## Electromotive Force (EMF)

In order for a current to exist, charge carriers must be motivated to move. Returning to our water tower analogy, gravity acting on the difference in height of the top water to the bottom, generates a potential force that can be exploited to move water and turn the water wheel:

This pressure at the valve is known as electromotive force (EMF) and is expressed as a difference between the top and bottom of the water path and represents a potential for work.

When water (or charge carriers) is flowing through the valve, the amount of force is expressed as voltage, in volts (`V`).

Another way to think about voltage is to imagine that a battery provides an engine to lift charge carriers up to a particular energy level. For example, a standard AA cell has the power to lift charge carriers up to `1.5V`:

### Relative Voltage

It’s important to note that this force is relative; even if the water towers were at different altitudes, the pressure at the valve is still dependent on the voltage drop from the top of the water to the ground.

This means that current flowing from a `10V` source to `5V`, the potential difference is still only `5V`. The amount of force differential between those two points would be the same as `5V` flowing to `0V`.

A minor, technical point, is that while EMF exists whether current is flowing or not, voltage only applies when a current exists.

### Common Source Voltages

Voltage amounts vary greatly around the world, and for different uses, consider the following sources:

Source Voltage
High power transmission lines `230kV` - `500kV`
North American Household Mains `~117V` (`110V` nominal)
European Household Mains `~220V` (`230V` nominal)
AA Battery `1.5V`
USB Power `5V`
Netduino Digital IO `3.3V`
Low-power Peripherals `1.8V`

For most of the circuits that we’ll explore here, voltages of `5V` and `3.3V` are the most common. However, many logic circuits control higher voltage circuits that power motors, heaters, and other loads that do heavy lifting and are typically powered by household mains of `110V` and `220V`.

## Next - Wattage

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