The opposition to alternating current flowing through a solenoid is known as reactance which is found in two forms:

• Capacitive reactance, caused by the capacitive effect in the circuit
  
• Inductive reactance, generated by the rising and collapsing of flux in a solenoid or coil causes a back emf in the circuit.

When a wire (conductor) is passed through a magnetic field an emf (electrical potential) is generated in the wire see the illustration in fig 10.

In fig 10e With the inductor connected to an electric circuit the loop is running parallel to the lines of force at position A,

In this position loop is not cutting across the lines of force and therefore has no emf generated and no current will flow.

As the coil rotates it cuts the lines of magnetic force at an increasing angle until at position B where having rotated through 90^othe coil is cutting the magnetic lines of force at the maximum angle the emf and consequently current flow generated is at maximum.

As the coil continues to rotate, cutting the lines of magnetic force at an decreasing angle the emf generated falls, reducing the current flow until it is again 0.

The coil continues to rotate until it is again cutting the magnetic lines of force inducing an negative emf with a current flow in the opposite direction until with the coil cutting the magnetic field at maximum angle the generated emf is again at a maximum but negative peak with a maximum negative current flow. The coil continues on to the start position at A the emf will decrease until it again reaches 0.

Illustration of simple ac generator, the coil is rotating through the magnetic field set up by the permanent magnets.

As the coil passes through the vertical position it travels along with the magnetic flux,

Fig 10a

Fig 10b

Fig 10c

Progressing the coil continues on it cuts the field at a shallower angle and the induced voltage reduces to zero Then  increasing in a negative direction as the coil continues on its course.