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In this quiz we will consider a series of electrical devices in everyday use which share the common feature of some kind of remote interaction, usually through the magnetic field. This means that different parts of the working apparatus do not have to be in electrical or mechanical contact with each other, but can act upon each other at a distance (though in most cases these distances are short). In every case there will be the same 4 possible answers, the device might operate by Magnetic Induction, by Capacitance, by Electromagnetic Waves or by non-Electromagnetic means (this last is basically a none-of-the-above answer). In considering which answer is correct in a given case, take note of your experience in using the device. Does it require movement of some kind to make it work? If so, it might be making use of magnetic induction. Can it work at quite a considerable range? If so, it may be an electromagnetic wave.
ATM machine with Bank Card • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – A Traditional ATM cards have a strip of magnetic tape on the back, whose magnetic field is read by the ATM by measuring tiny currents induced in the card reader circuits of the machine as the card is fed through the slot in the reading device. The movement of the magnetic strip means that the magnetic field through the circuits is changing. This is why your card will sometimes not be read properly if you push the card in too slowly. It simply doesn’t move fast enough to induce a detectable current.
H-ITT devices • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – C The H-ITT devices which we use for this quiz are obviously quite long range, because some of you are sitting 10 meters away from the receivers. This would generally be too long range for any induction or capacitance effect. It is no surprise to find that H-ITT devices use electromagnetic waves to communicate across the classroom, specifically infra-red waves. Infra-red waves are also used in television remotes. Such high-frequency, short-wavelength waves (compared to radio waves) are useful for “line-of-sight” applications of this kind, but they do not diffract around corners very well. For this reason you have to take care to point your H-ITT remote right at one of the receivers on the wall.
Toaster • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – C A Toaster obviously heats bread at a distance, as it seems clear that the heat to sear the bread is not coming through the metal of the toaster in a form of heat conduction. But might it not still be a non-electromagnetic process? Specifically could it be due to convection of heat through the air, as in an oven? Note that the toaster is not enclosed, as an oven is. There is little time for the air around the toast to become hot, and if it did the heat would dissipate around the room pretty quickly. What is happening is that infra-red radiation is emitted by the toaster filaments and this heats the bread.
Stud Finder • A stud finder is not a dating service aimed at ladies, but a device which will (supposedly, I’ve never had much luck with them) find out where your wooden studs or supports are behind the dry walls in your house. How is this achieved? • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – B One has to move the stud finder along the wall, so it is tempting to think in terms of magnetic induction. But we know that the wooden struts are not good conductors, nor are they responive to magnetic fields. Interestingly the stud finder is simply an unusual form of capacitor. Instead of parallel plates facing each other the stud finder has two plates side by side. The electric field looks as illustrated below. Recall that the capacitance of a capacitor depends on the electrical permittivity of the material between the plates. Wood has a different permittivity than air does, so if there is a stud present behind the dry wall, the device may be able to sense this change in capacitance and identify the location of the stud. Electric Field Lines Stud Dry Wall Plate 1 Plate 2
Video Cassette Tape • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – A The tape in a video cassette is essentially just like the magnetic tape on the back of your ATM card. As it moves past the heads or magnetic readers in the VCR the changing magnetic field induced by the tiny little magnetic particles embedded in the tape induces currents in the electric circuits inside the heads. The specific orientations of the particles on the tape induce slightly different currents, which are interpreted as an analogue signal by the circuitry in the VCR. Note that the tape must be moving in order for this to work, again a sign that we are dealing with induction.
Compact Disk • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – C A compact disk (or a DVD, which works on the same principle) works very differently from a VCR on a number of levels. First of all the signal or information is encoded digitally, rather than in analogue fashion, which means essentially that it is encoded as a series of binary numbers and therefore the reading device must include a computer capable of interpreting the code. In a CD the code takes the form of tiny little pits in a layer of metal beneath the clear plastic surface of the disk. These pits are read by a laser, which is to say an electromagnetic wave operating at visible light frequencies. The disk reader fires the laser at the disk and detects the scattering back from the pits. The pits are the right size to diffract visible light, which is why you will frequently see a rainbow effect when you shine white light of a CD. A DVD is different from a CD only in so far as it operates with a higher frequency (lower wavelength) laser (640 nm versus 780 nm for the CD laser), which allows smaller pits to be seen, and so one can fit more of them, and therefore a greater quantity of information, on the disk.
Electric Guitar • An electric guitar does not need to have its strings electrically connected to any circuit (it can be strung just like any other guitar). How is the signal from the strings read by the “pick-ups” in the guitar? • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – A Although there are a number of different kinds of pick-up device used in electric guitars, by far the most common method used is that of magnetic induction. The strings on an electric guitar are made from a ferromagnetic material. The pickups contain a permanent magnet with coils of current-carrying wire wrapped about them. The permanent magnet induces some magnetism in the instrument strings, and then when the strings are plucked their motion induces a current in the wires, which is then sent to an amplifier. Thus the strings of the instrument are not themselves required to carry any electric current.
Mobile Phone • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – C Mobile phones operate using microwaves, meaning short wavelength radio waves, to transmit signals between the phones and nearby aerials. Because microwaves are not very long waves they lack the ability to go around large objects, so mobile phone companies must put up a lot of aerials to get good coverage within towns. In addition the phones have a fairly low watt output, so their range is poor. Microwaves, of course, are also used in microwave ovens. In this case the power output is very high and advantage is taken of the fact that a certain wavelength of microwave is easily absorbed by water molecules, causing them to heat up. Since the rest of the food is transparent to the waves, the food is cooked inside and out at the same time. Obviously the same thing would happen to our flesh if exposed to microwaves (in fact weapons have been built which would make one feel as if one was burning from a great distance), so microwave ovens must be well shielded. To do this one must surround the oven with metal, which prevents the electromagnetic waves from passing through, as a Faraday cage.
Pendulum Clock • A) Magnetic Induction • B) Electric Capacitance • C) Electromagnetic Waves • D) Non-Electromagnetic Means
Correct Answer – D It might not appear that a pendulum clock involves any kind of remote force, but it does. A pendulum clock does not work in space, it requires the presence of the Earth to make it go. But of course it is not an electromagnetic force which operates on the pendulum bob as it swings but the gravitational force of the Earth.
