GCSE Science Energy 1A

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Heat Radiation

Heat energy tends to flow from a hotter object to its cooler surroundings 

  • Heat energy can be transferred by radiation, conduction or convection
  • Heat radiation is the transfer of heart energy by infrared radiation 
  • Conduction and Convection involve the transfer of energy by particles 
  • Conduction is the main form of heat transfer in solids 
  • Convection is the main form of heat transfer in liquids and gases 
  • Infrared radiation can be emitted by solids, liquids and gases 
  • Any object can both absorb and emit infrared radiation, whether or not conduction or convection are taking place 
  • The bigger the temperature difference between a body and its surroundings, the faster energy is transferred by heating   
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Heat Radiation II

  • All objects continually emit and absorb infrared radiation, this is emitted from the surface of an object 
  • An object that's hotter than its surroundings emits more radiation as it cools down, an object that is cooler than its surroundings absorbs more radiation as it warms up 
  • The hotter an object is, the more radiation it radiated in a given time 
  • Dark, Matt surfaces absorb infrared radiation that falls on them better than light or shiny surfaces, they emit more radiation too at any given temperature 
  • Light, shiny surfaces reflect a lot of the infrared radiation that falls on them 
  • Solar hot water panels contain water pipes under a black surface
  • Radiation from the Sun is absorbed by the black surface to heat the water in the pipes 
  • This water can be used fro washing or pumped to radiators to heat the building 
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Kinetic Energy and Conduction

Kinetic Energy describes how particles move in solids, liquids and gases. The energy an object or particle has because of its movement is called its kinetic energy 

  • The three states of matter, solids, liquids and gases have the same particles but they are arranged and the energy it has are different 
  • Solids - strong forces of attraction, hold the particles together in a fixed regular arrangement, they don't have much energy which causes them only to vibrate about in their fixed positions 
  • Liquids - there are weaker forces of attraction, the particles are close together but can move past each other and form irregular arrangements, they have more energy as they move around in random directions at a low speed 
  • Gases - there are almost not forces of attraction, have a lot more energy than in solids or liquids - they are free to move and travel in random directions and at higher speeds 

When you heat a substance, the particles are given more kinetic energy, they vibrate and move faster, this is what causes solids to melt and liquids to boil 

 

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Kinetic Energy and Conduction II

Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles 

  • This process continues throughout the solid and some of the extra kinetic energy is passed all the way through the solid, causing a rise in temperature, this increases the heat radiating from its surface 
  • Conduction is faster in denser solids because the particles are closer together and will collide more often and pass energy between them. 
  • Metals conduct because the electrons are free to move inside the metal 
  • Because the electrons can move freely, this is a much faster way of transferring the energy through the metal, than slowly passing it between neighbouring atoms 
  • This is why heat energy travels so fast through metals, making them good conductors  
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Convection

  • Gases and liquids are free to move about this allows them to transfer heat energy by convection
  • Convection occurs when the more energetic particles move from the hotter region to the cooler region and take their heat energy with them  
  • Convection cannot happen in solids because the particles are not able to move around 
  • Convection currents are all about changes in density 
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Condensation and Evaporation

  • Condensation is when gas turns to liquid, when a gas cools the particles in the gas slowdown and lose kinetic energy, the attractive forces between the particles pull them closer together 
  • If the temperature gets cold enough and the gas particles get close together that condensation can take place, the gas becomes a liquid 
  • Water vapour in the air condenses when it comes into contact with cold surfaces 
  • Evaporation is when particles escape from a liquid
  • Particles can evaporate from a liquid at temperature that are much lower than the liquid's boiling point 
  • Particles near the surface of a liquid can escape and become gas particles if the particles are travelling in the right direction to escape the liquid and the particles are travelling fast enough to overcome the attractive forces of the other particles in the liquid
  • The fastest particles are most likely to evaporate from the liquid, when they do the average speed and kinetic energy of the remaining particles decreases 
  • The decrease means that the temperature of the remaining liquid falls, the liquid cools  
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Condensation and Evaporation II

  • The rate of evaporation will be faster if 
    • Temperature is higher, the average particle energy will be higher, so more particles will have enough energy to escape 
    • Density is lower, the forces between the particles will be weaker, so more particles will have enough energy to overcome these forces and escape the liquid 
    • Surface Area is larger, more particles will be near enough to the surface to escape the liquid
    • Airflow over the liquid is greater, the lower the concentration of an evaporating substance in the air it's evaporating into, the higher the rate of evaporation
      • A greater airflow means air above the liquid is replaced more quickly, so the concentration in the air will be lower
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Condensation and Evaporation III

  • The rate of condensation will be faster if 
    • Temperature of the gas is lower, the average particle energy in the gas is lower so more particles will slow down enough to clump together and from liquid droplets 
    • Temperature of the surface the gas touches is lower
    • Density is higher, the forces between the particles will be stronger. Fewer particles will have enough energy to overcome these forces and will instead clump together and from a liquid 
    • Airflow is less, the concentration of the substance in the air will be higher and so the rate of condensation will be greater 
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Rate of Heat Transfer

