[DoctorBeaver (OP)]

No, not those little courses you have to take when you start a new job!  []

sophiecentaur's reply in another thread (http://www.thenakedscientists.com/forum/index.php?topic=13615.0) inspired this question.

I know inductors are used in electronic circuits and, I believe, hand-dryers use induction to "sense" when someone's hands are present. But what actually is induction? Is it a force in its own right or something to do with the em force? How does it work and what does it do? What do inductors do in a circuit?

[techmind]

Hand-dryers usually sense the reflection of an infra-red light source (LED - much like in a TV remote-control) to turn on the dryer. Moving on...

Inductors and induction are about creating a magnetic field from a current-flow. Any current-carrying wire generates a magnetic field, but when you make a coil of that wire you get a much more useful strength of field.
Energy can be stored (temporarily) or transmitted in a magnetic field.

Inductors are used in electronic circuits to smooth power-supplies and the like - they can "buffer" fluctuations in current-flow (and similarly they are used in filters to prevent unwanted radio-frequency energy escaping down the mains cable). You wire an inductor in series with a "load", and it passes DC and low-frequency currents, but attenuates high-frequency ones.

If an inductor is wired in parallel (or series) with a capacitor, you get a circuit which resonates (or strongly absorbs) at a specific frequency. These arrangements are useful in radio circuits. The energy in a tuned-circuit effectively "sloshes" back and forth between the magnetic field of the inductor, and energy stored in the capacitor.

Finally you can use induction to transport energy as a magnetic field, typically by placing a second coil near the first coil. By passing an AC current in the first, you generate a changing magnetic field. Changing magnetic fields in turn induce a current in nearby conductors, so you generate a current in the second coil. This is basically how transformers work. In the case of an "induction hob" for cooking, the AC magnetic field generated by the appliance induces very large currents in the base of the pan (known as Eddy currents, because they flow round in little circles, the base of the pan being a planar conductor), and the high current causes the pan to heat up.

[lyner]

If you have a conductor in a changing magnetic field, a voltage is 'induced' which makes a current flow. There is a sort of 'circularity' here; the current induces a magnetic field and the field induces a voltage and the resulting current causes a magnetic field etc. etc.
The induced current is in a direction which is 'such as to oppose its cause' i.e. it produces a force against what is causing it. It's a bit like the reaction force from a mass which you are trying to push against. This is why, when you wind a generator by hand, you have to push harder as you increase the electrical load and the generator delivers more current.

When an em wave hits the surface of a metal the changing fields 'induce' currents on (near) the surface. These currents produce (induce) a new wave and the energy is 'reflected' back off the surface. This is why metals reflect so well.
I guess the word 'induced' just means 'caused'.

[DoctorBeaver (OP)]

Techmind & sophiecentaur, thank you both very much. I understand it now.

Incidentally, I was told about induction in hand-dryers by an electronics engineer about 20 years ago. Maybe designs have changed since then.

[lyner]

I thought that hand dryers detected the change in Capacity as you bring your hand closer. There are many ways of killing a cat, though.

[techmind]

I thought that hand dryers detected the change in Capacity as you bring your hand closer. There are many ways of killing a cat, though.

In the case of "touch" switches (or very close proximity not-quite-touch) that is true. Some hand dryers do work that way, and it was more popular a few years back.
Many newer ones where you just hold your hands underneath them usually work with IR.

[lyner]

I looked under one today and couldn't see any likely 'hole' for an IR source. I will have to study one more closely but must try to avoid getting caught hanging around the Gents' too long.

[Soul Surfer]

When you pass an electrical current through a wire you actually create a magnetic field.  Coiling up the wire can increase the field by concentrating the flowing current in one place.  The creation of this magnetic field requires energy and the collapse of the field when the current is turned off releases energy.  The faster this happens the larger the peak of energy thats why if you make a coil of wire and connect it to a 1.5 volt battery you get a spark (and a little shock) when you suddenly disconnect the coil from the battery.  The "Back EMF" can rise to hundreds of volts when the battery is disconnected.  This is part of electromagnetic induction.  If you have another wire or coil close to the first coil, voltages and currents will also be induced in this by the creation and collapse of the magnetic field.  That is a very brief primer on induction.

[lyner]

The induced emf is proportional to the rate of change of the magnetic flux.  If you break the circuit quickly, the change is rapid - hence the high voltage spike.