SFUSD 2005

Explorations in Electrostatics, magnetism, and Light

Part 1

Electrostatics

Here's an exploration that is so much fun it can even partially cure "senioritis."

Rub a PVC rod with wool and it becomes negatively charged.

Rub a small piece of plastic grocery store bag with wool and it becomes negatively charged.

The repulsive force between the PVC and the plastic can be used to levitate the plastic against gravity.

Flying Hydra

Scotch Magic Tape can be used to determine the sign of an electric charge.

Put two piece of tape together with the sticky side of one attached to the smooth side of the other, pull them apart and the tape that had its sticky side against the smooth side will become positive and the other negative. Bring these tapes near an object with an unknown charge, look for attraction and repulsion to determine the unknown charge.

If both tapes attract an object with an unknown charge then that object is uncharged. Charged objects attract uncharged objects.

Tape Electroscope

You can rub a styrofoam board with wool and use an aluminum pie pan with a styrofoam cup handle to create high voltages. The high voltage can be used to make a spark to your finger. You can feel, see and hear the spark. A true example of hands-on electricity.

Electrophorus

Magnetism

Disk magnets can be used to show that magnetic fields are needed to explain some motions of magnets.

At first the only rules needed to understand magnets were that like magnetic poles repelled, unlike poles attracted and that both types of poles attracted iron objects.

As experiments became more sophisticated it became necessary to include a rule that the force became weaker as the distance between the poles increased.

Here is a simple exploration that can be explained by the forces between magnetic poles

Magnetic Atmosphere Model

Then it became necessary to create the idea of magnetic fields. The magnetic pole created a field that exerted forces on other magnetic poles.

Here is a simple exploration that requires magnetic fields to explain its results. Oh, it's also fun!

Magnetic Oscillators

Light

Light is a wave in the electromagnetic field.

Electric fields spread out from electric charges, magnetic fields spread out from magnetic poles.

If you accelerate an electric charge, you create a transverse wave in the electric field. The changing electric field in this wave creates a changing magnetic field resulting in an electromagnetic wave.

Pull apart two pieces of scotch magic tape, which have been stuck together with the sticky side of one against the smooth side of the other, one tape becomes positively charged and the other negatively charged. wave the positive tape back and forth and the negative tape held nearby will also wiggle back and forth. By wiggling the positive tape back and forth once per second you are creating a 1 hertz electromagnetic wave that then moves the negatively charged tape.

Tape Electromagnetic wave

At some frequencies near 10^15 hertz this electromagnetic wave becomes visible to humans and is called light.

Part 2 More Light 21 April 2005

Polarized light

Since light is a transverse wave the electric field must oscillate perpendicular to the direction of motion of the wave. This direction of oscillation is described as the direction of polarization of the light.

Light polarization can be modeled as waves on a string or a spring.

Model of polarization of light.

Light also is seen to have different colors when detected by a human eye. The color perceived is related to the wavelength or mixture of wavelengths of the light.

Spectrum of Light

Diffraction gratings can be used to separate light into its component wavelengths.

Various light sources can be examined with a diffraction grating.

Spectra Exploration

A compact Disk contains a metal surface with a spiral track of pits embossed into it. The tracks are so close together they can diffract light.

Here are some explorations using compact disks as diffraction gratings.

View a spectrum in a Compact Disk

Compact Disk Spectra viewing intro

Project a Rainbow with a compact disk

Build a spectrometer with a compact disk

Commercial diffraction gratings are now available at relatively low cost they can be used to project a spectrum of light using an overhead projector.

Project a Spectrum

 Scientific Explorations with Paul Doherty © 2005 15 April 2005