Physics (Energy Transfer by Heating)

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Infrared Radiation

Infrared Radiation is energy transfer by electromagnetic waves

All objects emit infrared radiation

The hotter the object is, the more infrared radiation it emits in a given time

Radio waves, microwaves,infrared raditation and visible light are part of the electromagnetic spetrum

Hot object emits more radiation than it absorbs (cooling down)

Cooler objects absorbs more radiation than it emits  (heating up)

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Surfaces and Radiation

Dark, matt surfaces emit more infrared radiation than light, shiny surfaces

Dark, matt surfaces absorb more infrared radiation than light, shiny surfaces

Light shiny surfaces reflect more infrared radiation than dark, matt surfaces

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States of Matter

The particles in a solid are held next to each other in fixed positions

The particles in a liquid move about at random and are in contact with each other

The particles in a gas move about randomly and are much further apart than particles in a solid or a liquid

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

Materials that have larger spaces between their particles conduct heat more slowly- these are called insulators

Meatals contain free electons which collide with themselves and ions, transfering kinetic energy faster

Non-metals electrons are held in atoms, transferring energy takes place because the atoms vibrate and shake eachother, which is less effective

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Convection is the circulation of a fluid (liquid or gas) caused on by heating it

Convection only occurs in liquids or gases

Heating a liquid or gas makes it less dense so it rises and the cooler liquid or gas replaces it

Fluids rise when heated because the particles move about taking up more space

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Evaporation and Condensation

Cooling by evaporation of a liquid is due to the faster moving molecules escaping from the liquid

Evaporation can be increased by:

  • Increasing the surface area of the liquid
  • Increasing the liquids temperature
  • By creating a draught of air across the liquids surface

Condensation can be increased by:

  • Increasing the area of the surface
  • Reducing the temperature of the surface

When the particles with the most kinetic energy evaporate, the average speed and kinetic energy decreases; This means the temperature of the remaining liquid cools

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Energy Transfer by Design

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

  • the shape, size and type of material of the object
  • the materials the object is in contact with
  • the temperature difference between the object and its surroundings
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Rate of Heat Transfer

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 e.g radiators

Heat sinks are devices designed to transfer heat away from objects they're in contact with, e.g. computer components. They have fins and a larg surface area so they can emit heat as quickly as possible.

If two objects had the same temperature and surface area but different volumes, the object with the smaller volume will cool more quickly- as a higher proportion of the object will be in contact with its surroundings.

For humans, the hair on their skin "stands up" to trap a thicker layer of insulating air around the body. This limits the amount of heat lost by convection

When humans are too warm, their body divert more blood to flow near the surface of your skin so more heat can be lost by radiation.

Animals in warm climates have larger ears than those in cold climates to help control heat transfer.

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Specific Heat Capacity

Specific heat capacity is the amount of energy needed to raise the temperature of 1kg of a substance by 1'C. Water has a specific heat capacity of 4200J/kg'C

The greater the mass of an object, the more slowly its temeperature increases when heated

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

  • The energey suppiled to it
  • Its mass
  • Its specific heat capacity

Storage heaters use off-peak electricity to store energy in special bricks

E= m x c x ϴ

Energy (J) = Mass (kg) x Specific heat capacity (J’C-1 kg-1) x Change in temperature(‘C)

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Energy Efficiency in the Home

Payback time = intial cost/annual saving

  • Cavity Wall Insulation- reduces covection and radiation across the gap. Pockets of air in the foam reduces heat transfer by conduction
  • Loft Insulation- reduces heat loss by conduction and convection
  • Draught Proofing- reduce heat loss due to convection
  • Hot Water Tank Jacket- reduces conduction and radiation
  • Thick Curtains- reduces heat loss by conduction and radiation

U-values tell us how much energy per second passes throught different materials

Heat transfers faster through materials with higher U-values

The lower the U-value the more effective the material is as an insulator

Solar heating panels do not use fuel to heat water but they are expensive to buy and install

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