Magnetism and Magnetic Levitation Trains

Does the US have them ? How do they work ?What do they have to offer ?

1. Japan and Germany have demo versions. Vmax = 350 mph.2. There is some talk of maglev trains in Southern Calif,

Baltimore-DC, and in Virginia

What does a magnet do ? It can deflect a compass needle.

This can’t be explained by mechanical forces. Can electrostatic forcesexplain this ?• No, these forces are in the wrong direction and occur even if the compass needle, magnet are Neutral !!!

So…we must postulate a new type of force, with new type of ‘charges’ (which we call poles). Let’s call this force ‘Magnetism’.

N S

What’s in a Magnet ? What if you cut it up ?

N S

You get many magnets with N, S poles !Experimentally, no single magnetic poles have been seen!Just dipoles.

What is the source of magnetism ? What makes a magnet ?

This can pick up paper clips !? How ? It’s ‘magnetic’.

Current flowingthrough wires.

current loop with current i

N

SA bar magnet

current loop with current i

N

S

A bar magnet

What’s the difference between a current loop and a bar magnet ?

They can produce the same magnetic field !Magnetism is intimately related to flowing currents.

3 Methods of Magnetic Levitation

1. Magnets on Track and Train – Repulsion produces levitationbut equilibrium is UNSTABLE.

2. AC Levitation – AC Electromagnet on Train, on Metal Tracksproduces levitation but energy immediately decays away.

3. Dynamic Levitation – Magnet on Train, on Metal Tracksproduces least energy dissipation by allowingthe train to initially move fast enough.

See Class Discussion Notes on Electromagnetic Induction and Lenz’s Law