P1.1 Transfer of Energy by Heating

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1. Where Heat Energy Transfers to

Heat energy always flows from hotter materials of higher temperatures to cooler materials of lower temperatures. 

The bigger the temperature difference between an object and its surroundings (eg. a mug of hot coffee in a cold room) the faster the heat energy is transferred from the hot material to the cold material  (eg, heat from coffee transfers to air)

The bigger the temperature difference, the bigger the rate of heat energy transfer. 

Heat energy can be transferred by conduction, convection or radiation. 

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2. Conduction

Conduction is the transfer of heat energy through particles vibrating against each other in a solid, or the collision of particles in a fluid.

Conduction is the main mode of heat transfer in a solid.

In a solid, the hotter particles vibrate more strongly so there is more kinetic energy. Those particles bang agaisnt other cooler particles with less kinetic energy and so tranfer the heat energy from a higher temperature region to a lower temperature region. 

Usually, the denser the solid, the better it is as a conductor.

In materials where the particles are futher apart (eg, gas) the rate of heat transfer/ conduction is reduced.

Metals are particular good conductors due to free electrons. Some of the electrons are free to move around within the solid, meaning they can rapidly transfer kinectic energy by particles. The 'hot' electrons in the higher temperature region collide with the 'cold' electrons and so tranfer the heat energy (kinetic energy). This is much faster than the particles vibrating in non-metals. 

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3. Convection

Convection occurs only in fluids. It involves heat transfer via particles but in bulk movement of liquids or gases. 

Convection occurs when warmer less dense fluids rises and is replaced by cooler more dense fluids falling. This is called a convection current. When a material is heated, the particles have more kinect energy, so move faster and move further apart from each other. This means the material expands and becomes less dense. It is the change in density that causes convection to occur. 

This is how hot water is produced in water tanks and saucepans etc, where convection currents flow from a heating element and heats all the water. The heating element must be near the base of the tank/saucepan otherwise convection currents can't occur. 

Radiators heat up whole rooms mainly due to convection. Warm air rises from the radiator and cool air is drawn in at the base of the radiator to replace it, so a convection current occurs that gradually heats up a whole room. 

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4. Radiation

Heat radiation is emitted by all materials, gases, liquids or solids. The hotter a material is,  the more strongly it gives out heat radiation call infrared radiation. An object that is hotter than its surroundings will emit more radiation than it absorbs. An object that is cooler than its surrounding will absorb more radiation than it will emit. 

Dark,matt surfaces are good absorbers and good emitters of radiation. Solar panels for hot water consist of pipes carrying water to be heated under the black surface to efficiently absorb infrared radiation. The pipes are made of copper which allow efficient conduction of the surface heat energy to the incoming cold water.

Light, shiny surfaces are poor absorbers and poor emitters but good reflectors of radiation. They can keep heat in to keep things warm or minimise heat radiation in to keep things cool. 

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5. Vacuum flask

Designs of a vacuum flask are to reduce heat transfers

The vacuum prevents heat lost by conduction and convection

The plastic lid reduces heat lost by convection, conduction and evaporation

The inside is silvered to reflect radiation, reducing heat lost by infrared radiation

The glass vessel is supported by insulating material

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6. Condensation and evaporation

Condensation is when a gas is cooled low enough to allow the attractive forces to be strong enough to attract the particles together as a liquid. This only happens if the kinetic energy of the particles is low enough. The lower the temperature, the smaller the kinetic energy.

Evaporation is when the highist kinetic energy particles of a liquid escape from the surface, They overcome the attractive forces of the bulk of the paricles. The more kinetic energy a liquid surface particle has, the more likely it will become a gas particle. As the highest kinetic particles escape, they leave the slower lower kinetic particle, the bulk of the liquid will cool. A cooling efect accompanies the evaporation of a liquid. Sweating cools you down because of the evaporaion of the water from your skin. 

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7. Factors Affecting Rate

The rate at which an object transfers energy by heating depends on: surface area and volume; the material the object is made of; the material the surface the object touches. 

Cooling fins maximise the transfer of heat by having a large surface area.

The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by the object. 

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8. Animal adaption

Hair and fur can trap insulating air, minimising heat loss by conduction or convection in cold convection. Conservation of energy is essention to survive in cold climates

Animals in cold climates have smaller ears than those in hot climate. They have a smaller surface area meaning heat is lost by radiation at a lower rate. 

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9. Kinetic Particle Theory

There 3 states of matter: gases; liquids and solids

Gases: have almost no force of attraction between particles; most kinetic energy; free to move around and move at high speeds; no fixed positions; 

Liquids: have weak force of attraction; particles rather close together but are free to move around; they move at a slower speed than gas particles; free moving particles have some kinetic energy 

Solids: have strong force of attraction; held together in fixed positions in a regualr arrangement;  have least amount of kinetic energy

When a solid is heated, the kinetic energy of the particles is increased until they have enough kinetic energy to weaken the interparticle bonds to allow melting. With further heating above the melting point, the particles at the surface with the highest kinetic energy can escape the surface (evaporate) or vapourise in the bulk of the liquid at the boiling point.   

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10. U-Value

The U-values measures how effective a material is as an insulator.

Materials with low U-values are good insulators (Poor conductors)

Materials with high U-valuers are poor insulators (Good Conductors)

Solar Panels may contain water that is heated by radiation from the sun. The water is used to heat buildings. it has a high heat capacity and can store a lot heat energy

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11. Specific Heat Capacity

The specific heat capacity is the amount of energy it takes to heat 1kg of a substance by 1*C

E = m x c x θ

E = energy transfered (J)

m = mass (kg)

c = specific heat capacity (J/kg*C)

 θ= temperature change (*C)                                                     

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12. Insulating Buildings

Loft Insulation - Cheap and Effective - Thick layer of fibreglass wool over loft floor, reduces conduction and convection of heat lost through roof - payback time a few years

Cavity wall insulation - Rather expensive - insulating foam between brick walls, reduces convection (possibly conduction) and radiation across the walls - payback time a few tears

Double glazing - expensive, longer- insulating air trapped between glasss panes, reduces heat loss by conductino and convection - long payback time

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13. Heat Sinks

Heat sinks are devices designed to transfer heat away to keep another device cool. Metal plates with fins are fitted in computers to stop electronic components overheating. Copper conducts heat away quickly

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