h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

is it possible to power cars in a similar fashion to trams, at least in inner urban environments? i am thinking underground power similar to scalextric cars.

what are the pros and cons of this?

pros - reduction in emissions in the locality of the car
cons - infrastructure cost.
lack of mobility (how to plumb in for parking spaces,
junctions etc)
Possible safety issues through sheer amount of electrified
tracks and what jo public might do with them!
Emissions from powerstations to supply it
Cost of supply and how to apportion who pays what.

Not insurmountable but certainly problems.

Far better would be overhead grid like they used for bumper cars. This way you solve the mobility issue *and* it would be much more fun.

Could you perhaps do it by induction: having a coil underneath the roads?

not sure of the mechanics (electrics!) but if you are talking about a mechanism to induce an electric current without contact then you'd be talking a big magnetic field wouldn't you?

Which would have obvious problems from the magnetism but also it would be non-discriminate in its induction - leading presumably to lots of claims as iPods and so forth were burnt out.

The field would be highest when the car is drawing power from it, so most of the time (with no load) the field would be pretty small or non-existent.

They've done this with trains, though using a linear induction motor, basically one long coil stretching under the track. Difficult to see how this could be applied to cars that should be able to go anywhere. If you used a load of individual coils, the car would only be able to draw a little power from each as it passed over, then onto the next one. Not sure that would work.

interesting - how does that work? Can't quite get my head round how the field modulates itself based on how much current is being drawn by the motor? What's the feedback mechanism?

p.s. not doubting you, just interested as to how it is accomplished!

Hmm. Put simply, I guess, the current through an electrical device depends on its impedance (assume same voltage). Higher impedance, lower current; infinite impedance (open circuit), no current. Therefore current drawn depends on load.

AC circuits can incorporate devices with air gaps like capacitors or transformers that block DC but let a varying field on one side induce a current on the other. But induction works both ways; a low impedance on the 'load' side of the gap still appears as a low impedance on the 'supply' side. To AC, it appears to be a short circuit. Stick your toothbrush on its charger, and suddenly there's a low impedance coil on the load side instead of some air, and the current will flow. Without the toothbrush, very little current flows through the supply coil, therefore the field is very small.

The best way to visualise it is just to ignore the air gap I think, and pretend there's a piece of wire there. Does that make any sense