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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
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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.
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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.
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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.
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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
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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.
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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.
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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
- body.
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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.
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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.
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how is energy transfered by conduction
- Heating a material causes vibrations in the particles;
- these vibrations are passed on.
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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
- another.
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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
- energy.
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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.
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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
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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.
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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.
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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
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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.
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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.
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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
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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).
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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
- rate
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define heat sink
a device with a high specific heat capacity that can absorb energy from another object at higher temperature
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