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describe two uses for devices that emit infrared radiation and detect infrared radiation
(a) Any two from: heating rooms, heating the body for physiotherapy/treating sporting injuries, heating the hair in a hair salon, cooking, or any similar sensible use. (b) Any two from: detecting intruders/burglar alarms, detecting criminals at night, finding people in collapsed buildings , or any similar sensible use
which will emit more infrared radiation , a glass of milk taken from the fridge or a cup of tea ? explain your answer
- The cup of coffee
- will get colder, because it is warmer than its surroundings so energy will be
- transferred from the coffee to the surroundings; the cola will get warmer,
- because energy will be transferred to it from the warmer surroundings.
- Eventually, both drinks will reach room temperature. Once they have reached the
- temperature of their surroundings no more energy will be transferred.
you can buy insulated mugs to keep hot drinks hot . expalin what would be the best colour for an insulated mug
- White: energy from the hot contents of the mug will be
- transferred to the mug; if the mug is white it will absorb less of this energy from
- the hot drink, and emit less of the absorbed energy to the surroundings.
cars of many different colours are parked in the sun . which cars will be the hottest inside explain your answer
- Black cars, or cars of other dark colours; dark colours
- are better at absorbing energy than light colours.
which mug will emit more infrared radiation over a period of 5 minutes , a white mug full of hot tea or a white mug full of lukewarm tea
- The hot drink will emit
- more infrared radiation than the lukewarm drink
why can gases be compressed , but not solids or liquids
- There are large spaces between the particles in a gas,
- so they can move together if the gas is compressed; in liquids and solids the
- particles are already very close together and cannot be pushed any closer.
explain the best weather for hanging out washing to dry
- A warm breezy day: the warmth will mean the water in the
- washing is warmer, and so more particles will have enough energy to become a
- gas; the breeze will move the evaporated particles away so they cannot condense
- back onto the washing.
explain how sweating keeps you cool
- When sweat evaporates it takes its energy from the
- liquid that is left on the skin. This cooler liquid absorbs energy from the
if you were hiking , what effect would water have on your body temperature if you got wet ? explain your answer
- Water on their skin and clothes will evaporate and this
- will make them feel colder.
alcohol has a lower boiling point than water . explain why a drop of alcohol on your skin feels colder than a drop of water
- The lower boiling point of alcohol compared with water
- means that the particles do not need quite as much energy to escape from the
- liquid and form a gas; more particles will therefore be able to evaporate from
- the alcohol, and so the remaining liquid will lose more energy and feel colder.
how is energy transfered by conduction
- Heating a material causes vibrations in the particles;
- these vibrations are passed on.
why are liquids and gases poor conductors
- Their particles can
- move around and are very spread out, so they do not pass on energy to one
explain why materials that are good electrical conductors are usually also good at conducting by heat
- Materials that conduct electricity have electrons that
- can move freely within the material; these electrons also help to transfer
many glass houses have double-glazed windows . These have two sheets of glass with an air gap between them . Why does double glazing provide better insulation than single glazing
- Air is a poor conductor, so the air trapped between the
- two sheets of glass improves the insulating properties of the window.
what is convection
- Convection is the way
- that energy is transferred in fluids. When part of a fluid is heated the
- particles move around faster and the heated fluid expands. This reduces its
- density and so it rises. Cold fluid moves in to
- take its place, forming a convection current
why doesnt convection happen in solids
- Convection can only happen if particles are free to move
- around; the particles in a solid are in fixed positions.
if the heating is turned off on a day in winter , why is the coldest place in a room likely to be under a radiator
- Energy will be transferred from the room through the
- window, and this will mean the air by the window is cooler than the rest of the
- air in the room; cooler air is more dense, and will sink.
explain why the cooling element in a freezer is usually at the bottom
- The element cools the
- air in the freezer, the colder air is more dense and sinks; having the element
- at the top allows the whole freezer to be cooled by convection
explain why the coldest part of a fridge is usually at the bottom
- Air cooled by the cooling elements sinks to the bottom
- of the fridge.
explain why smoke detectors are fitted to the ceilings rather than walls
- Smoke is hot and less dense than air. It will rise to
- the ceiling, so a detector on the ceiling will probably detect smoke before one
- on a wall.
why does the piope for the hot water taps come out of the top of the water tank , not the bottom
- The hottest water is
- least dense and so will be at the top of the tank
explain which features of the vaccum flak reduce energy transfer by (a) radiation , (B) conduction (C) convection
- (A) The silver coatings
- on the glass reduce energy transfer by radiation, because light coloured, shiny
- surfaces are poor radiators.
- (B) The glass and plastic are both poor thermal conductors;
- energy cannot be conducted in a vacuum, so the vacuum in the space between the
- two walls also reduces the energy transferred by conduction. (c) The vacuum in the space between the
- two walls reduces convection losses, as convection only happens when there are
- particles that can move (answers could also include the glass and plastic, as
- convection cannot occur in solids).
many vacuum flasks have steel walls instead of glass how would this affect the rate of cooling of hot coffee in the flask , explain your answer
- The coffee is likely
- to cool faster, as steel is a better thermal conductor than glass
would a vacuum flask keep a cold drink cold on a hot day
- Yes; it will reduce
- the amount of energy that can be transferred from warm air to the cold drink.
explain why air moving over an object can increase its rate of cooling
- A hot object will transfer energy to the surrounding
- air. The rate of transfer is greatest when the difference in temperature is
- greatest. If the air is still, the air very close to the object will warm up
- (by conduction) and reduce the rate of energy transfer. If the air is moving,
- this warmed air will be moved away and replaced with cooler air.
how can solar panels help to reduce heating bills
- The panel heats water, which can be used for washing or
- to heat the house; if this energy is obtained from the Sun, this means that
- less gas or electricity is needed for heating.
how can someone work out which type of insualtion would be the most cost effective
- They can work out the
- payback time, which is how long it takes to save the money that it cost to
- install the new insulation; the shorter the payback time, the more cost
- effective the insulation.
modern houses usually have cavity walls . Explain why they are now built with a layer of insulation between the two walls
- Cavity walls with insulation between the two parts have
- a lower U-value than cavity walls without the insulation. This means that they
- transfer less energy. Houses built like this should have lower energy bills.
why does it take longer to boil a kettle full of water than one only half full
- There is a greater
- mass of water in the full kettle, so it takes more energy to raise its
- temperature by the same amount. If the rate at which energy is transferred to
- the kettle is the same, it will take longer to heat the larger mass
explain why stand alone radiators are filled with oul , instead of being filled with air
- Oil has a higher
- specific heat capacity than air, so it can store more energy.
explain why heat sinks have fins , and why the metal used should have a high specific heat capacity and be a very good thermal conductor
- The fins increase the
- surface area of the metal and so increase the rate at which energy is transferred
- to the air. A high specific heat capacity means that the metal can absorb a lot
- of energy without its temperature reaching the temperature of the component it
- is cooling. The metal must be a good conductor so it can transfer the energy
- away from the component and towards the fins. The faster the energy can be
- transferred to the fins, the better the temperature difference between the
- component and the heat sink can be maintained and so the faster the cooling
define heat sink
a device with a high specific heat capacity that can absorb energy from another object at higher temperature