See this article on Wikipedia, for explanation and history of world electrical systems.
http://en.wikipedia.org/wiki/Mains_electricity
Volts
In the US the voltage is about 220 volts. This voltage is based upon the average voltage of the AC sine wave. It can peak as high as 240 volts or as low as 210 volts. So these different voltages are actually referring to an average 220 volt system.
Europe's power voltage typically ranges from 220V to 240V. Different countries may have varying voltages within this range, so it is important to check the specific voltage requirement for the country you are visiting.
Power supply voltages vary with the amount of load placed on them. In many countries the supply voltage has a nominal voltage and a plus or minus tolerance. In the UK the nominal voltage is 230 v but the allowable variation is -6% +10%, in other words 216-253. At my house the voltage is usually between about 236 and 253 volts.
The voltage source that is applied to them is the difference between AC and DC light bulbs.
No.
Yes, most power banks are dual voltage and can be used in multiple countries with different electrical systems.
if you are traveling out of the country a voltage converter is a neccessity. Because a lot of other countries use electricity on a different voltage.
Generators generate and transformers step up or step down the voltage
voltage and frequency both are different quantity.. don't mix it...
Voltage Tarnsformer transforms only voltage. While power transformer transforms both voltage and current.
voltage = the electrical "pressure"current = the electrical "movement rate" or "flow rate"
Scroll down to related links and look at "AC voltage and frequencies in the world".
It varies in different countries round the world but most use 240 volt mains.
Volts
different countries
A short circuit is an abnormal connection between two nodes intended to be at different voltages. A voltage circuit is caused intentionally for the purpose of voltage sensing. A ground circuit occurs between a phase and the ground.