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class code: SPS21 teacher: Mr. Elert
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Physics A: Problem Set 19: Magnetism

recommended reading

High Marks: 3:114–3:122
Barron's Let's Review: 10.1–10.3
physics.info: Magnetism
Wikipedia: Magnetism, Magnetic field
Mr. Machado: 19 Magnetic Fields
Reference table with select items highlighted, page 1 Reference table with select items highlighted, page 2 Reference table with select items highlighted, page 3 Reference table with select items highlighted, page 4 Reference table with select items highlighted, page 5 Reference table with select items highlighted, page 6

homework

  1. Draw the magnetic field around the following arrangements of magnets. Start each field line on a given blue dot and extend it until it hits a magnet or it hits the edge of the bounding box. (The magnets inside one bounding box do not affect the field inside any other bounding box. All arrangements are assumed to be suitably far apart.)

    Isolated bar magnet

    Isolated horseshoe magnet

    North-south together

    North-north together

    South-south together

    You should have done an activity like this in physics lab and made drawings that look something like these. The last drawing is what you'd get if one magnet was stronger than the other, which does happen from time to time.

    Isolated bar magnet

    Isolated horseshoe magnet

    North-south together

    North-north together

    South-south together

  2. Two small, lightweight, pivoting magnets are placed near one another with their north poles facing inward as shown in the top view diagram below. What final arrangement will these two magnets have?

    Cartoon representation

    Draw your solution in the space provided below.

    blank space

    Any arrangement where opposite poles are adjacent is a possible outcome. There are four.

    Two compasses in a row aligned south-north, south-north Two compasses in a row aligned north-south, north-south
    Two compasses side by side aligned north up, south up Two compasses side by side aligned south up, north up
  3. Using a strong magnet, it is possible to pull the iron out of a fortified breakfast cereal. It is not possible to do this to the iron in your blood. (I don't care what you saw Magneto do in that X-Men movie, it can't be done.) Why is this? How is the iron in breakfast cereal different from the iron in human blood?

    The short answer is, the iron atoms in fortified breakfast cereal are near enough to one another to behave like a magnet. The iron atoms in human blood are too far apart to behave like a magnet.

    This is a question about ferromagnetism. A magnet sticks to a piece of iron because the iron atoms adjacent to each other line up. (The outermost electrons of these iron atoms align their spins.) This phenomenon is known as ferromagnetism. Any one iron atom is a tiny magnet and many aligned iron atoms make a big magnet.

    But here's the problem. In order for any two iron atoms to align, they have to be close enough to communicate. In a solid chunk of iron, the nearest neighbor of any iron atom is another iron atom, which is why a chunk of solid iron can be pulled by a magnet. The iron in fortified breakfast cereal is a chunk of solid iron that has been ground down into a fine dust. Even though the particles are small, the dust is still just a collection of closely packed iron atoms. Closely packed iron atoms, no matter what the particle size, can still become a magnet.

    The iron in human blood is not like the iron in breakfast cereal. It is a part of a much larger molecule called hemoglobin. Hemoglobin is an example of a meatalloprotein — a large, complex, organic molecule with a few metal atoms buried deep inside. For every one atom of iron there are more than 2,000 other atoms that are not iron. In addition, blood is mostly water (water, salts, sugar, proteins, dissolved gases, etc.). The iron atoms are not close enough to "communicate" so they can't align. No alignment, no magnet. No magnet, no pulling blood out of the body.

    Here's a simple experiment you can try yourself with a strong magnet and a small piece of iron or magnetic steel — like a paperclip. Hold the magnet on one side of your hand and the paperclip on the other. If the magnet is strong enough, the paperclip right will stick to your hand. (Actually, the paperclip is sticking to the magnet. Your hand is just in the way.) I have done this demonstration in class many times.

    Now hold the same magnet next to your hand and wait for the blood to move toward the magnet. You will wait forever. Let me know when forever has arrived.