Convection D. Crowley, 2008
Convection To know how energy can travel by convection
Housing • Why do houses in hot countries have floor tiles, yet countries in colder countries have thick carpets? • Heat energy is transferred from homes via conduction through the walls, roof, windows and floor • Floor tiles allow heat energy to be transferred easily, useful in hot countries • In colder countries it is important to conserve as much heat as possible, so laying carpet acts as an insulator (reducing the amount of heat transfer via conduction)
Conduction • Heat is transferred when there is a difference in temperature • How does heat get transferred in conduction? • What state of matter is conduction the most important form of energy transfer? • In conduction energy is passed from atom to atom • It is the most important type of heat transfer in solids • Vibrating particles pass on their extra vibrational energy to their neighbouring particles
Convection • Heat can also be transferred via a process known as convection • Watch the demo of the potassium manganate in water when it is heated… • Can you explain what is happening – a particle box of what is happening to the particles before and after heating my help
Convection • The particles in liquids and gases can move from place to place • Convection happens when particles with a lot of thermal energy in a liquid or gas move, and take the place of particles with less thermal energy • Thermal energy is transferred from hot places to cold places by convection
Insulation • Why is it important to have roof insulation in a house? • Loft insulation, such as a thick layer of fibreglass wool laid out across the whole loft floor reduces heat loss via conduction • The material also prevents air circulating along the loft floor, reducing heat loss by convection
Convection • Cut out the statements below, and stick them into your book in the correct order
Convection • A purple crystal is dropped into the water. • The purple crystal starts to dissolve and colours the water purple. • The beaker is heated underneath the purple crystal. • Heat energy is conducted through the glass of the beaker and starts to heat the water. • The water particles that are being heated start to move faster. They take up more space. • When the particles take up more space, the water is less dense than the water around it, and it starts to rise. You can see this happening because some of the purple dye is carried upwards with the water. • The rising water transfers some heat energy to the cooler water around it. Eventually it becomes the same temperature as the water around it, which is now a little warmer than it was to start with. • Cooler water moves in to take the place of the warm water. More of the purple crystal dissolves. • The cool water warms up and starts to rise. A convection current has formed. • Eventually the convection current will spread heat energy (and the purple dye!) through all the water in the beaker.
Hot Air Balloon • How can you make a hot air balloon? • Why does the balloon rise – what is happening to the particles?
Hot Air Balloon • Hot air balloons rise because of convection currents • As the heat is applied some of the particles move faster and further apart, making them less dense (so they rise to the top of the balloon, lifting it up) • As these particles move up and away from the heat they cool off, becoming more dense, so must be re-heated to keep the balloon rising