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OHP Circuit Board

What it shows:

This demo allows a lecturer to play around with various DC circuits on the overhead projector.

How it works:

A removable template of 26cm × 17cm plexiglass has a set of 6mm diameter tightly wound springs of length 1cm fixed at 5cm intervals (reminiscent of those Radio Shack® n1000-in-1 electronics kits). Standard resistors and 5cm lengths of 22AWG wire clip into these springs to form a circuit, and the template is then rested on a parent board consisting of two transparent meters (figure 1). These are...

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Bird on a High-Voltage Transmission Line

What it shows:

Why doesn't a bird sitting on a high-voltage wire get electrocuted? This demonstration addresses that question and serves as a model of the situation.

How it works:

The important concept conveyed is that there needs to be a voltage difference across a conducting medium for current to flow through the medium. In this situation the conducting medium is a bird sitting on a high-voltage wire. The voltage on the wire is the voltage of the whole length of wire with respect to the ground. Although the bird on the...

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Hand Battery

Copper and zinc plates connected by micro-ammeter; your hand completes the circuit.

Clean copper and zinc sheet metal stock is cut into strips or pads. Alligator clip a zinc strip and a copper strip and plug the leads into a modern multimeter. DC 2V scale should cover the 1.09V that we expect from a zinc/copper battery at standard conditions of 1 Molar electrolyte and room temperature.

Our fingers are a network of electrolytic conductors, with more or less conductance (depending on moisture and salt). Pressed to the metal strips, adjacent fingers complete the circuit and...

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Hair Raising

What it shows:

A Van de Graaff generator will apply a charge to its dome and anything else in contact with the dome. Should that object be a person, they obtain a net surplus of charge (be it positive or negative). It is especially noticeable with hair, as each individual strand is repelled from every other and from the scalp.

...

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Induced Electric Dipole

What it shows:

Bringing a charged rod close to neutral dielectric polarizes the dielectric's surface charges. Here a pile of Styrofoam puffs are polarized and attracted to a charged rod.

How it works:

The neutral puff experiences a non-uniform electric field from the rod. Although there are polarized charges of both kinds, because (figure 1) the field is stronger near the rod due to the concentration of positive charges, there is a net attraction. On a dry day they'll jump to meet the rod.

figure 1....

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Triboelectric Effects

What it shows:

As long ago as 600 B.C., the Greek philosopher Thales knew that amber, when rubbed, would attract bits of paper and other light objects. Many other substances have this same property and can be electrified by rubbing. The kind of electrification (positive or negative) depends on the substances used.

...

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Musical Bottle

A beer bottle becomes a Helmholtz resonator when air is blown across its mouth.

musical bottle

Siren Discs

What it shows:

Demonstrate musical intervals, the relation of pitch to frequency, and autocorrelation in psycho-acoustics.

How it works:

A 25 cm diameter metal disk has a number of concentric rows of regularly spaced holes. When rotated at a uniform speed while blowing air at a row of holes, a musical note is produced by the sequence of regular puffs of air issuing from successive holes. The frequency is determined by the speed of rotation and the known number of holes.

The numbers of holes in the successive rows are 24, 27...

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Sonometer

What it shows:

The effect of length, tension, diameter, and kind of material on the pitch of a vibrating string is demonstrated. One may also show the harmonics of a vibrating string.

How it works:

The sonometer is a long hollow wooden box along the top of which are stretched one or more strings rigidly attached to the box at one end, with provision at the other for changing their tension. If there is just one string, it's known as a monochord. The monochord illustration is from John Tyndall's book entitled Sound, (...

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Organ Pipes

Selection of single organ pipes, open and close-ended, to blow through.

organ pipes

Doppler Tuning Forks

Run towards the blackboard carrying a tuning fork...

What it shows:

Waves emitted from a moving source are Doppler shifted to higher frequencies when moving towards the observer, and shifted to lower frequencies when moving away from the observer. In this situation the source is moving away from you, but the raised frequency sound is reflected back interfering and causing beating.

How it works:

All you need is a tuning fork (say 896Hz, see comments), a reflective surface like a blackboard, and plenty of room to take a run at...

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Doppler Whirler

A high-pitched alarm on the end of a rope is whirled about the head.

What it shows:

Doppler shift of sound emitted by an object moving in a circular orbit, with the pitch clearly changing as the object move towards, away or perpendicular to the line of the observer. Useful as an analogy to the redshift and blueshift of spectral lines from a rotating astronomical source such as a planet or binary star system.

How it works:

We have a Powerhorn™ Security System buzzer attached to a 1.5m length of nylon cord. Swing it in a...

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