1. While the tape is made of Mylar, the same material used in plastic helium balloons, the coating on that Mylar is special. Aluminum-coated Mylar is common in helium balloons but it won't work in magnetic recording tape. Why not?
Answer: Aluminum is not an intrinsically magnetic metal and cannot be magnetized permanently.
Why: While aluminum atoms are magnetic, they lose their intrinsic magnetism when the coalesce to form solid aluminum. Since it contains no magnetic domains or microstructure, aluminum cannot be magnetized in the way that iron or steel can.
2. Television images are built dot by dot, with millions of dots being illuminated each second. To record video, the recorder must store brightness and color information for roughly 5 million screen dots each second. That's a lot of recording. If the patch of tape surface needed to record a dot is 0.001 millimeter wide and if the dots are recorded side-by-side, one after the other, about how long is the tape strip that's required to record just one second of video? (Note: You must calculate this length yourself. The value in the book is slightly different and won't be accepted.)
Answer: About 5 meters
Why: 5 million dots, each 0.001 millimeters long will occupy 5,000,000 * 0.001 mm or 5,000 millimeters. That's 5 meters or about 16 feet.
3. Moving that much tape past a stationary recording head
each second would be difficult and a 2 hour movie would
require miles of recording tape. So instead of moving the
tape past the recording head, the VCR moves the head past the
tape. Actually, there are either 2 or 4 tiny, delicate
magnetic heads sitting on the surface of a shiny metal
cylinder about the size of a tuna can. This cylinder spins 30
times a second so that the heads sweep across the almost
stationary tape. The cylinder is tilted at a slight angle and
the tape is wrapped part way around it so that as the
cylinder spins, the heads sweep diagonally across the tape.
After each sweep, the tape advances about a millimeter so
that the heads sweep across a fresh diagonal on the tape. The
figure below shows three sweeps of the head across the tape:
The tape is pushed very gently against this cylinder as it
spins. Why is it important to minimize the pressure between
the spinning cylinder and the almost stationary tape? (hints:
(1) VCR's have limited lifetimes and doubling this pressure
will halve the lifetime of a typical VCR and (2) don't worry
about magnetism for this question)
Answer: To minimize sliding friction between the tape and heads.
Why: Sliding friction causes wear to the surfaces involved. Since the heads slide across the tape at about 5 meters per second, they travel thousands of kilometers during a typical year's use. Even though this sliding involves relatively smooth, clean surfaces, it still causes those surfaces to wear. The tape keeps changing so its wear isn't a problem. But the heads gradually wear out and must be replaced. Since that replacement costs a significant fraction of the VCR itself, many people simply discard the VCR when its heads fail.
4. The VCR consumes power in order to keep the recording head cylinder turning. Using words like force, distance, and work, show that power is needed to keep that cylinder turning as long as there is tape in the machine. (This question has nothing to do with magnetism.)
Answer: To keep the head moving across the tape against the slowing force of sliding friction, the VCR must push the head forward. That head moves forward a distance in the direction of the VCR's push, so the VCR does work on the head. Since it continues to do work, every second, the VCR consumes power twisting the cylinder and thus pushing the head across the tape.
Why: Any system in which one surface is dragged across another consumes power. It is perpetually producing thermal energy, so it perpetually needs new energy to continue. Anything that consumes some energy every second, indefinitely, is consuming power (energy per time).
5. Each recording head contains a tiny coil of wire wrapped around a small iron ring with a gap in it. During recording, the VCR pushes currents through this coil of wire and the tape nearest the ring's gap becomes magnetized. In a couple of sentences, explain why sending a current through the wire coil causes the tape to become magnetized.
Answer: The current is magnetic and it magnetizes the iron ring. The poles that form on either side of the gap produce a strong magnetic field that enters the tape and magnetizes it.
Why: The ring and its coil are a tiny electromagnet. When current flows through the coil, the electromagnet becomes active and its poles appear on the edges of the gap. A strong magnetic field appears around this gap and that field magnetizes the nearby tape.
6. During playback, the gap in a head's iron ring moves past various magnetized patches of tape and currents are produced in the head's tiny coil of wire. In a couple of sentences, explain why moving the head past the recorded tape causes these currents to flow in the wire coil.
Answer: The tape's magnetic field magnetizes the iron ring. As the tape's magnetization changes, so does that of the ring and its changing magnetic field induces currents in the wire coil.
Why: The tape's moving magnetic field ultimately produces the electric field that pushes charges through the wire coil. But the iron ring helps out by extending the tape's magnetic field inside the wire coil, where it is more effective.
7. A bulk videotape eraser exposes a tape to a very strong magnetic field--one that reverses directions rapidly as it gradually gets weaker and weaker. How does this process erase any recorded information from the tape?
Answer: The strong, fluctuating field randomizes the magnetization of all the tiny magnetic particles on the surface of the tape.
Answer: In a recorded tape, the tiny magnetic particles are systematically magnetized in one direction or the other, in ways that represent the video information. But exposure to a strong, fluctuating, and diminishing magnetic field randomly flips all of the magnetizations so that they contain no more information. While all the magnetization align with the field at its peak, the particles gradually stop following the fluctuating field as it diminishes. Since the particles drop of alignment with the field at different times, there is no telling just which way any particle is magnetized. They end up randomized nicely.
8. Videotapes should never be stored where they can get very hot, in large part because the plastic case and tape may deform. But there is also the possibility that the magnetic recording will be erased. How do high temperatures erase magnetic recordings?
Answer: Thermal energy can flip the magnetizations of the magnetic particles and randomize their magnetizations. The video information they represented is thus lost.
Why: Thermal energy can damage the order of just about anything. In this case, the thermal energy destroys the order associated with carefully recorded magnetizations. The magnetizations of the tiny magnetic particles are scrambled by the thermal energy so that they no longer represent anything.