The particles are closer together in lessdense substances
Colder air sinks because it is more dense
Convection happens in solids, liquids and gases
Particles are closer together in more dense substances
To understand how heat can be transferred by convection , the idea of density is important. If water, oil and air are mixed up, they will settle out in order of density - which one will rise to the top? The air is least dense and the water is the most dense – it depends on how far apart the particles are. Thermal convection
The movement of hotter areas in a liquid can be seen using potassium permanganate as a dye: H E A T This cycle is called a convection current. Explain how the convection current moves using the idea of density ? The diagrams on the left will help you. Thermal convection
Convection currents can’t occur in solids because the particles are held in fixed positions - but can they occur in gases? Place a candle at one side of a litre beaker. Place a piece of card down the centre, leaving a gap of 2cm at the bottom. Make some smoke with smouldering spills and watch the path of the smoke. Can you explain why this happens? Thermal convection
When shaft mining was first used to mine coal, convection currents caused by an underground fire were used to ventilate the shafts: Why do you think miners don’t use this method anymore? Thermal convection
Convection currents in a pan of boiling water.
4. Radiation Heat can move by travelling as Infra Red waves These are electromagnetic waves, like light waves, but with a longer wavelength. This means that infra red waves act very much like light waves: They can travel through a vacuum They travel at 300,000,000 m/s They can be reflected They can’t travel through opaque materials. Energy : heat transfer
Paint 4 thermometer bulbs with the following colours Black White Silver Red Place the thermometers into very hot water for 1 minute. Take it out of the water, start the stopwatch and read the temperature. Take the temperature every 30 seconds and put the answers in the results table on the next page. 4. Radiation Energy : heat transfer
Which colour radiated most heat? Black 4. Radiation Energy : heat transfer
Either : a) Draw a bar chart of your results. Put temperature change on the y axis and colour on the x axis. OR b) Draw a line graph of your results. Put temperature on the y axis and time on the x axis. Draw 4 lines on the graph, one for each colour. 4. Radiation Energy : heat transfer
2. Why does take-away food often come in aluminium containers? 3. Why do elephants have big ears? 4. Radiation Energy : heat transfer 1. How does a cup of tea lose heat by conduction, convection, evaporation and radiation?
5. A cup of tea takes up to 30 minutes to go cold. This depends on the colour and shape of the cup. Design an investigation to find out the best shape and colour of a cup to keep tea warm for longer. 4. Radiation Energy : heat transfer
Thermos (vacuum) flask This means that hot liquids placed inside the bottle lose hardly any heat and stay hot for very long times. Also, cold liquids … even ice cream … stay cold for longer as the heat cannot easily get into them. Thermos, (or vacuum), flasks are very good at slowing heat from moving into or out of them. They vastly reduce how quickly heat energy moves.
Why is it important that this is insulated? Why is it important this is designed like this? Why is it important that this is tight fitting ? with hollow container walls and no particles inside i.e. vacuum Why is it important this is silver in colour? Glass bottle or stainless steel (unbreakable ?) Tight fitting insulated bottle cap (lid) Silvered (bright and shiny) inside and outside covering Thermos lid/cup Insulated placers to keep the bottle safely and firmly in place, and away from the outside wall of the container
Insulated so that there’s hardly any heat transfer by conduction
- So no heat transfer by conduction or convection
Tight fitting so that there’s less (almost none) heat transfer by convection
this limits the amount of heat transfer (absorption and emission) by radiation Glass bottle or stainless steel (unbreakable ?) Tight fitting insulated bottle cap (lid) Silvered (bright and shiny) inside and outside covering with hollow container walls and no particles inside i.e. vacuum Thermos lid/cup