A relay is a magnetically-operated switch that operates on which principle?

A relay is a magnetically-operated switch that operates on which principle?

• A. Magnetic repulsion
• B. The solenoid principle
• C. Thermal expansion
• D. Piezoelectric effect

Answer: (B)

Relays operate by converting electrical energy into magnetic force using a coil, which is the solenoid principle. When current flows through the coil, it creates a magnetic field that pulls a ferrous armature, moving the switch contacts to open or close the circuit. This magnetic attraction lets a small control current switch a larger load current, with electrical isolation between the control and the switched circuit. Other mechanisms listed don’t fit the typical relay action: thermal expansion relies on heating and is slow and imprecise; the piezoelectric effect involves deformation of crystals under voltage rather than magnetic attraction; magnetic repulsion is not the usual means by which a relay moves its contacts—attraction from the energized coil is what drives the operation.

Relays operate by converting electrical energy into magnetic force using a coil, which is the solenoid principle. When current flows through the coil, it creates a magnetic field that pulls a ferrous armature, moving the switch contacts to open or close the circuit. This magnetic attraction lets a small control current switch a larger load current, with electrical isolation between the control and the switched circuit. Other mechanisms listed don’t fit the typical relay action: thermal expansion relies on heating and is slow and imprecise; the piezoelectric effect involves deformation of crystals under voltage rather than magnetic attraction; magnetic repulsion is not the usual means by which a relay moves its contacts—attraction from the energized coil is what drives the operation.