14.4 Electric Motors
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. Because 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 right-hand rule on each side of the conductor to prove to confirm this fact.
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 .
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Note: 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-CheckAnswer the following self-check (SC) question then click the "Check your work" bar to assess your response. |
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?
Contact your teacher if your answer varies significantly from the answer provided here.
SC 1.
The coil's momentum moves it past the gap where the current has switched directions, as previously described.
Watch and ListenTo 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 video above.
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ReadRead "The Electric Motor" on pages 608-609 of the textbook.
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Self-CheckAnswer the following self-check (SC) question then click the "Check your work" bar to assess your response. |
SC 2.
Answer the "Concept Check" question on page 609. Hint: The answer is in the DC Motor Operation simulation you looked at earlier.
Contact your teacher if your answer varies significantly from the answer provided here.
SC 2.
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.
ReadRead "The Generator Effect" and "Faraday's and Henry's Discoveries" on pages 609-611 of the textbook then review "Inquiry Lab" on page 612 of your textbook. |
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