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Eddy Currents at LN2 Temperature

What it shows:

A rectangular block of copper (measuring 6"×6"×2"), offers VERY little resistance to eddy currents generated by dragging a magnet across its surface. Thus the Lorentz force between the eddy currents and magnetic field is quite strong and you can feel a sizable drag force. Dropping a magnet onto the surface likewise produces a sizable Lorentz force, as evidenced by the damped motion of the magnet's fall. The effects are quite dramatic at liquid nitrogen temperature.

How it works:

Copper has a positive temperature...

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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 Levitation

What it shows:

It's impossible to magnetically levitate an object with static magnetic fields. However, it's posible to levitate a magnet with another hand-held magnet by taking advantage of eddy currents.

How it works:

A rectangular block of copper (6"×6"×2") is stacked on top of another one (6"×6"×1"). They are separated by 1" plastic spacers. A rectangular bar magnet (2"×2"×½") is placed in the space between them. When a second magnet is lowered from above, the two magnets attact each other. However, rather than "jumping up"...

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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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Hand Cranked AC Generator

Observe the induced current in a gimbaled coil as it rotates in Earth's magnetic field.

What it Shows

A changing magnetic flux through a circular coil of wire induces a current in the wire. By spinning a circular coil of wire at constant frequency and measuring the induced voltage across its ends we can find the local direction and magnitude of the Earth's magnetic field as it passes through the coil. The commutators of the coil are configured to produce an alternating current.

...

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Oersted's Experiment

What it shows:

Oersted showed that an electric current produces a magnetic field. His experiment is repeated here on a suitable grand scale.

Oersted's Experiment

How it works:

The current carrying wire in this case is a tubular 22mm diameter copper...

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Jumping Wire

What it shows:

A current carrying wire in a magnetic field experiences a force at right angles to both the field and current directions. The wire will jump up or down, depending upon the current direction.

How it works:

On a microscopic scale, the electrons in the wire experience a Lorentz force due to the magnetic field,



the force perpendicular to both field and velocity vector. On the...

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RC Time Constant

Charging and discharging of a 10µF capacitor with variable time constant.

What it shows:

The growth and decay of current in an RC circuit with a time constant chosen so that the charge and discharge is visible in real time.

How it works:

By choosing the values of resistance and capacitance, a time constant can be selected with a value in seconds. The time constant τ is given by

τ = RC

To obtain useful values, we chose three resistors 100K, 200K and 400K in series with a 10µF capacitor, giving time constants of...

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RL Time Constant

What it shows:

The growth and decay of current in an RL circuit with a time constant visible in real time.

How it works:

By choosing the values of resistance and inductance, a time constant can be selected with a value in seconds. The time constant τ is given by

τ = L/R

We chose two resistance values, 4.7K and 10K coupled with a 45kH UNILAB 1 induction coil giving time constants of 9.5sec and 4.5sec respectively.

The circuit is set out on a 1.0 × 0.5m plywood board. The actual...

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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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Giant Capacitor

What it shows:

The basic principles of the parallel plate capacitor made large.

How it works:

The capacitance C of a simple parallel plate capacitor is given by


the ratio of the magnitude of the charge Q on either conductor to the potential difference between the...

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