為什麼電阻值是V/I?

You may think the electricity as the water supply system.

Voltage = WATER PRESSURE
Resistance = THE STONE IN THE RIVER
Current = THE SPEED OF WATER FLOW

We can prove the equation with respect to the Current(I)

The Current (I) increases when the voltage is increased,just like the water flow faster if the water pressure is set to a very high level

Also

The Current (I) decreases when the resistance is increased,just like the water flow slower if there are to many stone in the river to block the water to flow easily

From Above

I is DIRECTLY PROPORTION TO V and
I is INVERSELY PROPORTION TO R.

we get I=V/R,and we get V=IR by changing the subjects

A voltage V sets up an electric field, and such field drives charges to flow, which is the current.

Just imagine that if the electric field is set up by a voltage V in free space, charges would be accelerated to infinitely high speed (in the classical sense). That is , an arbitrary voltage gwould ive rise to an infinately high current in free space. The ratio V/I is becomes zero.

Now, in a conductor, the flow of charges are inhibited by atoms present in the conductor ( the reason is due to collisiosn between charge carriers and atoms) . The charge flow. therefore, would slow down and reach a constant value. Such consntant flow of charge is the current. The ratio V/I now has a certain finite value.

In this sense, you would be able to visualize that for a given voltage V, the ratio V/I indicates the "difficulty" with which charges flow through a conductor. Such ratio is termed "resistance". In free space, charges flow freely without any inhibition, its "resistance" is thus zero. In a conductor, there is a finite value.

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