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.