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Wire-resistance

by Edmund Hazzard (Submitted: 09/08/2008)

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WHAT IS IT?

This is a model of electrical resistance in a wire. It is intended to help one visualize how electric current heats a wire when moving electrons collide with atoms. It was developed for 5-8 graders.

This model is used by the Concord Consortium in the UDL (Universal Design for Learning) project. To see the model in the context of an activity, go to http://udl.concord.org. Find the Intermediate Electricity Unit (Grades 56) and go to the activity called "Heat a wire".

HOW IT WORKS

The wire is composed of electrons (blue dots) and atoms (larger white dots). If voltage is applied, the electrons are accelerated to the left or right, depending on the polarity of the voltage.

When an electron is on the same patch as an atom, it "collides", which in this model means that it transfers a portion of its energy to the atom and heads off in a random direction. Each atom can accumulate energy in this way, balanced by a constant cooling rate. The atom becomes redder as its energy or temperature goes up.

HOW TO USE IT

The 'voltage' slider can apply voltage in either direction.

The 'collision-chance' slider changes the chance that an electron will "collide" with an atom if they are on the same patch. If collision-chance is 0, the electrons go right by the atoms without interacting.

The 'watch one electron' button allows one to follow the path of an individual electron.

This is a wrap-around model. Electrons that leave one end of the wire show up on the other end, going in the same direction.

THINGS TO NOTICE

The graph plots average behavior of the electrons and atoms.

Atom temperature is the average temperature (energy) of the atoms. The units have no meaning.
Current is the average x-velocity of the electrons. Note that it can be positive or negative. Note also that if the electrons are all moving randomly, the current will be zero.

THINGS TO TRY

Run the model and change the voltage slider. Notice the effect on the motion of the electrons. Also notice the effect on their average behavior in the graph.

See if you can find a relationship between the voltage, the current, and the temperature of the atoms.

Run the model, apply a set voltage, and change the collision-chance slider. If the value is zero, what happens? This would be equivalent to the wire having no resistance at all.

EXTENDING THE MODEL

The cooling rate of the atoms could be changed into a slider. This would be like a hot wire that you blow on to cool it, which is one way to make a measuring device for wind speed.

It has not been confirmed that this model follows Ohm's Law. See if it does. See if the exact rule for collisions has an effect on this relationship.

NETLOGO FEATURES

Note that the x-axis is displayed by plotting x=0.

CREDITS AND REFERENCES

This model was written by Ed Hazzard at the Concord Consortium (www.concord.org) for the Universal Design for Learning (UDL) Project, in March 2008.

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