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Demos

Thompson’s Apparatus

This is probably the best experiment I have to demonstrate that electrons are particles.

Your students could draw a free-body force diagram to show the electric and magnetic forces on the electrons and perhaps even work through the algebra to find he charge to mass ratio themselves.

The final path taken by the electrons is not flat. This is because the electric field causes the electron to follow a parabolic path while the magnetic field causes it to follow a circular path over the same distance and the two together give the path as observed.

Electrons are particles

The camera man was a bit distracted by the spark on the high voltage generator and it would have been nice to see where the magnet on a stick was with respect to the beam of electrons. However this demonstration does show that the beam bent downwards when the magnet was behind the screen and upwards when the magnet was in front of the screen. So, reversing the direction of the magnetic field reversed the direction of the force on the electrons. Therefore the electrons interact with the magnetic field as though they are charged particles. The particle nature of matter!

Electrons Fired at a Maltese Cross

You need to view this video with the lights off as the glow on the tube is quite faint.

It was thought that this was a good demonstration of the particle-like nature of electrons because of the clear and crisp edges of the shadow. However the de Broglie wavelength of an electron is so small that even if there are diffraction effects they would not be easily seen with this equipment. Still in an historical context I think the demonstration is useful.

Van de Graaff Generator

I love playing with the Van deGraaf Generator. It feels a bit like magic. The only bit of the explanation missing from the video is why the tissue paper that jumps back and forth between the large aluminum dome and the smaller earthing dome, is initially attracted to the charged dome. The tissue paper should not have had any initial charge so there should not have been an initial force to make it move.

Charging By Induction

Trying to demonstrate charging by induction using a small conical flask at the front of the lecture theatre has never worked very well, even when using the camera on the video overhead projector so this video was created. If there was time it might be interesting to get students to create their own electroscopes. And embed them in an interactive artistic piece of work.

Resonance In A Rotating Tube

I must admit I am not sure how this works. In the video I propose that it is the speed of the air moving through the tube that changes the frequency but now I am not so sure. So let’s try and work it out step by step (logically).

What we know: The faster you spin the tube the higher the frequency it resonates at.

What we infer: The faster the tube spins the faster the air is flowing across the mouth of the spinning end of the tube. The end with the fast moving air will produce a zone of lower pressure compared to the more stationary end.

Resonance & Standing Waves

Standing waves on a string form from the sum of two waves, one moving down the string and the reflected wave coming back. Normally these two waves would pass along the string and the standing wave pattern would not be seen. The amplitude of the two waves would add up to give a total which would change over time as the two waves move in opposite directions. However the frequency of the speaker is the critical factor here. If it is providing pulses at just the right frequency standing waves will form.

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