CIE IGCSE Physics (0625) D.C. Motor Study Notes - New Syllabus
CIE IGCSE Physics (0625) D.C. Motor Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of D.C. Motor
Key Concepts:
- Turning Effect on a Current-Carrying Coil in a Magnetic Field
- Operation of an Electric Motor (DC Motor)
Turning Effect on a Current-Carrying Coil in a Magnetic Field
Turning Effect on a Current-Carrying Coil in a Magnetic Field
When a coil carrying current is placed in a magnetic field, it experiences a force on each side due to the interaction between the magnetic field and the current. These forces create a turning effect (torque) on the coil, causing it to rotate.
Why Does the Coil Rotate?
- The current in opposite sides of the coil flows in opposite directions.
- Each side experiences a force due to the magnetic field (via the Lorentz force).
- The forces on the two sides are equal and opposite but act at different points — this causes a moment (turning effect).
Direction of Rotation
- Use the left-hand rule (Fleming’s Left Hand Rule) to determine the direction of the force on each side of the coil:
- First finger → magnetic field (B)
- Second finger → current (I)
- Thumb → force (F, direction of motion)
Factors that Increase the Turning Effect
The torque on the coil is increased by increasing any of the following:
- (a) Number of turns (N): More turns = more total force acting on the coil.
- (b) Current (I): Higher current = stronger magnetic force on each side.
- (c) Magnetic field strength (B): Stronger magnetic field = stronger force per unit current.
\(\tau \propto N \cdot I \cdot B \cdot A \cdot \sin\theta\)
Where:
- \(\tau\) = torque (turning moment)
- \(A\) = area of the coil
- \(\theta\) = angle between normal to coil and field lines
Application: Electric Motor
This is the basic working principle behind a simple electric motor, where a coil in a magnetic field rotates when current flows through it.
Example:
A rectangular coil with 50 turns, each of area \(0.01\ \text{m}^2\), carries a current of \(2\ \text{A}\) and is placed in a magnetic field of \(0.3\ \text{T}\). Calculate the maximum torque experienced by the coil.
▶️ Answer/Explanation
Use the torque formula: \(\tau = NIBA\sin\theta\)
Maximum torque occurs when \(\theta = 90^\circ\), so \(\sin\theta = 1\)
\(\tau = 50 \times 2 \times 0.3 \times 0.01 = 0.3\ \text{Nm}\)
Final Answer: \(\boxed{0.3\ \text{Nm}}\)
Operation of an Electric Motor (DC Motor)
Operation of an Electric Motor (DC Motor)
An electric motor converts electrical energy into mechanical energy. It works on the principle that a current-carrying coil in a magnetic field experiences a turning force (torque).
How a Simple DC Motor Works
- A coil is placed between the poles of a permanent magnet.
- When current flows through the coil, each side of the coil experiences a force (via Fleming’s Left Hand Rule).
- These forces act in opposite directions on either side of the coil, creating a turning effect (rotation).
Role of the Split-Ring Commutator
- The commutator is a metal ring split into two halves, connected to the coil.
- It reverses the direction of the current in the coil every half-turn.
- This ensures the torque on the coil remains in the **same direction** throughout the rotation, allowing continuous rotation.
Role of Carbon Brushes
- Carbon brushes press against the rotating split-ring commutator.
- They provide electrical contact between the external circuit and the rotating coil.
- This allows current to flow into the coil while minimizing friction and wear.
Example:
A student builds a simple motor with a single rectangular coil in a magnetic field. She notices the coil turns a little and then stops, rather than spinning continuously. What part is likely missing, and why?
▶️ Answer/Explanation
The motor is likely missing a split-ring commutator.
Without the commutator, the current direction in the coil stays the same, so after half a turn, the torque reverses and opposes the rotation. The coil gets “stuck.”
Conclusion: The split-ring commutator is essential to reverse current direction every half-turn to keep the coil turning in one direction.