Thermal (heat) energy transfer

Infrared radiation

electromagnetic radiation whose wavelength is greater than that of visible light; sometimes known as heat radiation

Radiation

energy spreading out from a source carried by particles or waves

Electromagnetic radiation

energy travelling in the form of waves

Conductor

a substance that transmits heat or allows an electric current to pass through it

Insulator

a subtance that transmits heat very poorly or does not conduct electricity

Explaining conduction on non-metals (glass rod)

- One end is being heated, the other end is cold. There is thus a temperature difference between the two ends, and heat flows down the rod. -At the end of the hot area, the atoms are vibrating a lot -At the cold end, they are vibrating much less -As they vibrate, the atoms jostle their neighbours. This results in each atom sharing its energy with its neighbouring atoms. -Atoms with a lot of energy end up with less and vice versa. -The joslting gradually transfers energy from the atoms at the hot end to those at the cold end. Energy is steadily transferred down the rod from hot to cold.

What is radiation really?

- all hot objects emit infrared radiation -this radiation can travel across a vaccum in straight lines -the hotter the object the more infrared radiation that it emits -this can help you to do some detective work -our skin detects the infrared radiation produced by a hot object. Nerve cells buried just below the surface respond to heat.

Why does water not conduct heat?

- its molecules do not contain free electrons to easily pass on their kinetic energy to their meighbours, so the heat can only be transmitteed throught the vibration of the particles.

An experiment to show that water is a poor conductor of heat.

-A lump of ice is trapped at the bottom of the test tube, held in place by a piece of metal gaze -The water is heated close to the mouth of the tube. -The water boils, while the ice remains frozen. - Heat has not conducted down to the bottom of the tube. -The water remain cold and the ice does not melt

Sea breezes

-During the day, infrared radiation from the sun heats up the land more than the sea. The air above the land gets hotter than the air above the sea -The hot air above the land rises because it is less dense than the surrounding air -cooler air from the above sea rushes in to take its place -this convection current causes a cool sea breeze to blow from the sea to the land

Consequences of thermal energy transfers

-Hot objects have a lot of internal energy -energy tends to escape from a hot object, spreading to its cooler surroundings by conduction, convection and radiation. -this can be a great problem as energy could be wasted -to keep energy in something that is hotter than its surroundings, we need to insulate it and knowing about conduction, convection and radiation we are able to produce effective insulation

Explaining conduction in metals

-Metals are good conductors of heat because they have electrons in their structure which carry this energy to the rest of the electrons in the structure -Electrons are freely moving and they carry this heat energy to the other electrons that are found in the structure

An experiment to show which metal is the best conductor of heat

-The metal rods are all of the same sizes -Each one of them has an equal amount of wax put at the end of each rod -They are all heated equally at the other end. - The best conductor is the metal on which the wax melts first

The difference between convection and conduction

-conduction: energy is transferred through a material from a warmer place to a cooler place without the material itself moving -convection: energy is transferred through a material from a warmer place to a cooler place by the movement of the material itself.

Characteristics of infrared radiation

-is produced by warm or hot objects -is a form of electromagnetic radiation -travels through empty space in the form of waves -travels in straight lines -warms the object that absorbs it

Investigating rates of radiation

-one matt black can is filled with hot water -another shiny silvery can is also filled with hot water -aboard is used to isolate the cans and a thermometer is placed into both of them

Solar panel

-the black surface of the solar panel absorbs infrared radiation from the sun -the heat is then conducted through metal pipes to warm up water -the warm water rises to the top of the storage tank by convection

Convection currents in a fridge

-the freezing surface is usually positioned at the top and the back, so that cold air will sink to the bottom. Warm air rises to be re-chilled

Factors which affect the rate at which energy is radiated from the surface of a hot object

-the temperature of the surface - a hotter surface radiates energy at a greater rate -the surface area - energy is radiated at a greater rate from a bigger surface area

Experiment to show convection currents

1. a few crystals of potassium permanganate are placed at the bottom of a beaker of water. 2. They dissolve and colour the water near them purple. When the water is heated, the purple water rises above the bunsen burner and moves across and then falls on the other side of the beaker before returning to the flame to be heated again. HOW DOES THIS WORK? - When water is heated, it gains kinetic energy which increases its energy and starts to move faster. - As this takes place the water molecules start to expand, that means its volume increases while the mass stays the same. We then say that its density decreases. Since it, density has decreased and is now less dense than its surroundings it rises upward - A less dense material is lighter and is pushed upwards by the surrounding denser material - The molecules of the liquid have more kinetic energy and therefore they move faster. As they move from place to place they take this energy with them. -As the warm water rises it pushed the cooler water across because it has more kinetic energy -As the water cools further, its molecules kinetic energy and move closer together. The density of the water increases. The cooler, more dense water sinks

Home insulation

Doubled glazed windows -the vacuum between glass panes cuts out losses by conduction and convection -reduces heat exchange Cavity wall -reduces heat losses by conduction -it is impossible to have a vacuum in a cavity and so energy could be lost convection Filling cavity walls with wool -further reduces heat losses by convection -the foam traps the air Loft and underfloor insulating material -prevents conduction of heat through floors and ceilings -contains fibres which trap the air Thick curtains and draught excluders -stops convection currents, and so prevents cold air from entering and warm air from leaving

Vacuum flasks

Stopper -this is made of plastic which is an insulator to reduce heat flow by conduction. -the stopper also stops heat flow by convection of air and stops heat flow by evaporation Gap between double walls -this gap contains no air and evacuated to reduce losses by conduction and convection Silvery surface -reduces losses made by radiation by reflecting back any infrared radiation.

What happens to your finger when you touch a metal spoon compared to a plastic spoon?

Metal spoon 1. When your finger touches a metal object, heat is conducted out of your finger and into the metal. 2. Because metal is a good conductor, heat spreads rapidly through the metal. 3. Heat continues to escape from your finger, leaving it colder than before. 4. The temperature sensitive nerves in your fingertip tell your brain that your finger is cold. So you think you are touching something cold. Plastic spoon 1. When you touch a plastic object, heat conducts into the area that your finger is in direct contact with. 2. However, because the plastic is a good insulator, the heat travels no further. 3. Your finger, therefore, loses no more heat and remain warm.

Conduction

The transfer of heat energy or electrical energy through a material without the material itself moving

Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of radiation

a surface that is a good reflector is a bad absorber -Shiny or white surfaces are the best reflectors of heat radiation the worst absorbers) -matt black surfaces are the best absorbers of thermal radiation (the worst reflectors) -Matt black are the best emitters

Why is convection currents important

convection is the only method of thermal transfer of energy in fluids. -although thermal energy can be conducted through a liquid, this is generally a slower method compared to convection currents, when the fluid itself moves, carrying energy with it

convection currents

is a movement of a fliud that carries energy from a warmer place to a colder place

Convection

the transfer of heat energy through a material by movement of the material itself

What is thermal energy

thermal energy is energy transferred from a hotter place to a colder place - in other words from a high temperature to a lower temperature. -it requires a temperature difference in order for the energy to be transferred