P1b Flashcards
(26 cards)
Why is trapped air in a material a very good insulator?
The air can’t move so the heat has to conduct very slowly through the pockets of air, as well as the material in between. Clothes, blankets and cavity wall foam insulation all work by trapping pockets of air - to reduce convection, you need to stop the fluid moving.
Which materials absorb and which reflect infrared radiation?
It is reflected from a light coloured, smooth and shiny surface and absorbed by a matt black, dull or rough surface.
How can absorption and reflection of infrared radiation be applied in everyday situations?
Cooler objects will absorb the heat radiation emitted by hotter things, so their temperature increases. You can feel heat radiation, for example if you’re indoors and the sun shines on you through a window.
Matt black solar panels absorb as much heat as possible with a shiny inner surface to keep absorbed heat in.
Shiny foil behind radiators reflects radiation back into the room rather than heat up the walls.
Fridges are shiny white to help reflect it.
Lining a grill pan with shiny foil reflects the heat radiation back onto the bottom of the food being grilled, so the food is cooked more evenly.
What is conduction?
Process where vibrating particles pass on extra kinetic energy to neighbouring particles, occurring mainly in solids.
How is energy transferred by conduction?
In a solid, the particles are held tightly together. So when one particle vibrates, it bumps into other particles nearby and quickly passes the vibrations on. 0Particles which vibrate faster than others pass on their extra kinetic energy to neighbouring particles. These particles then vibrate faster themselves. This process continues throughout the solid and gradually the extra kinetic energy (or heat) is spread all the way through the solid. This causes a rise in temperature at the other side.
Which materials conduct heat really well?
Metals conduct heat really well because some of their electrons are free to move inside the metal. Heating makes the electrons move faster and collide with other free electrons. transferring energy. These then pass on their extra kinetic energy to other electrons, etc. Because the electrons move freely, it is a much faster way of transferring energy than slowly passing it between jostling neighbouring atoms.
Which materials don’t conduct heat?
Most non-metals don’t have free electrons, so warm up more slowly, making them good for insulating things. That’s why metals are used for saucepans, but non-metals are used for saucepan handles.
Do liquids and gases conduct heat?
Liquids and gases conduct heat more slowly than solids - the particles aren’t held so tight together, which prevents them bumping into each other so often. So air is a good insulator.
What is convection?
When the more energetic particles move from the hotter region to the cooler region and take their heat energy with them, occurring in liquids and gases.
How is energy transferred by convection?
When you heat up a liquid or gas, the particles move faster, and the fluid (liquid or gas) expands, becoming less dense. The warmer, less dense fluid rises above its colder denser surroundings. As the warm fluid rises, cooler fluid takes its place. As this process continues, you actually end up with a circulation of fluid (convection currents). This is how immersion heaters work.
How does convection happen in radiators?
Radiators in the home rely on convection to make the warm air circulate round the room. The heated less dense air rises, the warm air displaces cooler air, the cool denser air falls and the cool air flows to fill the gap left by the rising, heated air.
What is radiation?
How we get heat from the sun. Heat is radiated as infrared waves - electromagnetic waves that travel in straight lines at the speed of light. All objects emit or absorb heat radiation.
How is radiation different from conduction and convection?
It doesn’t need a medium to travel through, so it can occur in a vacuum, like space. This is the only way that heat reaches us from the sun.
It can only occur through transparent substances, like air, glass and water.
The amount of radiation emitted or absorbed by an object depends to a large extent on its surface colour and texture.
How can heat loss be reduced in homes by loft insulation?
Fibreglass wool laid across the loft floor reduces conduction through the ceiling into the roof space. The insulation material traps air. Air is a poor conductor of heat. The fibres of the insulation also prevent the trapped air from circulating and causing a convection current to be set up.
How can heat loss be reduced in homes by double glazing?
Two layers of glass with an air gap between reduce conduction as air is a poor conductor of heat. Heat transfer by convection currents is also reduced by making the gap very narrow.
How can heat loss be reduced in homes by cavity wall insulation?
Two layers of brick with a gap between them reduce conduction but you still get some energy lost by convection. Squirting insulating foam into the gap traps pockets of air to minimise this convection. Insulating materials are bad conductors and so this reduces the heat loss by conduction.
What does source mean in terms of energy loss?
Sources are things that emit energy e.g. radiators. They can also waste energy if they’re not efficient.
What does sink mean in terms of energy loss?
Sinks are things that transfer and waste or lose energy e.g. windows/computers.
What are everyday examples of energy saving in the home?
Using energy saving light bulbs instead of normal ones so they waste less energy.
Using a hot water tank jacket reduces conduction.
How do curtains at windows keep homes warm?
They reduce conduction and radiation through windows
How is draught proofing energy saving?
Strips of foam and plastic around doors and windows stop hot air going out, reducing convection.
How does payback time work?
It costs money to buy or install insulation, or buy more efficient appliances, but it also saves you money, because your energy bills are lower. Eventually, the money you’ve saved on energy bills will equal the initial cost - the time it takes is called the payback time. If you subtract the annual saving from the initial cost repeatedly then eventually the one with the biggest annual saving must always come out as the winner.
Payback time=initial cost/annual saving
What is the different forms of energy that come out of a machine?
Useful machines are only useful because they convert energy from one form to another. The total energy output is always the same as the energy input, but only some of the output energy is useful. This is because some of the input energy is always lost or wasted, often as heat. The less energy that is wasted, the more efficient the device is said to be.
What is the equation for efficiency?
Efficiency = useful energy output/total energy input (x100%)
