Study Guide

Electromagnetism In the Real World

  • Engineering

    Liquid Crystal Displays

    Staring at a screen? We are. But what kind of screen?

    Sadly, not the silver screen (it would take too much film). But there are computer screens (and television screens) that have what’s known as a liquid-crystal display, or LCD for short. LCDs use concepts related to electric fields to do their magic. We’ve been using LCDs for years in calculators and watches, but these days we use them in color to full advantage.

    Liquid crystals are, as their name implies, part solid and part liquid. Each point of display It’s a bunch of tiny crystals in a fluid, in a random orientation. When an electric field is applied, however, the crystals align.

    Rotating the liquid crystals changes the light polarization, which changes the image, which is pretty cool. We’ve read, an electric dipole consists of two opposite charges of equal magnitude. The liquid crystals act as dipoles. When placed in an electric field, the dipole experiences a torque which aligns it with the E field. By applying an external electric field, we rotate these crystals as we see fit and alter their optical properties. And that’s how we get to watch the original Star Wars films with incredible detail.

    It’s not just spinning crystals and electric fields. LCD screens require the help of polarization filters, which either transmit or block the light depending on the crystals’ orientation. Think of polarization filters like blinds covering a window: if the crystals are aligned perpendicularly to the E field in the light, then no light can pass. Several techniques may then used to produce different colors of images, allowing us to look at pictures on our screens in color.

  • Astronomy

    The Northern Lights

    Like most planets, Earth has a magnetic field. Thank you, Earth’s molten iron core for making that happen for us. The magnetic field protects the planet from harsh radiation and dangerous solar winds—like we said, thank you. The magnetic field easily deflects these solar winds, mostly made of electrons and protons spewed out from the sun.

    Remember the Lorentz force? Charged particles moving in magnetic fields experience a force perpendicular to them both, and in the case of the earth it means away. Occasionally though, energetic particles do make it through though and they become trapped moving in helical motion along the field back and forth between the north and south poles. These ions comprise the Van Allen radiation belts.

    Not that we’ve ever heard of that. Here’s a picture.

    When ions from the belts collide with the atmosphere of the Earth, fireworks occur. Particles in the atmosphere become ionized or excited. This produces the emission of bright lights in the sky, depending on what emissions and absorptions are taking place.

    For example, oxygen atoms will emit green or reddish brown light, depending on the amount of energy released as an energized electron returns to its ground state. The difference in energy levels corresponds to different wavelengths of electromagnetic waves, or light.

    Nitrogen, as a second example, emits blue light when absorbing an electron after having lost one to ionization. In the same atom, if an energized electron returns to the ground state, then it emits red light. Nitrogen is some 80% of the atmosphere, so the Northern Lights frequently show these colors in addition to those from oxygen.

    No matter the colors, we guarantee a breathtaking show full of spectacular colors, and definitely worthy of taking a trip up north for the aurora borealis, or down south for the aurora australis.

  • Pop Culture

    Magnets

    Everyone collects things: pet rocks, shot glasses, Shmooints…Some people like to collect magnets and stick them all over their fridge. Magnets can be fun, fancy, frivolous, and even useful, but how do magnets work, exactly?

    Some matter becomes magnetized when exposed to a magnetic field, meaning it consists of tiny magnetic dipoles, each with magnetic moments, m = IA. These dipoles will all align with each other in the same direction as the field surrounding them. In this way, magnetization is more or less magnetic polarization.

    There are several kinds of magnetization, though. There’s paramagnetism, dimagnetism, and ferromagnetism.

    Paramagnets are all about unpaired electrons. Paramagnetism refers to the magnetic state of an atom. The spins of unpaired electrons make the atom have magnetic dipoles, which experience a torque under the influence of a magnetic field. Under these conditions, the dipoles tend to align themselves parallel to the field.

    Diamagnetism is another magnetic phenomenon. It involves an electron’s orbital speed changing in response to a magnetic field, and forcing their dipole moments to point opposite the magnetic field.

    What about permanent magnetization, like the decorations that give life to our otherwise silent refrigerating companions?

    There are substances (ferromagnets) that retain their magnetization after the magnetic field is removed. Just like paramagnetism, ferromagnetism involves dipoles created by the spins of unpaired electrons. The difference is that in a ferromagnet, dipoles actually prefer to align themselves in the same direction as every other dipole in a small region. Monkey see, monkey do.

    This happens because the electric fields in these atoms don’t cancel out. Now, there are several small regions within a ferromagnet, true, and they may all have different regional orientations. However, as we apply an external magnetic field to a ferromagnet, the material will become magnetized with the magnetic fields of each region aligning themselves with each other. Because of their strong magnetic memories, ferromagnets remain as they were after the external field is dead and gone.

    This why our grocery lists stay on the door of the fridge.