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Lesson: Chapter 13 — Electromagnetism and electromagnetic induction

The DC motor 6 of 6

Exercise 13.2

Worked answers to exercise 13.2.

Question 1 — Fleming's left-hand rule

A student's left hand

  1. What is Fleming's left-hand rule used for?
  2. In the picture, what do the directions of fingers A, B and C show — the force, the magnetic field or the current?
  3. Using the rule, write what happens to the coil in cases (a) to (d).

The answer

  1. To find the direction of the force on a conductor carrying a current in a magnetic field.
  2. A, the first finger — the magnetic field; B, the middle finger — the current; C, the thumb — the force.
  3. In every case the coil turns, because the forces on its two sides form a couple. Case (b) has the poles swapped compared with (a), and case (c) has the supply reversed, so each turns the opposite way to (a). Case (d) has both swapped, so it turns the same way as (a).

Question 2 — the loudspeaker

A loudspeaker cross-section

  1. Name parts A, B and C.
  2. Write a special feature of the current entering at X.
  3. Explain how the loudspeaker works.
  4. Write the energy change in a loudspeaker.
  5. Write the job of each of A, B and C.

The answer

  1. A — the ring magnet; B — the cardboard cone; C — the coil.
  2. It varies with time in step with the sound wave to be produced.
  3. The varying current in the coil, in the magnet's field, feels a force that changes with the current, so the coil vibrates back and forth; the cone fixed to it vibrates too and sends sound waves into the air.
  4. Electrical energy → kinetic energy of the cone → sound energy.
  5. A gives the magnetic field; B moves the air to make the sound; C carries the current and is pushed by the field.

Question 3 — the DC motor

The parts of a DC motor

  1. What are P and Q?
  2. What are X and Y?
  3. When switch K is closed, write the direction of the current using the letters.
  4. Which way does the motor turn?
  5. Write the job of (a) the poles N and S, (b) E, (c) P and Q, (d) X and Y.
  6. What happens if (a) the battery is reversed, (b) the magnet is made stronger?

The answer

  1. The brushes.
  2. The split rings of the commutator.
  3. As in figure 13.15: P → X → A → B → C → D → Y → Q.
  4. Clockwise, the direction marked on the figure: the force on AB is down and on CD is up.
  5. (a) give the magnetic field; (b) supplies the current; (c) carry the current between the fixed circuit and the turning rings; (d) reverse the current in the coil every half turn.
  6. (a) The motor turns the other way (anticlockwise). (b) The forces are larger, so the motor turns with more force and faster.

The book labels the poles with the Sinhala letters උ and ද, which appear as V and U in its question text; they are the N and S poles.