## Thursday, January 19, 2012

### Flux capacitor

Back to the future is a wonderful sci-fi film that is funny and light-hearted. I have seen the movie many times and I always chuckled when they mention the term "Flux Capacitor".  It sounded like someone took two random technical words and mashed them together.  In the movie, an elderly scientist tells the protagonist that the Flux Capacitor is what makes time travel possible.  From high school physics, we know that a capacitor is an electronic device used to store energy, and it is used in almost every electronic device, whether they are analog or digital.  Examples where capacitors are useful include audio filters, power line regulators, and dynamic memory (DRAM).  The basic design of a capacitor consists of two plates with a dielectric in between.  Applying a potential difference on the plates create an electric field between the plates where electrical energy is stored.  The strength of the electric field E is the potential difference V across the plates divided by the distance d between the plate. The electric flux Φ in the capacitor is the strength of the electric field E multiplied by the area A of the plate.  For a fixed voltage drop V across the plates,  Φ = VC/ε is proportional to the capacitance C.  Thus one can argue that the normal capacitor that we all know is in fact a flux capacitor!

In recent years, there have been advances in creating ultracapacitors or supercapacitors, i.e. capacitors that can store a large amount of energy. Some of these capacitors that are available today can have an energy density of up to 30Wh/kg.  Imagine that we have 10 kg of such capacitors. If we can fully charge them and then discharge them within 0.89 milliseconds, we would have an average power surge of 300Wh /0.89x10-3s = 1.21348 Gigawatts which should be sufficient to activate the time machine!