Conduction and Convection Flashcards
(25 cards)
What is conduction?
Flow of heat through matter from places of higher temperature to places of lower temperature without movement of matter as a whole.
What materials are examples of good conductors?
Most metals.
What materials are examples of bad conductors?
Wood, glass, cork, plastics and fabric.
Why do metal objects below body temperature feel colder than those made of bad conductors?
Carry heat away faster from hand, even if all objects are exactly same temperature.
What other types of substances can conduct heat.
Liquids and gases but only very slowly.
What is convection?
Flow of heat through fluid from places of higher temperature to places of lower temperature by movement of fluid itself.
What are fluids?
Liquids and gases.
What are convection currents?
Streams of warm moving fluids.
How do convection currents arise?
When fluid is heated as it expands, becomes less dense and forced upwards by surrounding cooler, denser fluid which moves under it.
How is the inside of a buildings kept at a steady temperature above the outside?
Heating it at rate equal to rate at which it loses energy.
Where does the lose of energy by conduction mainly occur in a building?
Through walls, roof, floors and windows.
How can buildings be maintained at a steady warm temperature without burning fuels?
Wall and roof insulation.
What is the U-value?
Heat energy lost per second through specified heat conductor per square metre when there is temperature difference of 1oC between its surfaces.
What is the equation for the rate of heat energy loss through a conductor?
Rate of energy loss = U-value x Surface area x Temperature difference
What is the unit for the U-value?
W/(m2^oC)
What 2 substances are good for insulating a building?
Fibre glass
Plastic foam
How can conduction be demonstrated?
- Match fixed to end of each metal rod using melted wax.
- Other ends of each rod heated by burner.
- When temperature of far end reach melting point of wax, match drop off.
- Fastest match to drop means that metal is best conductor.
How can convection be demonstrated?
- Drop a few crystals of potassium permanganate down tube to bottom of beaker of water.
- When tube removed and beaker heated just below crystals by small flame, purple streaks of water rise upward and fan outwards.
What is radiation?
Flow of heat from one place to another by means of electromagnetic waves.
How can radiation be demonstrated?
- Some surfaces absorb radiation better than others.
- Inside surface of lid is shiny and other lid is dull black.
- Coins stuck on outside of each lid with candle wax.
- If heater midway between lids each receive same amount of radiation.
- After few minutes wax on black lid melts and coin falls off.
- Shiny lid stays cool and wax unmelted.
Where can radiation occur that conduction and convection cannot?
In vacuum as particles of matter not involved.
How heat reaches us from Sun.
What does radiation have all the properties of?
Electromagnetic waves
What happens when radiation falls on an object?
Partly reflected.
Partly transmitted.
Partly absorbed, raises temperature of object.
What is the first way that conduction can be explained by kinetic theory?
Occurs in metals that have large number of free electrons which wander about inside them.
When one part of metal heated, electrons there move faster (k.e. increases) and farther.
As result they jostle atoms in cooler parts, passing on energy and raising temperature of these parts.
Process occurs quickly.
