電磁線圈不接電能產生磁場嗎 ??? (20分)

As there is a current passing in a wire, then there will be a magnetic field. Let me take an example.

In the old kind of transformer we are using daily, the change in voltage is basicly using the effect as you stated.

In the primary coil, the coil is connected to a AC power supply, as the AC current pass to the coil, there will be a changing magnetic field produced.

In the secondary coil, it is connected to the eletric applicant, there is no power supply to the secondary coil. The secondary coil will 'feel' the changing magnetic field from the primary coil, this is called induction. Then there will be a induced emf in the secondary coil, then there will current flow in the secondary coil.

Any change in the voltage in a curcuit will induce a large electric and magnetic field.
And this electric and magnetic field will induce a current in a very short time. This is same as the using in transformer (but a steady AC current).

If you shut down the curcuit suddenly, then the change in voltage will be very large in a short time (even the voltage is small, by time is very very short, chnage in voltage will be large), then the induce magnetic field will be very large, and then the induce current will be also very large. It is true for your statement 把電路中斷也能產生時間很短的磁場, but need to add a strong magnetical filed in it.

The new electric applicance was added with a capacitor, which willl release / accept charges while the curciut is offed / on suddenly. The capacitor will increase the time of the change of voltage, then the change in voltage will be smaller, the back EMF is also smaller to protect the applicance from the damage of large back emf.

若把正在通電的電路中斷,電路上會產生Back EMF, 電壓有變動,所以在中間的鐵會產生瞬間的磁場