Consider an audio tape recorder in the following questions.
1. Suppose you send a constant current through the coil of a tape recorder's head. As the tape passes by the head describe the magnetization that is recorded on the tape.
Answer: The tape will be magnetized with all of the north poles of the magnetic particles pointing toward one end of the tape and all of the south poles pointing toward the other.
Why: A constant current through the coil will produce a constant magnetic field in the gap of the recording head. This will magnetize all of the particles in the same direction as they pass beneath the head.
2. You want to hear what the signal you recorded in question 1 sounds like so you rewind the tape and press play on the recorder. What do you hear and why?
Answer: You wouldn't hear anything. The constant magnetic field on the tape will produce a constant magnetic field in the iron ring in the playback head. Since the magnetic field is constant, no electric field will be produced to move charges in the coil.
3. Now suppose that you connect the coil of the recording head across a standard electrical outlet in your house. Describe the current that flows in the coil inside the recorder's head. Be sure to include both the amount (descriptively with words like more, less, large, small…) and the direction in your description.
Answer: As the current's direction alternates back and forth, its amplitude varies smoothly. The current goes from a large amount in one direction to a smaller amount to zero to a small amount in the other direction to a larger amount...
4. With the current from question 3 running through the recording head, describe the magnetization that is produced in the tape as it travels past the recording head. Be sure to include both the direction and depth of the magnetization in your description.
Answer: The magnetization also alternates directions and its depth varies smoothly.
Why: The direction of the magnetization in a recorded tape mimics the direction of the current used to record it and the depth mimics the amount or amplitude of that current.
5. You are using your tape recorder to play a tape with music recorded on it. When you press the pause button, the tape stops moving past the head, but the head continues to send whatever current is produced to the speakers. Why do you hear nothing instead of a continuous tone at the frequency (pitch) of the sound that was playing when you hit pause?
Answer: With the tape stopped, the magnetic field in the head is constant and no current is produced.
Why: To produce a continuous tone requires an alternating magnetic field, which produces an alternating current, which is then used to drive the alternating pressure fluctuations that we hear as sound.
6. Some magnetic tapes use tiny, elongated iron particles, which have only a single magnetic domain. Briefly explain why larger iron particles, which have many magnetic domains, cannot be used to store the information magnetically.
Answer: Although larger magnetic particles will be magnetized in the appropriate way as they pass beneath the recording head, they will relax and lose this magnetization as soon as they move out from under the head. A large iron particle is a soft magnetic material. To permanently record something magnetically, a hard magnetic material must be used.
There is a brief description of a video tape recorder at the end of chapter 12.5. In a video tape recorder the head does not remain stationary as the tape moves past. Instead, the head is on a spinning drum that sweeps it diagonally across the tape. Video information is written in diagonal stripes across the tape as it is slowly advanced under the spinning drum.
7. If you are watching a movie using your video recorder and you press the pause button, an image continues to appear in the television screen. Describe what the tape and head are doing while the video recorder is paused.
Answer: The tape stops, but the head continues to spin.
Why: When paused, a video recorder continues to read the magnetization recorded along one diagonal stripe on the tape so that an image is produced.
8. Most video recorders will automatically stop displaying the image if they have been paused for too long. This keeps the recorded from damaging the tape. Explain why leaving the recorder paused for too long will damage the tape by describing the forces between the tape and the head, which are gently pressed against one another.
Answer: There is sliding friction between the tape and the head as the head spins. Sliding friction wastes energy as heat, which may cause the tape to lose its magnetization. Sliding friction also causes wear, which may eventually rub a hole in the tape.