August 18, 2011

4.6 Energy Transfer

Recall that energy transfer may take place by conduction, convection and radiation.

Conduction
Conduction is the transfer of heat within a substance, molecule by molecule. So if you heat up the bottom of a solid; (i.e. a frying pan)the bottom layer of particles gain more kinetic energy, and transfer kinetic energy to the next lay, which in turn transfer energy to the layer above, and so on. As a whole, the heat travels through the solid from the heat source spreading out to the edges of the object.

Convection
Convection is the heat transfer by the mass movement of a fluid in a vertical (up/down) direction. Warm water is less dense than cold water, making cold water heavier than warm water. so if you heat up a beaker of water from the base, the warm water molecules at the bottom (heated by conduction) rise up to the surface, and the cooler molecules from the surface fall down to the bottom. These cool molecules are now closer to the heat source and so rise, and the previously hotter molecules fall back down to the base to, in turn, be heated and rise again, and so. This process of rising and falling creates a current in the fluid, which can be called a Convection Current.

Radiation is the emission of energy as waves rather than through particles as before. These waves are called Electromagnetic Waves, because the energy travels in a combination of electric and magnetic waves. This energy in these waves is released when these waves are absorbed by an object, for example, energy travelling from the sun to your skin (as infra-red waves). You can feel your skin getting warmer as energy is absorbed.

The image below shows all three concepts, in a way that should aid your understanding

August 14, 2011

4.5 Sankey Diagrams

Describe a variety of everyday and scientific devices and situations, explaining the fate of the input energy in terms of the previous relationship [see 4.4], including their representation by Sankey Diagrams.

In laymen's terms: Use a Sanky diagram (shown below) to explain how input energy is divided into "waste" and "useful" energy.

Filament Light bulb
The diagram above is a simple 'Sankey Diagram' it shows that when 100J of electrical energy is put into (in this case) a filament light bulb, only 10J is released as useful light energy and the remaining 90J is lost as heat. Using the previous relationship [from 4.4] we can see that:

$E = 0.1 = 10\%$

So a filament light bulb has an efficiency of 10%

Energy Saving light bulb

On the other hand the Sankey diagram above for a energy saving light bulb shows that out of 100J of electrical energy only 25J is lost, meaning that an energy saving bulb is 75% efficient which is much more efficient than a filament bulb

4.4 Efficiency

Recall and use the relationship:

$efficiency = \frac{useful\: energy\: output}{total\: energy\: input}$

Remember: Efficiency is generally measured as a Percentage (%) so in order to get the answer as a percentage you may need to multiply the answer by 100, otherwise you will result with a number between 1 and 0. And  energy, as usual, is measured Joules (J)

4.3 Conservation of Energy

Understand that energy is Conserved

The law of conservation of energy states that the total amount of energy in a system is constant over time (i.e. if you put 5J in you get 5J out). A consequence of this is that energy can neither be created nor be destroyed; and can only be transformed from one state to another.