Energy Resources And Energy Transfer - Part 2 (T4) Flashcards
What kind of surfaces are not good at emitting or absorbing infrared radiation?
- if a surface is bright or shiny, it will not absorb radiation very well because most of it will be reflected back from the surface (like light would be)
- a bright and shiny surface would also be a poor emitter of radiation because the radiation would be reflected back from the inside by the shiny or bright surface
What kind of surfaces are very good absorbers and emitters of infrared radiation?
Dark and matt surfaces
How do cooling fins work and where would they be used?
- cooling fins are often used on things like engines to help maximise surface area in order to maximise heat transfer
- they are sometimes painted matt black to maximise the amount of heat they radiate
- they illustrate the principle that the amount of heat radiated from a surface not only depends on the colour and reflectivity of the surface but also on the surface area
How do some creatures in hot climates have special features to help them keep cool? Use ears as an example..
- large ears radiate more heat than smaller ones would
- flapping them, like elephants would, also helps by increasing air flow over the surface
- they also often have more blood vessels in them than they need to have so as to help the cooling process even more
For many years the exterior walls of houses and larger buildings have all been built with an inner wall and an outer wall with a gap known as a ‘cavity’. What are the advantages and disadvantages of this cavity and how can one use it to keep their home warm AND damp free?
- the cavity keeps the house nice and dry, but at the same time allows far too much heat to escape as air can circulate by convection relatively freely with heat circulating from the warmer surface of one wall to the colder surface of the other, meaning heat is carried away from the inner wall
- the cavity can be filled, but the insulating material must not allow damp to cross the cavity
- most ordinary insulating materials cannot be used so houses built today have have the cavity filled with insulating panels made of glass fibre matting
- this is a lightweight, poor conductor that traps heat in
- the panels are usually surfaced with thin aluminium foil which reflects heat in the form of infrared radiation
What is Rockwool cavity wall insulation?
- this is a waterproof material that can be used to fill an empty cavity
- the cost of having it injected into all the external walls of a house is usually paid back in about five years by the reduction in heating bills, so it is a very cost effective measure
What two other common insulation methods are used and also more cost-effective than cavity wall insulation?
- draught proofing
- loft insulation
- loft insulation is extremely important in any house as it reduces convection losses dramatically
How should one keep warm in the winter?
- your outer layer must be wind proof, so coats with very fine weave material, leather or waterproofs
- the outer layer must fit tightly at the neck, waist and cuffs so warm air doesn’t escape and cold air can’t get in
- once you have stopped convection losses by these measures you just need plenty of insulation underneath to keep you snug…fluffy materials like wool, fleece or fur
- your feet lose most of their heat by conduction to the ground. Special foil-backed thermal insoles are very effective as they insulate your feet from the ground as well as reflecting back radiated heat
When we say that thermal energy is possessed by “hot” matter and that “hot” is a relative term, what do we mean?
Any matter above the temperature ‘absolute zero’ has some thermal energy
What exactly are “free electrons”? Give an example of a substance with a lot of free electrons..
- free electrons are electrons that are not bound to any particular atom in a substance
- metals usually have huge numbers of fee electrons per unit volume
- eg: copper has about 10 to the 29th free electrons in each cubic metre
Air and water are both fluids and allow heat transfer to take place by convection. However, they are not good thermal conductors as they are insulators. Give two examples of how their insulating properties are put to use…
- woollen clothing keeps you warm because air gets trapped in the fibres. The trapped air is heated by your body and forms a warm insulating layer
- in the same way, a wet suit keeps a diver warm as there is a thin layer of water trapped next to the diver’s skin
How do greenhouses work, and what is the “greenhouse effect”?
- infrared waves from the sun can pass through normal greenhouse glass
- the infrared waves heat up the ground which re-radiates the heat
- however the re-radiated heat is in the form of infrared waves with much longer wave lengths, which cannot pass back through the glass and are trapped
- carbon dioxide in the earths atmosphere acts in the same way, trapping the Suns heat and producing the “greenhouse effect”
How does double glazing work in windows?
- two layers of glass are used to trap a layer of air
- the thickness of this layer is important; too thin then the insulation effect is reduced but too thick and convection currents will be able to circulate and carry heat from the hotter surface to the colder one
- modern double glazing uses special glass to increase the greenhouse effect
What is the equation for gravitational potential energy?
GPE = m x g x h
GPE = gravitational potential energy in Joules m = mass of object in kilograms g = gravitational field strength in N/kg h = height above the ground in metres
What is the equation for calculating kinetic energy?
KE = 1/2 x m x v squared
KE = kinetic energy in joules m = mass in kilograms v = velocity (or speed) in m/s
