Electrons?

[Gareth56]

What "pushes" the electrons through the wires when I switch on e.g. an electric kettle? And do the electrons travel at a certain speed in wires?

 

Thanks

G56

[insane_alien]

potential difference.

 

it is analogous to pressure in a watter pipe. if one side is at 1.2bar and the other is at 1 bar the water will flow to the lower pressure. replace bar with volts and water with electrons and pressure with potential.

 

electrons tavel at a speed proportional to the potential difference.

[YT2095]

electrons tavel at a speed proportional to the potential difference.

 

I think that should be "Rate" rather that "Speed"

[insane_alien]

if it is for constant current then it would be speed. as in m/s

[Gareth56]

potential difference.

 

it is analogous to pressure in a watter pipe. if one side is at 1.2bar and the other is at 1 bar the water will flow to the lower pressure. replace bar with volts and water with electrons and pressure with potential.

 

electrons tavel at a speed proportional to the potential difference.

 

So where is the higher potential of electrons in my 240V wall socket and how does a voltage of however many volts "push" electrons along a wire?

 

Ta

[Klaynos]

if it is for constant current then it would be speed. as in m/s

 

It'd be a velocity infact... The drift velocity...

 

The potential difference will be between the negative and positive, the wall socket is AC so this changes ~50 times a second depending where in the world you are.

 

A voltage is another name for a potential difference, in this case the 240 is actually the RMS voltage...

[5614]

A potential difference (aka a voltage) is the force which acts on electrons and makes them move in circuits.

 

As for their speed: electrons tend to move very quickly, their exact speed depends on their thermal energy. Using:

[math]\frac{3}{2}kT = \frac{1}{2} m v^2 [/math]

k = Boltzman constant

T = temperature (Kelvin)

m = mass of an electron

v = velocity

a room temperature electron will be travelling at about [math]10^6[/math]m/s... very fast!

 

However in a wire (which has resistance) the fast moving electrons are always bumping into other things (other moving electrons, other electron that are still orbitting a nucelus etc.). I can't remember the numbers (I could calculate it...), but there are thousands(?) or millions(?) of collisions per second. After each collision the electron is then going to be travelling in a random direction (probably the wrong one, ie. against the current).

 

This means a slightly different velocity for electrons in a wire would be more useful. The drift velocity of an electron in a wire can be thought of as the average speed the electron goes in the right direction. It is given by:

[math]I = nAq v_d[/math]

I = current

n = number of charge carriers (normally electrons) per unit volume

A = cross sectional area of the wire

q = charge of an electron

v_d = drift velocity

 

To give you an idea: an electron travelling through a copper wire will travel at about 1mm/s... so it's actually quite slow.

[thedarkshade]

Jonathan mentioned resistance. You can refer to resistance as a barrier to electrons, and because of which electricity cannot be conducted with 100% efficiency.

 

The resistance increases with respect to temperature. The greater the temperature, the greater the resistance, the greater the electricity loss. Everything greater huh!!! And the other way around too.

 

Why a conductive decrease with increase in temperature? Well, heat is energy, and when electrons get more they move on to a higher energetic level, they move faster and the probability of collisions and the number of collisions is greater, so there is less efficiency on conduction. And the other way around too.

 

You can measure the resistance with temperature change by:

[math]R=R_0(1 + \alpha t)[/math]

 

You may have heard of superconductors.

The basic principle is that you cool them up to a point when rotation movements of electrons stop, and them all electrons move in a collective way. There are no individual trajectories and all electrons have the same direction. Resistance at that point is zero.

More about this on Wiki

I've heard something of high-temperature superconductivity too. Just google for that.

 

Cheers,

Shade

[5614]

You may have heard of superconductors.

The basic principle is that you cool them up to a point when rotation movements of electrons stop, and them all electrons move in a collective way. There are no individual trajectories and all electrons have the same direction. Resistance at that point is zero.

That is in fact quite wrong. But I am smiling because it reminds me of a mistake that I made, when I thought this was true. I was doing a science report on temperature and resistance (small temp -> less resistance). I was 15 at the time, I knew my teacher had done a PhD related to superconductors, so I decided to extrapolate my data to claim that at T=0 the resistance is also 0. Little did a young and innocent me know how very wrong that is!

 

The first problem is that we can never reach T=0. Note T is temperature in Kelvin. We can get very close to 0, but it is impossible to reach it (it is against the laws of physics, it is not an experimental limitation or a problem with our technology). But this is a minor side point.

 

The problem is that superconductivity is not explained by low temp -> little movement, it is explained by something totally different, known as BCS theory (named after the 3 people who discovered it - Bardeen, Cooper & Schrieffer). Basically it says that at low temperatures electrons pair up (to form Cooper pairs)... and because of how these electron pairs interact there is 0 resistance.

 

The "high" temperature superconductors mentioned work at around 70 Kelvin, or -200C or -334F - so they are quite cold too! However BCS theory doesn't seem to apply to them in quite the same way; at the moment there is no definitive theory that explains how high temperature superconductors (aka Type II superconductors) work.

 

If you want more then look up "BCS theory" and "Cooper pairs". Also a useful note if you do look this up is that Tk or critical temperature is the temperature when something interesting happens. Tk for a superconductor is the temperature at which the material starts superconducting.