There are many factors which affect the rate of heat transfer, different objects can lose or gain heat much faster than others even in the same conditions

  • Heat energy is radiated from the surface of an object 
  • The bigger the surface area, the more infrared waves that can be emitted from or absorbed by the surface, so the quicker the transfer of heat
  • If two objects at the same temperature have the same surface area but different volumes, the object with the smaller volume will cool quicker, as a higher proportion of the object will be in contact with its surroundings 
  • Other factors, like type of material also affect the rate of heat transfer. Objects made from good conductors transfer heat away more quickly than insulating materials. It also matters whether the materials in contact with it are conductors or insulators as if an object came into contact with a conductor, the heat will be conducted away much faster than if it was in contact with a good insulator   
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Rate of Heat Transfer II

  • Vacuum Flasks 
    • The glass bottle id double-walled with a vacuum between the two walls. This stops all conduction and convection through the walls
    • The walls either side of the vacuum are silvered to keep heat loss by radiation to a minimum 
    • The bottle is supported using insulating foam. This minimises heat conduction to or from the outer glass bottle 
    • The stopper is made of plastic and filled with cork or foam to reduce any heat conduction through it 
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Rate of Heat Transfer III

  • In the cold, the hairs on your skin stand up to trap a thicker layer of insulating air around the body. This limits the amount of heat loss be convection. Some animals do the same using fur 
  • When you're too warm, your body diverts more blood flow near the surface of your skin so that more heat can be lost by radiation
  • Generally, animals in warmer climates have larger ears than those in cold climates to help control heat transfer 
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Energy Efficiency in the Home

There are many things which you can do to a building to help reduce the amount of energy that escapes. 

  • Cavity Wall Insulation - foam squirted into the gap between the brickwork which reduces convection and radiation across the gap 
  • Loft Insulation - a thick layer of fibreglass wool laid out across the whole loft floor reduces conduction and radiation into the roof space from the ceiling 
  • Draught-Proofing - strips of foam and plastic around doors and windows stop draughts of cold air blowing in, they reduce heat loss due to convection 
  • Hot Water Tank Jacket - lagging such as fibreglass wool reduces conduction and radiation 
  • Thick Curtains - big bits of cloth over the window to reduce heat loss by conduction and radiation 

 Heat transfers faster through materials with higher U-values than through materials with lower U-values. So the better the insulator the lower the U-value 

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Specific Heat Capacity

Specific Heat Capacity tells you how much energy stuff can store 

  • Materials which need to gain lots of energy to warm up also release loads of energy when they cool down again. They can 'store' a lot of heat
  • This is the amount of energy needed to raise the temperature of 1kg of a substance by 1 degrees Celsius. Water has a specific heat capacity of 4200JkgDegrees Celsius 
  • Heaters have high heat capacities to store lots of energy 
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Energy Transfer

There are many types of energy but they all obey the same principle

Energy can be transferred usefully from one form to another, stored or dissipated. But it can never be created or destroyed 

Energy is only useful when it can be converted from one form to another 

  • Electrical devices - Electrical energy ---> Light, sound and heat energy 
  • Batteries - Chemical Energy ---> Electrical and heat energy 
  • Electrical generation - Kinetic energy ---> Electrical and heat energy 
  • Potential Energy - Elastic potential energy ---> Kinetic and heat energy  
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Efficiency of Machines

Most energy transfers involve some losses, often as heat 

  • Useful devices are only useful because they can transform energy from one form to another
  • In doing so, some of the useful input energy is always lost or wasted as heat
  • The less energy that is wasted, the more efficient the device is said to be 

No device is 100% efficient and the wasted energy is usually spread out as heat

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Efficiency of Machines II

It is called wasted heat because you cannot do anything with it

  • Useful energy is concentrated energy, the entire energy output by a machine, both useful and wasted, eventually ends up as heat
  • The heat is transferred to cooler surroundings, which then becomes warmer. As the heat is transferred to cooler surroundings, the energy becomes less concentrated it dissipates 
  • The total amount of energy stays the same, but as it spreads out it cannot be easily used or collected back in again 

Sometimes waste energy can actually be used

  • Heat exchangers reduce the amount of heat energy that is lost
  • They do this by pumping a cool fluid through the escaping heat
  • The temperature of this fluid rises as it gains energy 
  • The heat energy in the fluid can then be converted into a form of energy that is useful again 
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Energy Transformation Diagrams

The thickness of the arrow represents the amount of energy 

  • The idea of Sankey Diagram is to make it easy to see at a glance how much energy is being used usefully and how that compares with whats wasted
  • The thicker the arrow the more energy it represents, the smaller going off show the different energy transformations taking place 
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