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OHP RLC Circuit

What It Shows

The current in a circuit consisting of a capacitor, inductor, and resistor will oscillate back and forth as the capacitor charges and discharges.

How It Works

The circuit layout is shown in the figure below. Initially the knife switch links the capacitor to the battery. Switching to complete the LRC circuit allows the capacitor to discharge. The current I in the circuit increases, as does the magneic field B inside the inductor. When the capacitor charge is zero, I and B are a maximum (the energy of the circuit is now stored in the inductor). As the...

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Spectrum Piano

The visible part of the electromagnetic spectrum is represented by less than an octave of the keys; UV, IR, and microwaves are also indicated.

What it shows:

The keys of a piano are used to represent the electromagnetic spectrum, illustrating the narrow range of frequencies that constitute the portion visible to human sight.

How it works:

An old piano 1 with its center octave of keys (C4=261.6Hz to C5=523.3Hz) colored for the visible spectrum (the seven colors spread to...

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Electromagnetic Spear

What it shows:

Static 3-D stylized model of an electromagnetic wave, with two sets of sinusoidal fins at 90° representing the E and B fields.

How it works:

The wave packet model consists of a wooden spine with E and B fins of 1cm wooden dowels. A plastic arrowhead gives the spine a direction.

Figure 1. The Spear

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Skin Depth

What it shows:

The depth to which electromagnetic radiation can penetrate a conducting surface decreases as the conductivity and the oscillation frequency increase. This demo compares the skin depth of AM and FM radio frequencies, and shows just how small these distances are.

How it works:

An electromagnetic wave entering a conducting surface is damped and reduces in amplitude by a factor 1/e in a distance ∂ given by 1

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Magnetic Levitation

What it shows:

A magnet tethered over a spinning aluminum disc levitates due to induced currents in the disc.

How it works:

As the disc spins, electrical currents are induced in the aluminum as it moves with respect to the magnet. These induced currents create a magnetic field which, in accordance with Lenz's law, opposes the field of the magnet. The magnetic repulsion causes the rider to levitate about 1cm above the disc. Lenz's law also says that the induced field will oppose the motion that causes it. The magnet therefore tugs...

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Eddy Current Damping

What it shows:

A sheet of aluminum falls slowly between the poles of a magnet because induced currents in the sheet set up magnetic fields which oppose the motion.

How it works:

As the aluminum sheet falls between the poles of the magnet, eddy currents are induced in the metal. These currents set up their own magnetic fields, which through Lenz's law oppose the change that caused them. As the cause is gravity pulling the sheet to Earth, the sheet decelerates as it passes between the poles of the magnet, only to accelerate again...

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Ring Flinger Lenz's Law

What it shows:

A changing magnetic flux induces a current in a metal ring; the magnetic field due to this current opposes the primary field, repelling the ring and flinging it into the air. That's the simple "hand waving" explanation for the beginner student—a more accurate explanation follows.

How it really works:

The jumping ring is a vivid and popular demonstration of electromagnetic induction and is used to illustrate Faraday's and Lenz's laws. A conducting ring, placed over the ferromagnetic core of a solenoid, may levitate or...

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Back EMF

What it shows:

A sudden change in current in an inductor - resistor circuit produces a very large back EMF. If that resistance is a bulb, it will shine much brighter during the change than during DC flow.

E = -LdI/dt

How it works:

The circuit consists of a 6V bulb connected in parallel with a 10.5mH inductor coil as in figure 1. With the battery connected, the bulb burns at its rated 6V. Disconnecting the battery sends the applied voltage and hence the current to zero. The rapidly collapsing...

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OHP Magnetic Lines of Force

What it shows:

The magnetic field lines of the Earth can be represented by the field lines of a bar magnet.

How it works:

The Earth's magnetic field is basically a magnetic dipole. It can therefore be represented to first approximation by the field of a bar magnet. The shape of the field lines can be highlighted by the sprinkling of iron filings, or by the use of plotting compasses. The latter method has the advantage of showing the variation of dip angle with latitude, with the lines of force running parallel to the surface of...

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DC Circuit Puzzlers to Ponder

The circuits are built into large wood boards that mount easily onto an easel. The boards are labeled using conventional symbolism. Bulbs and batteries are easily removable--just be careful not to burn your fingers on the hot bulbs!

Parallel vs Series Circuit

Which bulb will be brightest in each circuit?

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