Presentations

Beats

Two tuning forks with similar frequencies; one fork is variable in frequency to tune beating.

What it shows:

The interference of waves from two tuning forks of slightly differing frequencies gives rise to beating, that is, a modulated wave of frequency.

ν_b = (ν₁ - ν₂)

How it works:

Using two tuning forks of 256Hz, with one of the pair having small clamps (see figure 1) attached to the fork's limbs. These alter the fork's resonant frequency, and adjustment of the clamp...

What it shows:

Pure water is an electrical insulator. But provide an ionic compound in the form of salt, and you complete the circuit.

How it works:

A simple circuit with the mains supply connected to a 15W light bulb and two copper sheet electrodes (figure 1). The electrodes are placed in a 1500ml beaker containing distilled water. Distilled water is a very good insulator, with an autoionisation of 1:10^-7 (the proportion of molecules in H₃O⁺ + OH^- form) it has a resistance of...

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 = -L^dI/_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...

What It Shows

A large magnet with a small cylindrical gap allows a stream of liquid nitrogen to pass over and through. Poured liquid oxygen hangs between the poles in the strong field until it boils away.

What it Shows:

Detection of infra-red radiation by the rotation of a paddle-wheel vane inside a low pressure flask.

How it Works:

A radiometer consists of a partially evacuated flask containing a four bladed vane (see figure 1). One side of each blade is a matt black, the other silver. The black surface, being a better absorber and radiator of heat, warms the air above its surface more than the silver. The resulting higher kinetic energy of these air...

What it shows:

A beam of white light incident on a giant raindrop (simulated by a water-filled round flask) produces a full rainbow of colors. As with real rainbows, one can also see that the light intensity inside the rainbow is much greater than outside the rainbow.

How it works:

A Florence (round-bottomed) flask is completely filled with water and sealed with a rubber stopper. A Beseler slide projector ¹ serves as the sunlight. The light incident on the giant raindrop is refracted, reflected, and refracted once more, back in the direction of the...

What it Shows

Black body radiators in thermal equilibrium should emit the same spectrum of radiation; inside a muffle furnace at high temperature, objects should appear the same color, whatever their material.

...

Periodic Potential

What it shows:

Ball bearings simulate atoms in a lattice sitting at local potential minimums. Giving them energy excites the atoms and they oscillate about their equilibrium positions in these wells; only with large amounts of energy can they be truly dislocated.

How it works:

A piece of wood 100 × 25 × 2cm acts as the ‘potential’ structure of the lattice. The atoms, 3cm diameter ball bearings sit at the bottom of a cosine varying potential cut to about 10cm depth in the wood by a jig saw.The balls are held in the 2-dimensional...

Harvard Natural Sciences Lecture Demonstrations

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