Physics P1 Energy Transfer by Heating

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  • Created by: Phoebe
  • Created on: 06-06-12 11:23

1.1 Infrared Radiation

  • Infrared radiation is energy transfer by electromagnetic waves
  • All objects emit infrared radiation
  • The hotter an object is, the more infrared radiation it emits in a given time
  • If the temperature difference between the hot object and its surroundings is reduced, the rate of energy transfer decreases
  • The transfer of energy by infrared radiation does not involve particles
  • Infrared radiation can travel through a vacuum i.e. space
  • This is how heat reaches us from the Sun
  • A vacuum is a sealed compartment with no particles
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1.2 Surfaces and Radiation

  • Dark, matt surfaces emit and absorb more infrared radiation than light, shiny surfaces
  • Light, shiny surfaces reflect more infrared radiation than dark, matt surfaces
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1.3 States of matter

  • Flow, shape, volume and density are the properties used to describe each state of matter
  • The particles in a solid are held next to each other in fixed positions
  • A solid has a fixed shape and volume, and cannot flow
  • The particles in a liquid move about at random but remain in contact with one another
  • A liquid has a fixed volume but no fixed shape, and can flow easily
  • The particles in a gas move about randomly and are much further apart than particles in a solid or liquid
  • A gas can flow, and does not have a fixed volume or shape
  • The density of a gas is much less than that of a solid or liquid
  • In general, the particles of a gas have more energy than that of a solid or liquid
  • The particles in a liquid have more energy than those in a solid
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1.4 Conduction

  • Metals are the best conductors of energy 
  • Materials such as wool and fibreglass are the best insulators
  • This is because they contain air trapped between the fibres, and air is a poor conductor of energy
  • Conduction of energy in a metal is due mainly to free electrons transferring energy inside the metal
  • When a metal is heated at one end, the free electrons at the hot end gain kinetic energy and move faster
  • These electrons diffuse and collide with other free electrons and ions in the cooler parts of the metal
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1.5 Convection

  • Convection is the circulation of a fluid caused by heating
  • When a fluid is heated, it expands
  • The hot fluid becomes less dense and rises
  • The warm fluid is replaces by cooler, denser fluid
  • The resulting convection current transfers energy throughout the fluid
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1.6 Evaporation and Condensation

  • Cooling by evaporation of a liquid is due to the faster moving molecules escaping from the liquid
  • The average kinetic energy of the remaining molecules is reduced because the most energetic molecules have left

Evaporation can be increased by:

  • Increasing the liquid's temperature
  • Increasing the surface area of the liquid
  • Creating a draught of air across the liquid's surface

Condensation on a surface can be increased by:

  • Increasing the area of the surface
  • Reducing the temperature of the surface
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1.7 Energy Transfer by Design

The rate of energy transfer to or from an object depends on:

  • the shape, size and type
  • the materials the object is in contact with
  • the temperature difference between the object and its surroundings

To maximise the rate of energy transfer to keep things cool, we use materials that:

  • are good conductors 
  • are painted dark and matt
  • have the air flow around them maximised

To minimise the rate of energy transfer to keep things warm, we use materials that:

  • are good insulators
  • are light and shiny
  • prevent conduction, convection and radiation
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A Vacuum Flask

(http://t2.gstatic.com/images?q=tbn:ANd9GcR_G69IMOCJMQYTvUEt0DsdfVkhsEwh8LrCot89UOPHjAwEp_5NJQ)

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1.8 Specific Heat Capacity

  • The specific heat capacity of an object is the energy need to raise 1kg of a substance by 1C
  • The greater the mass of an object, the more slowly its temperature increases when it is heated

The rate of temperature change of a substance when heated depends on:

  • the energy supplied to it
  • its mass
  • its specific heat capacity
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1.9 Heating and Insulating Buildings

Energy transfer from our homes can be reduced by fitting:

  • loft insulation
  • cavity wall insulation
  • double glazing
  • draught proofing
  • aluminium foil behind radiators

U-Values tell us how much energy per second passes through different materials

The lower the U-Value, the better an insulator it is

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