Voltage Drop in Electric Circuits

[horsecandy911]

My textbook tells me that the voltage in an electric circuit only drops across a resistor. I don't quite understand this; doesn't this suggest that the electrons in the current are moving at a constant velocity? I would think that, since they are moving under the influence of an electrostatic force, they are being accelerated (and then slowing down when they encounter resistors). In that case, the potential energy would be being converted to kinetic energy, and the potential of points in the circuit would drop steadily as we moved along the circuit.

This raises the wider question of the velocity of electrons in a circuit. If they are moving at constant velocity, how do they get started moving? Is there a brief moment when the circuit is completed during which everything gets accelerated? Thanks for your help.

 

[RedX]

If you have a perfect conductor, then the tiniest of electric fields will set charges moving. Since the electric field is tiny, the voltage drop is tiny. So there is practically no voltage drop across a perfect conductor.

 

[schip666!]

I guess this is why all the real physicists look so shocked and dismayed when I use the water analogy to explain electricity...

They ain't no acceleration of electrons. In fact moving electrons are not really what makes electricity. It's the electrostatic field, which moves at the speed of light. Electrons drift around in the field tending in the direction of negative current flow. Here's a quickie description I found googling "speed of electrons in a wire":
http://www.Newton.dep.anl.gov/askasci/phy99/phy99092.htm

As to voltage drop. One needs to have a circuit to measure voltage. Lacking any other element, the voltmeter itself closes the circuit -- actually it measures current flow across a very high resistance and extrapolates that to be voltage. Aside from super-conductors, everything -- wire included -- has a finite resistance, which limits current flow and thus "creates" a voltage difference.

 

[Naty1]

My textbook tells me that the voltage in an electric circuit only drops across a resistor. I don't quite understand this; doesn't this suggest that the electrons in the current are moving at a constant velocity?

Electrons DO move at a constant DRFIT velocity...a slow speed when a constant voltage is applied. There IS a tiny "acceleration" as electrons are stripped from their orbitals by a newly applied emf (voltage) and begin to DRIFT slowly.

see here: http://en.wikipedia.org/wiki/Drift_velocity

but when an electron moves at one end of a circuit another begins to move at the other end almost the speed of light...so its SEEMS fast...like one car at the back of a train begins to move almost instantaneously as one at the front of the train begins to move...but the overall velcoity of the train is SLOW by comparison.

In a resistor the electrons are more tightly bound to their nucleus than in a conductor...so in a resistor it takes more voltage to break them free and keep them moving...So if a constant voltage is applied,say, to a piece of wood and a copper rod, very different currents will flow.

 

[vanhees71]

In fact in the stationary state, when there has been for a long time a time-independent voltage drop across a resistance, the current is also time-independent, and the electrons move at constant velocity.

Of course, there acts the electrostatic force on the moving electrons, but there's also a constant friction force which just compensates this electrostatic force. That's the microscopic picture of resistance!