Energy Transfer by Heating Flashcards
How can we detect infrared raditation?
With our skin - it makes us feel warm
What do all objects do?
Emit radiation
The hotter an object is…
…the more infrared radiation it emits in a given time
What is a vacuum?
A region that doesn’t contain any particles.
Conduction and convection cannot occur here, but radiation can
What does the transfer of energy by infrared radiation not involve?
Particles
What type of surfaces are good absorbers of radiation?
Dark-coloured, matt surfaces
If an object is a good absorber of radiation, we also know that it will be a good…
..emitter too. Physics works both ways!
The same is true of reflectors, they are also poor absorbers
What type of material is a good reflector and poor absorber of radiation?
Light-coloured, shiny surfaces
What are the properties of solid particles?
- Low transactional energy, but still vibrate (vibrational energy)
- Fixed positions due to strong intermolecular bonds
- Fixed + rigid shape
- Not compressable
What are the properties of liquid particles?
- Lower energy than gas, higher than solid
- Particles still in contact with each other due to fairly strong intermolecular bonds
- Can move around slightly and form random arrangements
- Therefore, can flow and fit shape of container
- Does not have a fixed shape
What are the properties of gas particles?
- High energy (high transactional)
- No fixed positions due to very weak intermolecular bonds
- Therefore they can move randomly and much faster
- No fixed shape and can flow
- Compressable
- Much less dense than solids + liquids
Draw a diagram showing the different states of matter
When does conduction occur?
In solids (and poorly in liquids)
What types of material are the best conductors and why?
- Metals are the best conductors.
- This is because they contain free electrons.
- When heated, these gain energy and move through the metal, transfering energy by colliding with other particles.
Describe how conduction works.
- When one end of a solid is heated, the particles at that end gain kinetic energy and vibrate more
- These vibrations pass energy onto neighbouring particles
- This process continues through the solid, passing on slightly less energy each time as some is maintained by each vibrating particle
- This is how energy is transfered through a solid (conduction)
What are insulators?
A poor conductor
What is a conductor?
A material/substance that conducts energy (via conduction) well…
Why are materials such as wool and fibreglass good insulators?
Because they contain trapped air.
Air is a poor conductor because it is a gas, so the particles are much less densely packed and therefore do not transfer energy by conduction very well at all.
Where does convection occur?
In fluids
What are fluids?
Liquids and gasses
(They can flow)
Describe how convection works.
- When a fluid is heated it expands and gains kinetic energy
- The fluid becomes less dense and rises
- As the fluid rises, it begins to loose some of its energy
- This causes it to sink again and become more dense
- It is then heated again and the process continues.
- This forms a convection current
What are convection currents responsible for in the real world?
Onshore and offshore breezes
Why does a fluid become less dense when it is heated?
Because the particles gain more kinetic energy and move apart.
This causes it to expand.
This means there is the same amount of mass in a larger area.
Why does evaportation take place?
Because the most energetic liquid molecules escape the liquid’s surface and enter the air/become a gas
How does evaporation also cause cooling?
- Evaportation takes place due to the most energetic liquid particles escaping the liquid’s surface and entering the air/becoming a gas.
- This means that the average kinetic energy of the remaining molecules decreases, so the temperature of the liquid also decreases
How can the rate of evaportation be increased?
- Increasing the surface area
- Increasing the temperature
- Creating a draught of air across the liquid’s surface
How can the rate of condensation be increased?
- Increasing the surface area
- Decreasing the temperature
The greater the temperature difference between an object and its surroundings…
…the greater the rate at which energy is transfered
What does the rate at which energy is transfered depend upon?
- The substances/materials the object is in contact with
- The object’s shape
- The object’s surface area
- The temperature difference between the object and its surroundings
Why might we want to maximise the rate of energy tranfer?
To keep an object cool
How can the rate of energy transfer be maximised?
- Use good conductors
- Use good emitters (dark, matt)
- Maximise air flow/draught
How can the rate of energy transfer be minimised?
- Use good insulators
- Use good reflectors (white, shiny)
- Prevent convection currents by trapping air in small pockets (this also prevents conduction)
Draw an label a diagram of a vacuum flask showing how it is a good insulator
- Plastic cap prevents conduction + convection
- Double-walled glass/plastic container prevents conduction + radiation00
- Plastic protective cover prevents conduction
- Hot/cold liquid
- Sponge pad for protection also prevents conduction
- Inside surfaces silvered to stop radiation
- Vacuum prevents conduction + convection
- Plastic spring for support
What does a vacuum flask do and why?
Reduces the rate of energy transfer to keep hot things hot and cool things cool
What is the specific heat capacity of a substance?
The amount of energy (Joules) required to raise 1Kg of it by 1°C (or K)
What does the rate of temperature change in a substance when heated depend upon?
- The amount of energy transfered to it
- Its mass
- Its specific heat capactity
What are U-values and what do they tell us?
The amount of energy per second that passes through a material.
The lower the U-value, the better insulator the material is
Why do most people want to minimise the rate of energy transfer out of their homes?
To reduce fuel/energy consumption, and therefore fuel/energy bills
How can heat transfer out of homes be reduced?
- Fibreglass loft insulation to reduce conduction
- Cavity wall insulation traps air in small pockets to reduce covection + conduction
- Double glazing reduces conduction
- Draught proofing reduces convection
- Aluminium foil behind radiators reflects infrared raditation back into the room and therefore reduces radiation
- Thick curtains reduce conduction and radiation
How do solar heating pannels work?
- They contain water that is heated by infrared radiation from the Sun.
- This water may then be used to heat buildings or provide domestic hot water.
- The pipes that contain the water are often painted black because it both a good absorber and emitter of radiation.
What are the benefits and drawbacks of solar heating panels?
- They are cheap to run because they do not use fuel
- They do not contribute to global warming because they do not produce greenhouse gasses when heating water
However:
- They are expensive to buy + install
- They do not heat water at night
