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# resistance Watch

1. what is resistance exactly? I feel so confused after reading this sentance 'the electrons in the wire filament of a light bulb move faster when they travel in the resistance of the filament that they do in the copper leads'. What does it mean? Doesn't resistance slow electrons down?
2. Pitched at GCSE level:

Think of resistance as something that tries to stop electrons getting from a to b and in the process forces them to give up energy in the form of heat and light (both forms of electromagnetic energy).

There is no way of knowing the velocity or path of an individual electron, but in bulk, the average path and velocity (current) of all electrons can be measured.

So resistance is a way of describing how difficult (or conversely successful) the journey from point a to b of the bulk of electrons will be.

Free electrons move through a conductor by hopping from one atom to another ONLY if there are free spaces in the outer shells of conductor atoms. i.e. the shell is not already full. The rate at which the hopping occurs is determined by the voltage difference at the ends of the conductor which applies a motive-force to all the electrons in the conductor.

So in a conductor, there are very many free spaces in the outer shells of its atoms and electrons are easy to pull from the outer shell either by an electric-force or a magnetic field. Atoms can move through the conductor with relative ease. The atoms are also evenly spaced in a regular lattice or grid, making jumping between atoms easier.

In a resistor, the individual atoms are increasingly unevenly spaced and there are fewer free spaces in the outer shells so less electrons cannot move so freely. Also the journey from a to b takes longer as the path for electrons is much more convoluted because of the uneven spacing of atoms.

Resistance is a function of temperature: At high temperatures, the resistance of a metal increases linearly with temperature. The individual atoms are jigging about far more, making the journey path from a to b longer. This means that for a given voltage, the successful journey of an electron from a to b becomes less likely, but more likely for it to give up its energy in the form of heat and light than in a cold conductor.
3. (Original post by celina10)
what is resistance exactly? I feel so confused after reading this sentance 'the electrons in the wire filament of a light bulb move faster when they travel in the resistance of the filament that they do in the copper leads'. What does it mean? Doesn't resistance slow electrons down?
It can get pretty complicated. I think that for now, it is best to tell you that the current in the circuit has to be the same everywhere. The resistance impedes electrons, so for the same number of electrons to flow by a point in a given time, you would need them to be quicker.
4. (Original post by Brister)
It can get pretty complicated. I think that for now, it is best to tell you that the current in the circuit has to be the same everywhere. The resistance impedes electrons, so for the same number of electrons to flow by a point in a given time, you would need them to be quicker.
So resistance can be seen as blocking one of the paths, and in order to find a path that isn't blocked the electrons would have to be quicker? Is this a correct way of thinking about it?
5. (Original post by celina10)
'the electrons in the wire filament of a light bulb move faster when they travel in the resistance of the filament that they do in the copper leads'. What does it mean? Doesn't resistance slow electrons down?
I would like to know where this comes from and the context of the statement.
What else is being discussed in that paragraph?
Taking something out of context can cause confusion.
There are other reasons why the electrons in the filament could travel faster (or slower) than those in the connecting wires.
6. (Original post by Stonebridge)
I would like to know where this comes from and the context of the statement.
What else is being discussed in that paragraph?
Taking something out of context can cause confusion.
There are other reasons why the electrons in the filament could travel faster (or slower) than those in the connecting wires.
'As described in spread 2.1.3, the electrons in the wire filament of a light bulb move faster when they travel in the resistance of the filament than they do in the copper leads.' This is what was in spread 2.1.3:

'Drift velocity in other materials:
A good conductor contains a huge number of conduction electrons that are free to move within the material. Metals such as copper are good conductors because they contain large numbers of these conduction electrons. They have large number densities. Insulators, on the other hand, have very few or no conduction electrons. The number density for good insulators is near to zero.

Semiconductors have intermediate properties. example include silicon, which is used in integrated circuits. Typically, they have number densities a million times smaller than metals such as copper. The number density of semiconductors can be altered by deliberately adding a small amount of impurity to the material. This is known as doping. The low number density of the semiconductor means that the conduction electrons travel much faster than they do in conductors.'
7. Well the paragraph you have quoted is saying that in a material with fewer free electrons (fewer conduction electrons), in order to carry the same current the electrons there would have to move faster than those in a conductor with more free electrons.

This question was asked here a couple of days ago and here is the explanation.
http://www.thestudentroom.co.uk/show....php?t=2303289

So logically, if this paragraph is related to the filament example, the implication in the text is that the filament has fewer free electrons.

The higher drift speed of the electrons in the filament could also be due to the fact that the filament wire is thinner than the conducting cable.
The formula for drift speed is v = I/nAe

So it will increase if A is less (thinner wire) or n is less (fewer available electrons)
for the same current.

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Updated: March 29, 2013
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