Making sense of Voltage Drop and Resistance

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theophilus88

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I don't know why this is bugging me so much, and I'm sure there's a simple analogy, but in terms of voltage drop, how does increasing a conductors size reduce the circuits resistance? Aren't we adding more resistance by increasing the size of the conductor? For Ohm's law, I've always used the analogy of water flow, pressure, and pipe size, but for voltage drop, this analogy doesn't seem to hold. Does anyone have a better explanation for how increasing a conductors size reduces voltage drop?
 
theophilus88 said:
Does anyone have a better explanation for how increasing a conductors size reduces voltage drop?
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When we say increasing the conductor size, we're talking about using a thicker wire, i.e., one having a greater cross-section area.

All conductors exhibit some resistance. The thicker or shorter the wire, the lower the resistance, and of course, the opposite is true.

If you picture a wire as a resistor, making the wire thicker is akin to placing more resistors in parallel, reducing the total resistance.
 
Forget the water-pipe analogies. They break down and yield bad results if you're examining it in any more detail than a middle-school science class.

Suppose we start with a piece of 14AWG copper THWN.
Think of it as 19 parallel resistors, each of which is 27AWG copper.

Now consider a piece of 8AWG, made of 37 strands of 27AWG, and calculate resistances in parallel.

If that's not helpful, look at conductance. (reciprocal of resistance)
 
IMHO, the water pipe analogy is perfectly fine and useful for this level of discussion, whereas the parallel resistor analog only works if you understand that already.

It's all about spreading a given load over different sized areas-

Voltage/pressure drop comes from resistance to flow, which both solid copper has for electricity and hollow pipes have for water (why? you're into physics here).
If there is no flow, there can't be any loss of pressure.
Once you have flow, there's "friction" to the movement and it takes energy to push past that friction- with water, it's the tendency of the liquid to cling to the pipe, with wire it's electrical resistance (physics again).
That energy is lost from what comes out the other end and appears as a loss of voltage/pressure, but only when there's flow.
When you expand the cross section of pipe or wire, there is now more area to carry the given water flow or electric current so that the flow per area becomes less and with less flow per area there is less energy loss per area and there is more area, so the overall loss is less (that's just math).

(Maybe that helps, this is the kind of topic that can lead to a 50 message thread with most writers in violent agreement.)
 
theophilus88 said:
I don't know why this is bugging me so much, and I'm sure there's a simple analogy, but in terms of voltage drop, how does increasing a conductors size reduce the circuits resistance? Aren't we adding more resistance by increasing the size of the conductor? For Ohm's law, I've always used the analogy of water flow, pressure, and pipe size, but for voltage drop, this analogy doesn't seem to hold. Does anyone have a better explanation for how increasing a conductors size reduces voltage drop?
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No.
Look at how the resistance drops as the cross sectional area is increased.

As far as your water analogy….
Pump water from a 500gpm pump at 120 PSI (volts) through a 1/4” pipe(we’ll call that 12 gauge) How much water will you get in a minute? It won’t be 500gpm because of the voltage(psi) constraints and the resistance of the 1/4 pipe..

Now pump it through a 1” pipe…



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The pipe analogy is a simple and useful way to visualize the flow of electricity. The larger the wire (the less resistance in ohms), the easier it is for electricity to flow.
 
Oh ya the ohm guy, my boss.
The volt guy my dad.
Then there me getting the boot.
The expression expression on there face sums it up.
 
How does the water analogy "not hold up" here? The bigger the pipe the more water flow, at a given pressure.
 
Electricity doesn't exhibit turbulent & laminar flow (and variable resistance) with changing "velocity", elbows have no measurable effect, (at 50-60 Hz) and skin effect isn't at all analogous to pipe-wall friction.

But more relevant than that, my experience has been that using the water-pipe analogy leads to less insight & understanding than using the physics of conduction.
 
drcampbell said:
Electricity doesn't exhibit turbulent & laminar flow (and variable resistance) with changing "velocity", elbows have no measurable effect, (at 50-60 Hz) and skin effect isn't at all analogous to pipe-wall friction.

But more relevant than that, my experience has been that using the water-pipe analogy leads to less insight & understanding than using the physics of conduction.
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What you have stated is true.
However, I train linemen..
Sometimes physics need to be left out and just use the KISS principle.
 
imagine running down a crowded hallway, the narrower the hallway the more people you are going to run into, the wider the hallway the more room to move. every time you run into someone it creates heat, your forward motion is stopped and turned into heat. electrons moving striking electrons changes movement into heat. energy is not lost, it is just changed from one form to another, basically movement to heat. (sorry for the simplistic explanation, hope it helps)
 
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