What kind of ranges exist for potentiometers?
Potentiometers with R = 10Ω have 0-10Ω range.

It's always from 0 to nominal R (+/- tolerance).

Typical potentiometer (and other basic electronic parts) values are defined in IEC 60063. There are few number series - E6 and E12 are very common. In E6 series there are 6 numbers: 10, 15, 22, 33, 47, 68 numbers. E12 series contains 12 numbers. For example - number 47 can be used for 4.7Ω, 47Ω, 470Ω values.

If you need value very close to preferred value - you may have to choose from E96 series (there are 96 numbers), but potentiometers with these values are manufactured in low volumes and it may be hard or impossible to find them on the market.

Closest value to 50Ω in E6 (and E12) series is 47Ω.

You can read more about electronic parts values here.

There are linear and logarithmic potentiometers. Linear potentiometers have resistance proportional to slider position (slider at 50% gives 50% resistance etc), logaritmic have logaritmic scale.



Potentiometer used to limit current directly is bad idea for motor speed regulation, because typical potentiometers are not designed for high currents.

Voltage drop on potentiometer at relatively high current will cause relatively high power loss. At 50% you will waste 50% of power on potentiometer and this power will produce a lot of heat. That heat will damage or completely destroy typical potentiometer.



There are potentiometers for higher currents, but they are much more expensive.

For motor speed regulation you should use PWM regulator or at least some regulator with transistor to "amplify current from potentiometer".