Voltage is caused by having more electrons in one place than another. Remember, electrons tend to repel one-another, so places with many electrons will tend to push outwards towards places with fewer electrons, and electrons will naturally be transferred until the number of electrons is even everywhere. This flow of electrons from one place to another is called a current, and is what causes electricity.
It is true that in a DC circuit, there needs to be a complete circuit so that electrons can flow from the negative terminal to the positive terminal in the circuit, and jump back to the negative internally in the battery. If there were a break in the circuit (such as at a capacitor), the electrons would naturally flow from a place with more electrons (low voltage) to a place with less (high voltage) until they were evenly spread out, at which point the flow would stop. Thus, there can only be a current where there is a difference in voltage between two points and a path for electrons to flow.
This difference in voltage is sustained in DC circuits by constantly moving electrons from the positive terminal to the negative internally in the battery, so that there is always an excess at the negative. However, in AC the voltage changes from positive to negative continuously, so there is never any chance for electrons to swell up at one end or the other; the energy comes from electrons swaying back and forth. Thus, in AC circuits there doesn’t need to be a complete circuit.
Since voltage is relative, what happens if the voltage-wave from the power plant is always below (or above) 0-volts/ground? It’s true that this will cause current to flow in only one direction, but only for a moment; as electrons flow out of the power line and into the earth, the voltage around that point of earth drops. This continues until the average voltage between earth and the wire is 0, at which point the circuit operates normally.
Additional Reading:
- What’s the difference between “voltage at a point” and “voltage between two points?” Also, what is ‘ground?’
- (Un)Common Questions about Electricity
- How does jump starting a car work if current only flows when there’s a difference in voltage?
- Does electricity flow from positive (+) to negative (-) or from negative to positive?
- Do transformers obey Ohm’s Law?
2 Comments
Hi there,
In regards to this paragraph:
“the electrons would naturally flow from a place with more electrons (low voltage) to a place with less (high voltage) until they were evenly spread out, at which point the flow would stop”
Doesn’t the low voltage and high voltage need to be switched around? As I thought that more electrons = high voltage and less electrons = low voltage -> as current flows from high to low voltage
Thank you
Nope, what I wrote is correct. I covered the reason in another blog post.