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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).

http://physicsfootnotes.com/500-kv-substation-arc/

https://www.youtube.com/watch?v=hIkNY5xjy5k

The flow of charge

## Voltage sources

Water  doesn’t flow by itself unless going downhill

Hills create a drop.

Charge doesn’t flow by itself unless going “down voltage”,
i.e. down a change in “potential difference”.

Batteries or generators create a potential difference.

## What creates resistance?

Let’s create an analogy (more intro TBD)

Image and text from Dr. Richard Vawter, Dept. of Physics and Astronomy, Western Washington University.

## Analogy for blood-vessel-resistance

http://www.ck12.org/book/Human-Biology-Circulation/section/4.1/

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

http://www.ck12.org/book/Human-Biology-Circulation/section/4.1/

## Where does the water-flow analogy break down?

### 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

## Know what a circuit schematic looks like

Here we see a circuit:
A wire was broken, and reconnected with an ammeter inside the wire – so the ammeter feels the current flowing through it.
We also see a voltmeter being used. Nothing needs to be broken. We just put one lead on side of a component, and the other lead on the other side. The voltmeter then tells us the potential difference (voltage drop) across the component.

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## 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.