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THERMAL PHYSICS
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To Heat... Heating...
If you have read the other pages is this thermal physics section (and, if not... why not!?) you might have noticed the absence of the word "heat" as a noun.  
This is not an accident.  
In everyday speech, there is often confusion between scientific terms and the word heat is a good example.  
It is often used as synonymous with temperature (and probably other terms as well).  
   
In thermal physics, there is no such thing as heat (noun).  
There is thermal (or internal) energy.  
The word heat does, of course, exist but only as a verb: to heat or heating... hence the slightly strange title of this page.  
For example, energy naturally flows from a hot body to a cold body.  
When energy flows from a hot body to a cold body, we say that the hot body has heated the cold body.  
Energy naturally flows from a hot Bunsen burner flame to a cold beaker of water placed above it.  
The flame heats the beaker of water.  
   
This is discussed further when considering the First Law of Thermodynamics.  
   
The word heat, then, is associated with energy transfers and we will discuss here the three mechanisms by which thermal energy can be transferred from one place to another.  
   
1. Conduction  
This is the way thermal energy moves through solids (liquids and gases conduct thermal energy but, in general, much less well than solids).  
Thermal energy is conducted through solids by two mechanisms:  
1. Lattice* vibrations When the nucleus of one atom vibrates, energy is passed on to nearby nuclei through the inter-atomic forces (strong in solids).
2. Free electrons The random motion of free electrons transfers energy (by collisions). This is why metals are particularly good conductors.
 
   
* the regular arrangement of atoms in a solid is called a lattice.  
   
The rate of flow of energy (Q/t) through a solid depends on  
1. the temperature difference (ΔT) causing the heat to flow
2. the length (L) of the piece of solid
3. the cross-sectional area (A) of the piece of solid
4. the type of material.
 
   
Experiments show that  
 
 
 
and, combining these statements gives the following equation  
 
in which k is a constant called the thermal conductivity of material through which the energy is flowing.  
If you rearrange the equation, you will see that the units of k are Js-1m-1C-1   
which is equivalent to  Wm-1C-1 (or Wm-1K-1)  
   
The quantity ΔT/L is called the temperature gradient.  
Imagine we have a metal bar, the ends of which are maintained at different temperatures as shown below.  
If we could obtain readings of the temperature at different points along the bar, which is well insulated from its surroundings, a graph of temperature against position would be a straight line.  
This should not be surprising as the rate of flow along the bar is the same at all points.  
   
 
   
The flow of thermal energy by conduction is sometimes represented by lines of flow (unfortunately often referred to as lines of "heat" flow, which I will try to avoid for the reason explained above!).  
The flow lines inside the bar for the above situation would simply be parallel, equally spaced lines.  
   
 
   
However, if the bar is not insulated, energy can also flow to the surroundings.  
   
 
   
In this case the temperature gradient varies along the bar, as shown in the graph and flow lines would be something like in the next diagram.  
   
 
   
Notice that the lines are more dense where the rate of flow is greater.  
   
2. Convection  
Gases (and many liquids) have low thermal conductivity.  
However, energy can be transported through a fluid (liquid or gas) by convection currents.  
   
 
The temperature of the water nearest to the flame increases.  
This water expands and so its density is less than the water surrounding it.  
The higher temperature water therefore "floats" upwards transferring energy through the liquid.  
   
A similar situation exists when a room is heated by a (badly named) "radiator".  
Effects of convection currents in air can be observed if a source of smoke (or a piece of very low density plastic film) is held near a room heater.  
   
3. Radiation  
Energy can also be transferred from hot bodies to cold bodies by the process of radiating electro-magnetic radiation.  
All bodies emit energy in the form of electro-magnetic radiation, (sometimes called electro-magnetic waves).  
The rate of emission of energy depends on the temperature of the body and the nature of its surface.  
Similarly, all bodies absorb electro-magnetic radiation.  
The rate of absorption of energy depends only on the nature of its surface.  
The transfer of energy by radiation can be demonstrated as shown below:  
   
 
   
The temperature of the darker tube increases more rapidly than the temperature of the shiny tube.  
When the source of energy is removed, the two tubes return to their initial temperature in about the same time.  
Conclusion  
Dark coloured or dull surfaces both emit and absorb radiation at a greater rate (for a given temperature) than light coloured or shiny surfaces.  
   
On a sunny cloudless day try touching different coloured cars in a car park.  
Touch exposed "chromed" surfaces and compare their temperature with both light and dark coloured painted surfaces.  
The darker coloured surfaces are noticeably hotter.  
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