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Lorentz force

You all know about Force.

When you push someone you exert a force on them, that's exerting a force directly.

You can exert a force on an object in other ways.

The mass of the earth exerts a gravitational force on every object.

The earth's magnetic field exerts a force on a compass needle, making it point north.

When you comb your hair and it sticks to the comb, that's an electrostatic force.

There's another force that's in use all around you, but it's difficult to demonstrate. It's easy to demonstrate the force of gravity, just lift something up and let go!

First Cousins

Electricity and Magnetism are first cousins. They affect each other, can cause each other, and combine to create everything from light to microwave dinners to electric motors.



This schematic represents the railrun apparatus. It introduces a new component, the Capacitor:


Capacitors are a little like rechargeable batteries. They can absorb and store electrical energy for a while.

There are two important differences: Capacitors store much less energy than batteries, but they can store and return it MUCH more quickly.

The capacitors are wired in parallel. You may remember form last week's session with batteries that this means they share the load. In other words, the two capacitors can provide twice as much current than a single capacitor in its own.

When you press the CHARGE button the capacitors are connected to the battery through the resistor. They absorb energy from the battery and store it. The resistor slows down the rate at which they charge (not that the capacitors mind, but you could damage the battery!).

When you press the DISCHARGE button the capacitors are shorted out via the rails and the wire. The capacitors release their stored energy in a small fraction of a second, this means a large current flows through the wire!

The fancy version

When a charged particle moves through a magnetic field it experiences a force perpendicular to both the magnetic field and the motion of the particle.

The long version

An electric current is caused by electrons moving through a conductor. Electrons are tiny particles with a negative charge.

When it moves through the wire from right to left, with the magnetic field going up and down, it gets pushed forwards.

The short version

When I short the capacitor through the wire it'll move!