Module 4

Module 4—Magnetic and Electric Fields in Nature and Technology

Try This

TR 3. Complete “Practice Problems” 1 and 2 on page 605 of the textbook.

Module 4: Lesson 3 Assignment

Remember to submit the answer to A 2 to your teacher as part of your Module 4: Lesson 3 Assignment.

A 2.  A motor uses a coil of wire in a magnetic field to generate force. The motor draws a current of 9.50 A through the coil of wire and has a magnetic field of 1.75 T. If the motor is designed to generate 800 N, how long is the wire in the coil, assuming that all of the wire creates force?

The Direct Current Electrical Motor

The magnetic force acting on a conductor can be used to rotate a loop of wire. In this image, the electrons making up the current travel around the loop of wire. Since the direction of current is opposite on either side of the loop, so too is the direction of the magnetic force, as predicted by the hand rule. The sum of these two forces causes the loop of wire to spin. Use the left-hand rule on each side of the conductor to prove to yourself that this is the case.

A split ring commutator and a set of brushes reverse the current direction in the wire every half turn, ensuring the orientation of the magnetic force is always up on one side and down on the other. The magnetic force in this application is known as the motor effect.

Module 4: Lesson 3 Assignment

Remember to submit the answers to A 3 to your teacher as part of your Module 4: Lesson 3 Assignment.

A 3. What would happen to the direction of the magnetic force if the loop of wire were to undergo one half-turn without reversing the direction of the current in the loop?

Watch and Listen

Watch an animation of a simple direct current electric motor. Note that the animation uses conventional current, which is the movement of positives. You will have to use the right-hand rule to find the direction of the magnetic force. The black arrows show the direction of the magnetic force.

Note that when the coil is perpendicular to the magnetic field, the connection to the DC voltage is broken; the black line on the commutator is a gap that breaks the electrical connection to the battery. The boxes are brushes that create that connection when they touch the metal part of the commutator.

When this connection is broken, there is no current flowing and no magnetic force.

Self-Check

SC 1. How does the direct current electric motor keep moving when the connection is broken and there is no current flowing and no magnetic force?

Watch and Listen

To understand the meaning of the statement “the current changes direction,” focus on just one side of the loop of wire during a full rotation. Refer to the simulation DC Motor Operation for a more detailed schematic of the DC motor.

Read

Read “The Electric Motor” on pages 608–609 of the textbook.

Self-Check

SC 2. Answer the “Concept Check” question on page 609. Hint: The answer is in the DC Motor Operation simulation you looked.

The Generator Effect

Symmetry in nature has allowed scientists to make important discoveries. For example, the similarities between electric and gravitational fields led Coulomb to conclude that the same mathematical relationships described by Newton’s gravitational laws could be applied to electrostatic interactions as well. In another instance of symmetry, Michael Faraday (1791–1867) and Joseph Henry (1797–1878) understood that if electricity could produce magnetism, as Ørsted’s compass proved, then magnetism should be able to produce electricity. In other words, if moving charges produce a magnetic field, then a magnetic field should also be able to produce moving charges (current). The process should work both ways. Experiments conducted in 1831 by Faraday and Henry supported this theory.

Read

Faraday and Henry concluded that when a piece of conducting wire moves perpendicularly through a magnetic field, a current is induced. This is known as the generator effect or electromagnetic induction.

Read “The Generator Effect” and “Faraday’s and Henry’s Discoveries” on pages 609–611 of the textbook. Look over “Inquiry Lab” on page 612 of your textbook.