a bunsen burner. Add a few Potassium Permanganate crystals to one side on the bottom of the beaker using a straw. Heat the beaker on that side using the bunsen burner. (Watch this on You Tube a: Physics Online Convection Experiment - GCSE Physics)

1. identify infrared radiation as part of the electromagnetic spectrum

   Anotações:

   • Thermal radiation is part of the electromagnetic spectrum – infrared
2. describe experiments to show the properties of good and bad emitters and good and bad absorbers of infrared radiation

   Anotações:

   • To demonstrate the absorption of thermal radiation:  Take two conical flasks – one painted with silver paint, the other with black paint – and place thermometers and bungs in them Measure and record their initial temperatures Place the two flasks an equal distance from an incandescent light bulb (light bulb with a wire filament - a good source of radiation) and switch the bulb on After a few minutes switch the bulb off and record the new temperatures of the flasks(The black flask’s temperature should have increased by more) Turn to the next page for the experiment to show emission
   • To demonstrate the emission of thermal radiation: Fill the shiny beakers with boiling waterOnce each beaker reaches a set temperature (e.g. 90 °c) start a stopwatch and allow it to cool for a set amount of time (e.g. 10 minutes)After this time, take a new temperature measurement and record the change in temperature(The black beaker should have cooled by slightly more than the shiny beaker, because it emitted more thermal radiation)
3. recognise that thermal energy transfer by radiation does not require a medium

   Anotações:

   • Unlike conduction and convection (which need particles) infrared radiation is a type of electromagnetic radiation that involves waves. Because no particles are involved, radiation can even work through the vacuum of space. This is why we can feel the heat of the Sun even though it is 150 million km away from Earth.
4. describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of radiation

   Anotações:

   • Black objects are very good at absorbing thermal radiation (think about black leather seats in strong sunshine) but also very good at emitting it (when it goes dark those seats cool down quickly) (White is opposite) Shiny objects are poor absorbers and emitters, but good reflectors of radiation. Shiny objects reflect thermal radiation and so absorb very little. They also emit very little, though, and so take longer to cool down (Dull is opposite) Shiny things do not reflect heat (they reflect thermal radiation). Black things do not absorb heat (they absorb thermal radiation).
5. show understanding that the amount of radiation emitted also depends on the surface temperature and surface area of a body

   Anotações:

   • The temperature of the object (hotter = more radiation)The colour of the object (black = more radiation) The surface area of the object (greater surface area = more area for radiation to be emitted from)

1. identify and explain some of the everyday applications and consequences of conduction, convection and radiation

   Anotações:

   • Vacuum flask: The lid is plastic, which is an insulator. It reduces heat flow by conduction and also by convection of air. The gap between the inside wall and the outside wall of the flask contains no air (it is a vacuum) so there are no particles to pass on the heat by conduction or convection.  The silver surfaces reflect infrared radiation and reduce heat flow by radiation.  P.T.O.
   • Insulating the home: Loft insulation is made of fibres that trap air to reduce conduction and convection. Double glazed windows air (or argon gas) between the two layers of glass to reduce heat exchange by conduction or convection. P.T.O.
   • Sea breezes: During the day, infrared radiation from the Sun heats up the land more than the sea. The air above the land gets hotter than the air above the sea. The hot air above the land rises because it is less dense than the surrounding air. Cooler air from above the sea rushes in to take its place. This convection current causes a cool sea breeze to blow from the sea to the land.
   • Solar panel: The black surface of the solar panel absorbs infrared radiation from the Sun. The heat is then conducted through metal pipes to warm up water. The warm water rises to the top of the storage tank by convection.