A solenoid is a coil of wire designed to create a magnetic field when an electric current passes through it. This magnetic field can be used to convert electrical energy into mechanical energy, making solenoids useful in various applications, such as in electromagnets and valves. When the current flows through the solenoid, it generates a magnetic field that can attract or repel a movable metal core, known as a plunger. This movement can be harnessed to perform tasks like opening or closing switches, which is why solenoids are commonly found in devices like door locks and automated machinery.