Transfer of Thermal Energy

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Transfer of Thermal Energy by Mind Map: Transfer of Thermal Energy

1. Conduction

1.1. Process by which heat is transferred through a medium from one particle to another

1.2. Convection in non-metals

1.2.1. Heat is conducted by vibration of particles

1.2.2. One end of the rod is heated

1.2.3. Particles at that end gain energy and vibrate vigorously about fixed positions

1.2.4. These particles collide with less energetic neighbouring particles

1.2.5. During collision,some KE is transferred fo neighbouring particles

1.2.6. Neighbouring region of the becomes hot Thermal energy has been transferred without the transfer of particles

1.2.7. This energy transfer stops when thermal equilibrium is reached

1.3. Conduction in metals

1.3.1. Heat is conducted by free electrons and vibration of particles

1.3.2. When heated,free electrons at the end absorb Thermal Energy(hence gain KE)

1.3.3. These electrons are free to move in the spaces in between the particles

1.3.4. As they move,they collide with atoms in the cooler parts of the rod,making them vibrate more vigorously Some of the KE of the moving electrons is transferred to the atoms

1.3.5. Thermal energy is transferred via the motion of the free electrons The cooler end of the rod becomes hot

1.4. How does a blanket keep you warm?

1.4.1. The fibres of the blanket trap tiny pockets of air

1.4.2. Air is a poor conductor of heat

1.4.3. Heat loss from the body to the surroundings by conduction is reduced

1.5. Conductors

1.5.1. Used if heat is needed to flow quickly

1.5.2. Examples Cooking utensils Soldering iron Heat exchangers

1.6. Insulators

1.6.1. Minimize heat flow or heat loss

1.6.2. Examples Handles of cooking utensils Floor mats Wollen clothing Double-glazed windows

2. Convection

2.1. Transfer of thermal energy by means of convection currents in a fluid(liquid or gas),due to a difference in density

2.1.1. Does not occur in solid

2.1.2. Hot Lower Density(Rises)

2.1.3. Cool Higher density(sinks)

2.2. Convection in liquids

2.2.1. When a liquid is heated, it expands

2.2.2. Becomes less dense and rises

2.2.3. Cooler and denser liquid at the top sinks to replace it

2.2.4. The cooler liquid in turns get heated up,forming a convection current

2.3. Convection in gases

2.3.1. Occurs more readily in gases than in liquids Gas expands more than liquid when its temperature rises

2.4. Examples

2.4.1. Electric kettle

2.4.2. Household hot water systems

2.4.3. Air Conditioners Positioned high near ceilings Cool air from the air-conditioner is denser and thus sinks Warm air which is less dense rises to be cooled by the air-conditioner Sets up a convection current Cycle repeats until room air temperature is the same as the temperature set on the themostat

3. Radiation

3.1. Thermal energy from the sun

3.1.1. Reaches us by radation

3.1.2. Does not reach us by conduction/convection These involve molecules

3.2. Transfer of thermal energy in the form of electromagnetic waves such as infrared radiation without the aid of a medium

3.2.1. Can take place through vacuum

3.3. Factors affecting the rate of emission and absorption of infrared waves

3.3.1. Colour and texture of the surface Dull and black surfaces Good emitters and absorbers of heat Shiny and Silver surfaces Poor emitters and absorbers of heat

3.3.2. Surface temperature The higher the temperature of the object's surface relative to the surrounding temperature,the higher the rate of emission of infrared radiation

3.3.3. Surface area If 2 objects have SAME mass and density,the object with larger surface area emit or absorb more infrared radiation

3.4. Emitters

3.4.1. Good Cooling fins at the back of the refrigerator painted dull black

3.4.2. Bad Shiny metal teapot Food kept warm by aluminium foil

3.5. Absorbers

3.5.1. Good Solar heating panels are painted dull black to absorb heat

3.5.2. Poor Houses in hot countries are painted white to reflect radiation Light colours are chosen for clothes and cars in hot countries

3.6. Vacuum flask

3.6.1. Plastic stopper Reduces heat transfer by conduction,convection and evaporation

3.6.2. Trapped air Poor conductor of heat Conduction through trapped air above the liquid is minimised

3.6.3. Vacuum Vacuum between the double glass walls of the flask prevents conduction and convection through the sides of the flask

3.6.4. Double-Walled glass container with silvered surfaces Reflect infrared radiation back into hot liquid,or to the surroundings if the liquid is cold Heat transfer through radiation is minimised

3.6.5. Cork/foam plastic support Poor conductor of heat Used to prevent heat transfer by conduction

4. Thermal Equilibrium

4.1. Heat travels from a place of higher temperature to a place of lower temperature (Transfer of Thermal energy)

4.1.1. Hot objects contain molecules of higher kinetic energy

4.1.2. Molecules in hot objects lose KE by slowing down

4.1.3. Molecules in cold objects gain KE by speeding up

4.1.4. Molecules of both object have the same average KE Transfer of thermal energy stops

4.2. Objects with same temperature(same average KE) are said to be in Thermal equilibrium