Solenoid actuator principle: When a solenoid is energized, what physical cause moves the plunger?

Introduction / Context:
Solenoids are electromechanical actuators that convert electrical energy into linear motion. Understanding what actually exerts force on the plunger is key for troubleshooting and design in automation and control systems.

Given Data / Assumptions:

• Coil supplied with current, creating a magnetic field.
• Movable ferromagnetic plunger aligned with the coil axis.
• Standard solenoid construction with return spring omitted from consideration here.

Concept / Approach:

The energized coil establishes an electromagnetic field. The ferromagnetic plunger experiences a force pulling it into the region of higher magnetic field density to minimize magnetic reluctance. The cause is the interaction of the induced magnetic field with the plunger’s material, not merely the presence of voltage or current without a field.

Step-by-Step Explanation:

Verification / Alternative check:

Reversing current reverses polarity, but the plunger is still pulled in, demonstrating force depends on field magnitude and geometry rather than polarity for typical designs.

Why Other Options Are Wrong:

Permanent magnetic field is not necessary in standard solenoids. 'Varying voltage' and 'steady current' are electrical conditions; the physical mover is the resulting electromagnetic field. Electrostatic attraction is negligible here compared to magnetic forces.

Common Pitfalls:

Confusing cause (field) with electrical conditions (voltage/current). The field creates the force that moves the plunger.

Final Answer:

an electromagnetic field