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CIE iGCSE Co-ordinated Sciences-P4.5.5 The d.c. motor- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-P4.5.5 The d.c. motor – Study Notes

CIE iGCSE Co-ordinated Sciences-P4.5.5 The d.c. motor – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

Supplement

  • Know that a current-carrying coil in a magnetic field may experience a turning effect and that the effect is increased by increasing:
    (a) the number of turns on the coil
    (b) the current
    (c) the strength of the magnetic field
  • Describe the operation of an electric motor, including the action of a split-ring commutator and brushes

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Turning Effect of a Current-Carrying Coil

A current-carrying coil placed in a magnetic field experiences a turning effect (torque). This is the basic principle of the electric motor.

Why Does the Coil Turn?

  • When current flows through the coil, each side of the coil experiences a force due to the magnetic field (motor effect).
  • On one side, the force pushes upwards; on the other side, it pushes downwards.
  • These forces form a couple, causing the coil to rotate about its axis.
  • The direction of rotation can be predicted using Fleming’s Left-Hand Rule.

Factors Increasing the Turning Effect:

  • (a) Number of Turns on the Coil: More turns mean the forces add up → larger torque.
  • (b) Current: Increasing current increases the force on each side of the coil → stronger turning effect.
  • (c) Strength of Magnetic Field: A stronger field increases the force on the coil → greater turning effect.

The turning effect (torque) is proportional to:

\( \mathrm{Torque \propto N \times I \times B} \)

  • (\mathrm{N}) = number of turns,
  • (\mathrm{I}) = current, and
  • (\mathrm{B}) = magnetic field strength. 

Applications:

  • Electric motors in fans, washing machines, trains, and cars.
  • Motors in toys and small appliances.

Example :

A rectangular coil of 20 turns carries a current of 2 A in a strong magnetic field. What happens to the turning effect if the number of turns is doubled and the current is increased to 3 A?

▶️ Answer/Explanation

Step 1: Torque ∝ N × I × B.

Step 2: Original torque ∝ 20 × 2 = 40.

Step 3: New torque ∝ (40 turns) × (3 A) = 120.

Step 4: Ratio = 120 / 40 = 3.

Final Answer: The turning effect is three times greater than before.

Electric Motor

An electric motor is a device that converts electrical energy into mechanical energy (rotation) using the motor effect.

Basic Construction:

  • A rectangular coil of wire placed in a strong magnetic field.
  • A d.c. supply connected to the coil through a split-ring commutator and carbon brushes.
  • The commutator is a metal ring split into two halves, which reverses the coil connections every half turn.

Operation:

  • When current flows through the coil, the two sides of the coil (at right angles to the field) experience equal and opposite forces due to the motor effect.
  • These forces form a couple that produces a turning effect (torque) on the coil.
  • As the coil rotates and passes the vertical position, the forces alone would make it stop and swing back.
  • The split-ring commutator reverses the coil’s connections to the power supply every half turn → this reverses the current in the coil.
  • As a result, the forces on each side of the coil are also reversed, keeping the coil turning in the same direction continuously.
  • The brushes press against the commutator, ensuring smooth electrical contact between the rotating coil and the fixed circuit.

Role of Components:

  • Coil: Carries current and experiences the turning effect.
  • Magnetic Field: Provides the field needed for the motor effect.
  • Split-Ring Commutator: Reverses the coil connections every half turn, ensuring continuous rotation in the same direction.
  • Brushes: Maintain electrical contact between the external circuit and the rotating commutator.

Key Points:

  • The motor effect produces rotation because forces act on opposite sides of the coil in opposite directions.
  • Without the commutator, the coil would stop after half a turn.
  • Increasing current, number of turns, or field strength increases motor torque.

Applications:

  • Electric fans, mixers, drills, washing machines, trains, cars, toys.

Example :

Why is a split-ring commutator necessary in a simple d.c. motor?

▶️ Answer/Explanation

Step 1: Without a commutator, the coil would stop when it reaches the vertical position, because forces would then act in opposite directions to reverse the turn.

Step 2: The split-ring commutator reverses the current direction in the coil every half turn.

Step 3: This keeps the turning forces in the correct orientation to drive continuous rotation.

Final Answer: The commutator ensures the motor keeps turning in the same direction continuously.

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