Let’s start with practical application and leave scientific law out of it for a moment.
For clarity,
Voltage is a force (ElectroMotive Force EMF)
Current is a volume (electrons - electric charges)
Power is work performed.
If you push very hard on a pallet of bricks, you could move it. Lots of power.
If you push lightly on a single brick, you could move it. A little power.
Cute Lightbulb by ColiNOOB at Pixabay.com
Power
In a practical application, it is all about power. We can talk about Ohm’s law (and I will,) but look at power.
Power= Watts = Voltage x Current = Volts x Amps.
Example: A 1 horsepower motor. 1 horsepower = 746 watts.
At 100 volts, the motor draws 7.46 amps. 100 x 7.46 = 746 Watts = 1 horsepower.
Increase the voltage to 200 volts and the current required to produce 1 horsepower is 3.73 amps. 200V x 3.73A = 746W
At 50 volts the current draw increases to 14.93 amps. 50 x 14.93 = 746 watts.
No matter what you do, that 1 horsepower motor needs 746 watts of power. The current in amps required to do 1 horsepower of work is inversely proportional to the voltage or the electromotive force behind the current.
Example: A 150,000-Volt transmission line carries current to a city.
150,000 volts was chosen as an efficient level to carry the power required by the community. A lower voltage would require a higher current in the transmission circuit, which means the cable that carries the current would be thicker, because the lower voltage must still meet the power requirements of the city. There is a limit to cable size due to weight and expense and other factors. A cable 1 foot in diameter is not practical.
We fall back to our equation. Power (watts) = Volts x Amps.
If we increase voltage, the current drops to meet the same power requirement.
At a substation, we raise or lower the voltage with transformers to meet the voltage and current requirements of the transmission or distribution line and the end user of the current.
With these two examples we can see how the power needs of a single device don’t change, but also how power can be delivered with significantly different values of volts and amps and still deliver the same amount of power. Raise the voltage, the current need drops. Lower the voltage and the current need increases.
If we got 20 people (our force) to push and pull the pallet (high volume) of bricks, we would get a lot of work done. Or a someone not very strong (less force) could push a single brick (low volume) and not do a lot of work. We don’t need a lot of power to move a brick, but we do need a lot of power to move a whole pallet of bricks.
The same is true with electricity.
Scientific Law (edited for more clarity and simplicity)
Bear with me because we’re going to look at Ohm’s law for a moment.
Ohm’s law states that current in amps (I) is equal to the voltage in volts (V) divided by the resistance in Ohms (Ω) or I = V / R.
We can rearrange that to say V=I x R or R = V / I.
Ohm’s Law is empirical which means it was discovered by observation rather than mathematical theory. In other words, a guy named Georg Ohm produced a voltage and measured the current across a resistance (actually more complicated, but that is the gist of it.) He had a “Holy Smokes!” moment (or Heiliger Strohsack! since he was German) and came to the conclusion that voltage, current, and resistance were related in the equation we all know as Ohm’s Law (again, it was more complicated and this is simplified.) The results of numerous experiments in 1825 and 1826 were published in his book in 1827.
Georg Ohm (public domain image)
A resistance “resists” the flow of current. The higher the resistance, the less current flows. For R=10Ω and V=100 Volts, Ohms law states I(amps) = 100V / 10Ω = 10 Amperes. However, if you need more current to flow across that resistor, you can increase the force pushing those electric charges and as a result get more current to flow.
Consider a simple circuit with 1 resistor and a voltage source.
Image by Waveguide2 via Wikipedia
For this example, we will say that our voltage (V) can be turned up or down, and that the resistor (R) is fixed at 100 ohms.
For V = 100 Volts and R = 100 Ohms, then I = 100V / 100Ω = 1 Ampere.
For V = 1000 Volts and R = 100 Ohms, then I = 1000V / 100Ω = 10 Amperes.
Notice how the current went UP with the voltage. Use the same resistor and different voltages, and the current will change with it voltage. Higher voltage will equal higher current. That’s just simple math in an empirical equation.
Application
Ohm’s law is useful for determining the current through a circuit or figuring out which resistor to use to cause a certain voltage drop in a more complicated circuit or how much voltage to use to make the desired level of current flow.
Ohm’s law won’t suffice to address the question - Why does voltage increase when the current decreases? because by Ohm’s Law, with a fixed resistance, voltage increases or decreases with a similar change in current.
V=I x R.
If R = 100 Ohms, and I = 10 Amps, then V = 100Ω x 10A = 1000 Volts.
for I = 5 Amps, then V = 100Ω x 5A = 500 Volts. (Current decreased, so did voltage)
for I = 1 Amp, then V = 100Ω x 1A = 100 Volts.
In the Power Equation, more voltage means less current is required to the do same amount of work (power used). Power in watts = Volts x Amps.