# Why does voltage drop to zero over a resistor?

1. Mar 10, 2012

### robjow

How I am currently imagining it: In a simple circuit with just a battery and resistor the voltage of the battery sets the electrons off through the circuit at some speed and that speed is proportional to the voltage. The electrons then encounter the resistor.

From these forums my understanding of resistors is that the atoms in the resistor attract the electrons and work is required to push them past the resistors atoms. The electrons kinetic energy is transformed into light and heat energy, slowing the electrons down.

So why would the voltage drop totally across a resistor. If the resistance was small, wouldn't the electrons only need to expend a little bit of work and hence be slowed down just a little. If they are only slowed down a little wouldn't that mean there is still electrical pressure and hence voltage after the resistor?

I know that what I'm saying is not possible due to Ohms law, but I'd like to intuitively know what is going on.

2. Mar 10, 2012

### tiny-tim

welcome to pf!

hi robjow! welcome to pf!
no

the number of electrons going past any particular point each second is the same all the way round the circuit

current means charge per time (past any particular point)

that's (obviously) proportional to electrons per time (passing any particular point)
it's like water flowing along a mill-stream …

when the water-wheel isn't obstructing the water, the water-speed is quite fast

when the water-wheel does obstructing the water, the water-speed is much slower, because the wheel is taking energy from the water,

and the speed of the whole stream is slower: the water doesn't suddenly slow down when it hits the wheel!

3. Mar 10, 2012

### rcgldr

Voltage is elecrical potential (link to wiki article below). The voltage in any circuit drops to zero by the time it reaches the negative terminal of a batter. Even in the case of a wire with no resistor, the wire is then the resistance, although it's very low, resulting in high current and a lot of heat dissipated. With a resistor, almost all of the resistnace is in the resistor, although there is still some resistance in the wires, so voltage isn't quite dropped to zero by the resistor except in an idealized model or a supercondutor. Since the resistor increases the overall resistance compared to just a wire, the current flow is much less and the amount of heat dissipated is also less (less power consumed), and almost all the heat dissipation occurs at the resistor.

http://en.wikipedia.org/wiki/Voltage