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?
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.)
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)
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.)
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.
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.
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?
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?