## Electromagnetic Induction: Falling Magnets

Imagine dropping a bar magnet vertically through a short fat solenoid connected to a sensitive galvanometer. We drop the magnet from a long way away. As the magnet falls it accelerates, getting closer to the coils, thus inducing an increasing EMF across the ends. When the magnet is in the solenoid, the flux cut by the leading pole of the magnet is opposed by the flux cut by the lagging pole so the EMF will first fall to zero then begin to be generated in the opposite direction, becoming a maximum as the leading edge leaves the coil on the way down. The EMF will fall to zero as the magnet accelerates beyond the coil and the flux linkage weakens. Notice the area of the leading edge and lagging edges are the same – a consequence of energy conservation.

## Capacitors for IG

The simplest capacitor consists of two metal plates separated by an insulator or dielectric. We can pour + and – charges on to it by connecting the plates to opposite ends of a battery. We usually roll up the layers of insulator and conductor into a ‘swiss roll ‘ to save space.  Just like milk bottles store milk, capacitors are components that are used to store electrical charge and are used with a resistor in timer circuits.  When electrical power is supplied to a circuit that includes a capacitor – the capacitor charges up, in other words, a voltage develops across the plates. When power is turned off the capacitor discharges –  its electrical charge leaks away slowly through a resistor.

This is useful in timers, also smoothing circuits where a current is changing and we want to smooth out the bumps. (see below)

Take a look at  this link

When a switch opens the current tries to keep going by jumping across the gap creating a spark. (You can see this through the switch cover plate when you turn off a mains light switch).

Adding a resistor – capacitor network across the switch absorbs the energy of the spark. This protects the contacts from being worn away.

## Nobel Physics and the Fate of the Universe

The Nobel Prize for Physics 2011 was announced today. Three astrophysicists join a galaxy of numinous people, from Einstein to Heisenberg to Feynman for the big award in physics. They were looking at cosmic lighthouses, distant supernovae – the only objects bright enough to be observed at such vast distances – and they  have concluded that the energy they emit is too faint, or, fainter than it ought to be.

So, the Universe isn’t just expanding, but its rate of expansion is increasing. The radius of the ball that is the Universe isn’t just getting bigger at a steady rate, the rate itself is increasing. So, energy from distant objects is being smeared out over a wider and wider area hence the light arriving on every square metre of Earth is less intense.

What does this mean? It means that the inflationary force of expansion of the early Universe plus other forces due to dark matter are bigger than the gravitational attractions wanting to pull all the bits together.  It also means that Einstein’s fudge factor in 1917 to prevent the collapse of the Universe was probably right. It also implies that 70% or more of the Universe is dark matter energy which exerts repulsive (antigravity) forces. So, overall density is decreasing (ω<1) and the dark and the cold will surely come…

But, not yet.