# How does a current do useful work?

1. Aug 9, 2006

### pivoxa15

So many things are electrical and (all of them?) involve a current to do useful work such as power a computer or digital watch. But how does moving electrons do useful work in devices? What is the underlying mechanisms or basic principle that allow moving electrons to power so many amazing devices?

2. Aug 9, 2006

### Andrew Mason

Electrons do work when they move through a potential difference. That is the principle. How that energy is harnessed is what differs between devices.

If electrons move through a potential difference, creating a current which in turn creates a magnetic field and that field interacts with another magnetic field to produce a force, and that force causes a motor armature to turn, the electrons produce useful mechanical work.

If electrons are passed through a resistance and heat it up, they can produce light or heat - also useful work.

It is very similar to gravity: When a mass falls through a gravitational potential difference, it does work. How you harness that energy is what differs: catapult, hydro-electricity, pendulum clock, etc.

AM

3. Aug 9, 2006

### rbj

one thing to give you a feeling for why we can do such amazing and fast operations with electronics, is that electrons (and similarly charged particles, like "holes") have a very, very high "charge to mass ratio". now since you cannot compare charge to mass directly (it's like "comparing apples to oranges"), there are these sorta universal units called Planck units (that are defined so that the constants that are properties of free space just go away) and in Planck units, the charge on an electron is roughly about 1/11 of a unit charge but the mass of an electron is about 20 sextillion times less than the unit mass.

so there isn't hardly any mass to resist pushing them around, but there is sufficient charge to push these electrons around real good. so, mechanically, electrons can be shoved around at very high speeds and accelerations very easily.

now, if you leave radio out if it, virtually all of the models we have that are used in electronics can be replaced with a hydrodynamic model with water instead of electrons, pipes instead of wires, valves instead of transistors or vacuum tubes, and some other analogs. but this hydrological computer would be horribly large and damn slow. the reason for this is because water is much, much harder to shove around than electrons.

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