Electric / Magnetic forces

  • Electric fields cause electric charges to move.
  • A moving electric charge is called an electric current.

    Reading assignment:
    Chapter 6 of Richard Muller's "Physics for Presidents" - Part 1, Part 2, Part 3 -- brief reading quiz on Friday over this
    Chapter 8 of Art Hobson [course textbook]

    Electric and Magnetic forces turn out to be connected.

    They're connected with light too.

    How is the electric force different from gravity?

    Bring: pvc pipe, length of nylon rope, leather gloves, magnet --- or van de Graaf and balloons.

    Reading quiz

    1. If an electron (negative charge) and a proton (positive charge) are placed 1 cm from each other, there is an attractive electric force between them that is...
      1. smaller than the gravitational attraction between them.
      2. approximately the same as the gravitational attraction between them.
      3. larger than the gravitational attraction between them.
      4. There is a force, but it's repulsive, not attractive.

    2. Mary Shelley wrote Frankenstein, a fictional account in which Dr. Frankenstein tries to bring a dead body to life with lightning. Her story was inspired by the real-life research of Luigi Galvani who passed electricity through the leg of what animal? When it jerked, he thought he had brought it to life.

    3. There is a connection between electricity and magnetism. All magnetic field arise due to
      1. circulating electrical currents.
      2. the destruction or creation of electrons in nuclear reactions.
      3. magnetism would not exist without lightning.

    4. Thomas Edison and Nikola Tesla had a feud over...what?

    Do exercise with van de graaf generator where you map out the forces around charged dome.

    Mapping out the electric field

    Sketch the direction of force on a charged balloon near the charged dome of a van de Graaf generator...


    ...a picture of something that can't be seen.

    Ejiri in the Suruga Province, by Hokusai, Katsushika. See also Jeff Wall's "A Sudden Gust of Wind".

    The electric force

    • Charged objects exert a force on each other that decreases with larger distance.
    • Sorta like magnets.
    • Actually, sorta like gravity too.

    • Objects that are charged have one of two kinds of charge which we call + positive and - negative.
    • Sorta like N and S magnet poles.

    • Two objects with the same kind of charge repel each other (+ & +, or - & -). While objects with different charges attract each other (one + and one -).
    • Sorta like magnetic N and S poles.

    • You can map out a field around a charged object by looking at the force on another charged object.
    • Magnetic fields point in the direction of a force (on a compass needle--another magnet).
    • Electric fields point in the direction of a force on an another electric charge. Electric fields make charges move.

    Electric force

    $$F_e = 9\times 10^9\frac{q_1\cdot q_2}{d^2}$$

    $q_1$ and $q_2$ are in "Coulombs"
    and can be positive or negative.

    Charge of 1 proton is $q_p = + 1.6\times 10^{-19}$ C.
    electron: $q_e=-1.6\times 10^{-19}$ C.

    Gravitational force

    $$F_g=6.7\times 10^{-11}\frac{m_1\cdot m_2}{d^2}$$

    $m_1$ and $m_2$ in kg
    and are always positive.

    The force of gravity between a proton and an electron is $10^{-39}$ times the electric force between them.

    Force field

    It's possible to map out the force at different points in space and draw different kinds of pictures. This one shows the force of gravity calculated at each point on a square grid close to the red "mass".

    Sometimes we just draw the direction of the force, and not how intense it is...

    Such a picture might depict the "force field" of

    • a charged object, or
    • a mass.

    It is rather strange that we have a way of "pushing" on something without being in contact with it.

    We say that a charged object sets up an electric "force field" around itself. A second charged object reacts to the field.

    A force field is not an explanation for why another charged object feels a force. It's more a means of keeping track of the forces and studying them.

    "Empty" space is not quite so empty any more!

    Suggested exercises

    Conceptual exercises- Chapter 8: 19, 21, 36, 38, 39

    Problems- Chapter 8: 15