Section 1 review
A magnetic field is created whenever current flows, and a device that turns electrical energy into magnetic energy is called an electromagnet. If current flows through a straight length of wire, the magnetic field at any point is tangential to a circle that is centered upon the wire. A solenoid is a long coil of current-carrying wire, and it generates a magnetic field that resembles a bar magnet’s. Electric motors rely on the attractive and repulsive forces between permanent magnets and electromagnets. For an electric motor to efficiently operate, the electromagnets’ poles must periodically alternate between N and S. The motor then can use electric current to generate torque and perform mechanical work.

For more information to research the development of the concept of the electromagnetic force, see
  • Hidden Attraction: The History and Mystery of Magnetism by G. L. Verschuur,
  • A History of Electricity and Magnetism by H. W. Meyer, and
  • The Electric Life of Michael Faraday by A. W. Hirshfeld.
Read the text aloud
electromagnetism, electromagnets, polarity, solenoid, electric motor, commutator, armature

Review problems and questions

  1. Research and describe the historical development of the concept of the electromagnetic force. Read the text aloud Show
Which pattern will you see?
  1. You look down upon a current-carrying wire with a cardboard square around it. (Positive current is approaching you.) You place four magnetic compasses on the cardboard stage. Which pattern will you see? (Recall that each red tip has a N polarity.) Read the text aloud Show
Which way will the blue tip of the compass needle point?
  1. You have been given an unusually colored magnetic compass. As shown in the top diagram, you place it next to a properly labeled bar magnet to determine what the compass’s white and blue tips represent. Next (bottom diagram), you bring the compass up toward one end (marked “?”) of a solenoid, which is carrying current as shown here. Which way will the blue tip of your compass needle point? Read the text aloud Show

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