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Electric current

This video clip was captured by the maintenance foreman of the 500 kV Eldorado Substation near Boulder City, Nevada. It shows a three-phase motorized air disconnect switcher attempting to open high voltage being supplied to a large three phase shunt line reactor. The line reactor is the huge gray transformer-like object behind the truck at the far right at the end of the clip. Line reactors are large iron core coils (inductors).




The flow of charge

Caution analogies

Electrons flow through metal like water flows through pipes. What do we know about analogies? If A is like B, then logically A is not B. They have some behaviors in common, but in others ways are different. (Otherwise A and B would not be analogous, but rather identical.)

For this chapter, we’ll examine simple aspects of electricity that can be explained using the water flow analogy.

There are cases where the analogy breaks down. At the end of this lesson we’ll begin to see why and how.

Electricity Water Analogy

Another image:

34.2 Electric current

So – water flows through pipes? Sure – pipes are hollow.  Yet metal wires are not hollow – they’re solid metal: where is “room” for the electrons to flow? Electrons don’t need room – atoms are mostly empty space. So electrons flow like this:

Another analogy.

In water pipes, water usually only flows in one direction. In electric circuits, there are two different systems: DC and AC.

AC – Alternating Current – This is what the flow of electrons looks like in AC.

DC – Direct Current – This is what the flow of electrons looks like.

Direct current


34.3 Voltage sources

Charges do not flow unless there is a “potential difference” – something that provides a potential difference is a voltage source.

Batteries and generators are voltage sources.

Distinguishing between current and voltage

34.4 Electric Resistance

Water flow analogy of DC electrical circuit

  • Dr. Richard Vawter, Dept. of Physics and Astronomy, Western Washington University.

A simple electrical circuit – consisting of a battery and a resistor – can be modeled by a pump to simulate a battery, and a paddle to simulate electrical resistance.

The vertical axis represents potential energy. This is reasonable since the gravitational potential energy, m·g·h , is proportional to the height h.

As the current turns the paddle it does work, and thus loses some energy – similar to electrical current flowing through a resistor.

Water Flow Analogy Simple circuit 1Water Flow Analogy Simple circuit 2

For two resistors in series:

Battery pumps charges up to a higher potential. Current then flows through the resistors, losing energy, returning to the starting level.

This starting level [is] the ground level.

Any time you connect a circuit to ground at some point, you are forcing the potential at that point to be the zero reference level;

[this is] similar to the zero reference level of gravitational potential.

The potential of any wire connecting the components are at the same potential anywhere along the wire.

This happens because the resistance of the wires is small compared to that of the resistors in the circuit.

Water Flow Analogy two resistors 1

Richard Vawter writes:

Two resistors in parallel have the same potential drop.
In this case both resistors are also in parallel with the battery so that potential drop across the resistors is same as the potential gain through the battery.

Amount of charge flowing through the two branches is not necessarily the same – unless both resistors have the same resistance.

More charge flows through the smaller resistor R2 – as water/charges take the path of least resistance.

If the two Rs are made of the same material, but have different values, then the drift velocity through the smaller resistor is larger…

Resistance is modeled by how hard it is to rotate the paddles.

The slope of the analog for the resistors (or the battery) is not significant because gravitational or electrical energy is path independent:

takes the same amount of work to lift a ball through a height h regardless of the angle of the slope.

Another analogy.

Water flow analogy complex 2 Water Flow Analogy complex 1

Richard Vawter writes:

R1 and R2 are in series with each other;

Their combination is in parallel with R3.

Current through R12 drops by the same voltage, as the current through R2, since they are in parallel.

There is less current flowing through the R12 branch since this branch has twice the resistance as R3 branch.

34.4 Analogy for resistance: blood-vessel-resistance

blood vessel resistance


Figure 3.5a The arteriole is completely open when the rings of muscle are relaxed.

Figure 3.5b When the rings of muscle contract the arteriole becomes narrower and less blood flows through.

Figure 3.5c Sometimes the rings of muscle can squeeze so tightly that very little blood flows through. This situation can occur if more blood is needed in one part of the body than another. This is how arterioles help direct the flow of blood


Our analogy here:

voltage = blood pressure (heart pumps blood, puts blood under pressure)

current = flow of blood

resistance = narrow blood vessels putting friction on the blood

What happens if the system loses water particles in one place, and they build up in another?

Can the same happen for electrons? Can charges be lost in one place, and then build up in another?  Yes – over time charges build up in one area of the world, and are lost in another.

cause of lightning electrical charges

But these charges strongly attract each other? So wouldn’t they eventually have enough pull to come back together? You betcha ->

Lightning BBC africa thunerstorm plasma

Extra: Why do water-flow analogies break down?

Water flows inside a pipe, but electricity doesn’t completely flow inside a wire.

Electric-magnetic fields flow outside wires, like this:

In a simple circuit, where does the energy flow? – William J. Beatty

Misconceptions spread by textbooks about Electricity: By William Beaty

Electricity apps Molecular expressions

Learning Standards

Massachusetts 2016 Science and Technology/Engineering (STE) Standards

HS-PS2-9(MA). Evaluate simple series and parallel circuits to predict changes to voltage, current, or resistance when simple changes are made to a circuit
HS-PS3-1. Use algebraic expressions and the principle of energy conservation to calculate the change in energy of one component of a system… Identify any transformations from one form of energy to another, including thermal, kinetic, gravitational, magnetic, or electrical energy. {voltage drops shown as an analogy to water pressure drops.}
HS-PS3-2. Develop and use a model to illustrate that energy at the macroscopic scale can be accounted for as either motions of particles and objects or energy stored in fields [e.g. electric fields.]
HS-PS3-3. Design and evaluate a device that works within given constraints to convert one form of energy into another form of energy.{e.g. chemical energy in battery used to create KE of electrons flowing in a circuit, used to create light and heat from a bulb, or charging a capacitor.}
HS-PS3-5. Develop and use a model of magnetic or electric fields to illustrate the forces and changes in energy between two magnetically or electrically charged objects changing relative position in a magnetic or electric field, respectively.

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