Speed of light?

[Tabby]

If you had a 230v 100W bulb, on the end of 1m of cable, how long would it take for a single electron to travel the length of the cable?

[steveb]

i think im right in saying it takes quite a long time, say a few minutes maybe 20 or 30, its only the energy which is passed along the cable very quickly

[fo3nix]

no, the electrons pass along the wire very quickly too. cant think of the speed off the top of my head, but i think its in the ballpark of the speed of sound.

definitely not as fast as the speed of light.

[brachiopod]

steveb, since an electric current is a flow of electrons, does that mean it will take 20 to 30 minutes for the bulb to light up after I switch it on ?

[kempie]

I tend to agree with steveb.

In order to understand why a metal conducts we have to know a little about the structure of metals. On a simple level we can imagine the metal as consisting of ions (metal cations) surrounded by electrons. These electrons are free to move through the structure of the metal. When there is no applied external electric field, the electrons move randomly. When an electric field is applied, however, the electrons will be attracted towards the more positive potential and the cations towards the more negative potential. It is only the electrons which are free to move and these migrate or travel towards the positive potential. The electrons are accelerated towards the positive potential and their energy (kinetic energy) increases. But as they move through the structure of the metal they collide with the metal ions, this has the effect of (a) slowing the electrons down and (b) transferring some of the electron�s energy to the metal. The speed that the electron travels, with all these collisions taking place, is typically 2 metres per hour. Of course in addition to slowing the electrons down, energy is transferred to the metal and this acts to heat up the metal. As the metal gets hotter and hotter, the metal ions vibrate more and more and so the number of collisions between ions and electrons increase, in effect the resistance to current flow increases. This is another characteristic of a metal; conduction decreases with increasing temperature.

However electricity is an apparent force and as such it is not that an electron moves over a specific distance that matters but rather that there is a net migration of these electrons. It is how quickly this migration is initiated which we observe when we 'flick the switch' and this is what tends to be called the Speed of Electricity, which is akin to the Speed of Light.

[steveb]

lol dont patronise me brachiopod, im studying a level physics. it does take a long time for electrons to move through a cable, as explained by kempie.

[Tabby]

Thanks Guys for some good answers. But it has got me thinking.... See my question on Super Conductors

Tabby.