Worksheet: Direct-Current Motors

In this worksheet, we will practice describing the use of a commutator in producing uniform circular motion from the output of a direct-current source.

Q1:

The diagram shows a direct current motor. The direction of current through the coil of the motor is shown. Which of the following diagrams correctly shows the direction of current through the coil when it has rotated counterclockwise by 90?

  • A
  • B

Q2:

Which of the following diagrams showing a direct-current motor correctly labels the poles of the magnet in the motor? The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor.

  • A
  • B

Q3:

When the coil of a direct current motor is aligned normally to the plane of the magnetic field of the motor, the force on the coil is zero. Which of the following most correctly states how a direct current motor can produce uniform circular motion despite there being two positions of the motor coil that occur during one complete revolution for which the force on the coil is zero?

  • ABefore the coil can move to a position where it is aligned normally to the plane of the magnetic field of the motor, it must acquire sufficient angular velocity to allow the coil to continue to turn past the position where the force acting on it is zero.
  • BWhen the coil is at a position where it is aligned normally to the plane of the magnetic field of the motor, the direction of the current supplied to the coil is reversed.
  • CWhen the coil is at a position where it is aligned normally to the plane of the magnetic field of the motor, the current supplied to the coil is slightly increased.
  • DWhen the coil is at a position where it is aligned normally to the plane of the magnetic field of the motor, the position of the magnets in the motor is slightly adjusted.

Q4:

The diagram shows two commutators that can be used in a direct current motor. Which of the following most correctly describes how a motor that uses the commutator with four terminals will operate differently than a motor that uses the commutator with two terminals?

  • AThe motor using the four-terminal commutator will have a force output magnitude double that of a motor using the two-terminal commutator.
  • BThe motor using the four-terminal commutator will produce reciprocal motion.
  • CThe motor using the four-terminal commutator will rotate at twice the frequency of a motor using the two-terminal commutator.
  • DThe motor using the four-terminal commutator will have a more uniform output force magnitude than a motor using the two-terminal commutator.

Q5:

Which of the diagrams showing a direct-current motor correctly represents the direction of the rotation of the motor? The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor.

  • A
  • B

Q6:

Which of the following diagrams showing a direct-current motor correctly labels the terminals of the motor? The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor.

  • A
  • B

Q7:

The diagram shows a direct current motor. The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor. Which of the diagrams correctly shows the paths of currents through each of the coils, from the positive terminal of the motor to the negative one?

  • A
  • B
  • C
  • D

Q8:

The diagram shows a direct current motor. The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor. The end of the coil closest to the north pole of the magnet connects to the positive electrical terminal and the end of the coil closest to the south pole of the magnet connects to the negative electrical terminal. Arrows showing forces that act on the coil are shown. The blue and red arrows are forces that might act on the coil when it is aligned in the plane of the magnetic field of the motor. The green and pink arrows are forces that might act on the coil when it is aligned normally to the plane of the magnetic field of the motor. Which color arrows correctly represent forces that would act on the motor?

  • ARed arrows
  • BBlue and green arrows
  • CRed and pink arrows
  • DBlue arrows

Q9:

The diagram shows a direct current motor. The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor.

What is the ratio of the current in the coil of the motor in position II to that in position IV?

What is the ratio of the current in the coil of the motor in position III to that in position I?

Q10:

The diagram shows a direct current motor. The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor.

At which position is the force on the coil of the motor at its maximum?

  • AIII
  • BIV
  • CII
  • DI

At which position is the force on the coil of the motor at its minimum?

  • AIV
  • BIII
  • CII
  • DI

Q11:

The diagram shows a direct current motor. The blue arrows shown represent forces that act on the coil. Which of the terminals 𝑎 or 𝑏 is the positive terminal of the motor?

  • A𝑎
  • B𝑏

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