What happens to the electrons when the voltage is increased? I know that when the current increases, there are more electrons passing by per second. And I know as well that voltage is some difference in charge between the anod and katod.
But, what I think I don't really understand is, what makes a light bulb glow stronger when the voltage is increased? Isn't it just that when the voltage increses in a circuit, so does the current. So, when the voltage is increased, the current increases and the ligt bulb blows stronger? Or am I wrong?
If I am, what does an increase in voltage actually do to the electrons in the circuit?
Thanks
 A: The current depends on the drift velocity of the electrons, in the metal lattice. 


I is the electric current
     n is number of charged particles per unit volume (or charge carrier density)
     A is the cross-sectional area of the conductor
     v is the drift velocity, and
     Q is the charge on each particle. 

The higher the voltage, the higher the current, from I=V/R .
In the Resistance of the lamp, the higher drift velocity  electrons will interact and transfer more energy to more of the atoms/molecules of the lattice in the resistor, which will translate to more photons as the atoms/molecules relax back.
A: 
voltage is some difference in charge between the anod and katod.

That's not really true. Voltage is a measure of the difference in electric potential between two points, this is related to the strength of electric-field in that location.

what makes a light bulb glow stronger when the voltage is increased?

You are talking about tungsten-filament incandescent light-bulbs rather than other types (CFL, LED etc)
The increased voltage drives a larger current through the resistive filament. This makes it hotter, which makes it brighter.

what does an increase in voltage actually do to the electrons in the circuit?

I believe it exerts a greater force on free-electrons in metals, which has an effect on their drift velocity (with the sort of materials and voltages you probably have in mind I don't think it recruits greater numbers of charge carriers)
A: The actual glow of the tungsten is a result of photons being released as tungsten energy state changes caused by the flow of electrons.  And so essentially the glow is directly caused by the amount of electrons passing through.  
If we're comparing electrical circuits to water flow, then the water flowing through the pipes would be the current, the size of the pipes represent the resistance, and the voltage would be the gigantic water tank source from which the water flows.
The more water in the tank, the more pressure those pipes have for water to flow through them, however you could have a huge water tank and only have a trickle flow of water if the pipes are too small.  The effect of increasing the water tank size will cause more water to flow through the pipes, and as we mentioned earlier is comparable to our lightbulb receiving more current and therefore glowing brighter.  What happens if we increase the water tank size and replace the pipes with smaller ones?  Increased pressure causes more water to flow, but the flow is limited by the size of the pipes, so they cancel each other out.
What does this mean?  Voltage in of itself does not increase light bulb brightness, but voltage with no increase in resistance does.  Ultimately what is representative of the brightness of the lightbulb is simply the current passing through, regardless of voltage or resistance.  Voltage and resistance only cause an increase or decrease in brightness because they directly influence the amount of current passing through.
