Help with understanding Current, Voltage and Resistance

Is my analogy correct:

With a highway as an example, would

• Resistance (ohms) be the number for lanes in one direction
• Current (amps) be the density of cars on the road
• Voltage (volt) be the speed limit of the road

What I find confusing is current and voltage. Current, if it can relate to density, could be a factor of voltage?

• This is a bad analogy. A good analogy is water flowing through a pipe between two potentials. Potential difference of two ends is like voltage , water speed is like current , and friction of the pipe is like resistance. – Standard Sandun Aug 1 '12 at 8:17
• @sandun dhammika What is two potentials? – Jason94 Aug 1 '12 at 8:42
• Imagine there's a water tank in a height. So we call that water have higher potential than water in ground level. – Standard Sandun Aug 1 '12 at 8:46
• @sandun - It's not because the water analogy is a common one that others would be bad. And it's not water speed that's the current, it's water throughput: if the flow goes from a thicker pipe to a thinner one the current will remain the same, but the speed won't. – stevenvh Aug 1 '12 at 10:44

Your analogy is quite good. It shows however a slight misconception about electron movement in conductors.

Electrons in conductors are not cruising along like cars on a road. It is more like bumper cars: they continuously accelerate, hit a obstacle, are stopped & deflected and begin again to accelerate. Also, the density of electrons is always the same.

A better picture would be this: Imagine a high-way with walls on the sides so cars cannot escape. The problem is, all over the road grows a forest with trees (i.e. atoms). The cars are pulled through the forest by bungee cords attached to the battery at the other end and continuously keep bumping into the trees.

1. Conductor: the highway with walls and a forest growing on the road. It is always filled to the brim with cars.
2. voltage: the tension in the bungee cord. It will not really make the cars travel through the forest faster as they still keep bumping into trees. It makes these collisions however a lot stronger.
3. resistance: the opposite of the width of the highway. The width itself would be conductivity.
4. current: the amount of cars going through the highway in a given time. Wider highway (i.e. less resistance) --> more cars.

That being said, there cannot be an exact analogy. Even mine has issues. How you best set up the analogy depends on what aspect of electricity you want to illustrate. It is unavoidable that other aspects are presented in a slightly skewed fashion.

• For #3 you could use the word “narrowness.” – mk12 Feb 17 '13 at 21:04

To check if it's a good analogy you use the relationship between the quantities as given by Ohm's Law:

$V = I \cdot R$

For a fixed resistance current is proportional to voltage. So for a given number of lanes the number of cars passing per hour should be proportional to the speed limit. That's OK: if you allow twice the speed then twice as many cars can pass per hour.

For a fixed voltage current and resistance are inversely proportional. So for a given speed limit the capacity should double when the number of lanes (resistance) halves. That's not true, but may be a question of definition. If we define resistance as 1/(number of lanes) it's OK again.

Finally, for a fixed current resistance is proportional to voltage. So for a given capacity, if we double the speed limit we'll need only half the number of lanes.

So it's a good model if you keep in mind that the resistance increases with decreasing number of lanes.

The water through a pipe in a slanting or vertical orientation seems to be a better analogy as compared to yours. By the virtue of having a slope or gradient, the pipe has two levels (Higher and Lower), so water or any thing would flow from a higher to a lower level.These levels can be thought of as the potential levels in a circuit.Due to difference in potential levels, there is a potential difference introduced which we better term as a voltage.So electrons would flow between two points only when there is a difference in potential between those two points just like water in the pipe. Next is current: it could be assumed as the flow of water through the pipe.More the number of water molecules flowing through the pipe more is the amount of water, similarly, more the number of electrons through the circuit or two points, greater is the current. Finally, resistance can be visualized as the friction offered by the pipe to the flow of water (as mentioned above by sandun dhammika). If the pipe has multiple blockages or a rough or coarse surface, then it would offer a greater friction to flow of water.Similarly if there is a greater resistance value associated with a circuit, a greater hindrance would be offered to the flow of current. I suppose this would sort the purpose well.

Most analogies fall down because of the notion of physical objects flowing along a path at various speeds. Electrons do not change speed! But the analogies are still useful in other ways. In my experience, you try a bunch of different ones and eventually discover one that makes the most sense to you.

• Someone will surely try to point out that the speed varies in different cables (velocity factor). But that does not affect ohm's law calculations. – gbarry Aug 1 '12 at 14:35
• Of course electrons change their speed. Their velocity is directly proportional to current (double the current and you double the speed.) But their velocity is actually determined by current density, and by the charge-density of that conductive material. Kewords: "drift velocity." Note that drift-velocity is a collective effect: the average velocity of a group of electrons. Analogy: the wind is a collective effect: the average velocity of a large group of air molecules. Individual velocity is not "current," not "wind," the individual velocity always high, even with no wind at all. – wbeaty Feb 2 '17 at 21:21

Here is another 'analogy'. It involves water, a firehose (or pipe), and a source of pressure.

The pressure pushing the water, gravity if a sloped pipe/water tower or a mechanical pump.

Pressure is voltage or potential difference (upper/lower end of the pipe/hose or height of the water tower).

The size of the fire (or, for that matter a garden) hose or of a pipe is comparable to resistance. Smaller hose-pipe is higher resistance. LARGER hose/pipe is lower-less resistance.

The volume of water able to travel through the hose/pipe is comparable to current. Picture / relate the volume of water as the ability to do work. i.e. fill troughs on a water wheel = weight turns the wheel or volume of water available to put out a fire. i.e. a garden hose (no matter the pressure) vs. a 2" fire hose. Electrical current is the capacity to do work, heater, motor, ......... and burn,shock, kill.

REMINDER, the greater the available current (with a sufficient voltage to overcome skin resistance) the more dangerous, but, once the voltage is high enough (70V +/-) to overcome skin resistance, less than one Amp, CAN KILL!

I suspect the vehicle analogy was intended to assist readers to visualize electron movement, but how electrons move, in my opinion, has little to do with current, voltage, and/or resistance.

• Do not sign your posts. If you want to disagree with other posts please downvote, dont put complaints in your answer unless you are going to explain and include references. I am not sure why you have put a safety warning with relation to current. I would say that something closer to 100mA current is probably the most dangerous until you get up into much higher currents that are more destructive, how does that apply here? – Kortuk Aug 4 '12 at 14:44