An electric bell has an electromagnet, an iron armature with a hammer, a spring, and a contact that…

Contributed by StemPy Lab

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

An electric bell has an electromagnet, an iron armature with a hammer, a spring, and a contact that the armature touches.

Describe, in steps, how the bell rings while the push button is pressed.
The hammer strikes the gong $18$ times a second. How long does one strike and return take, in milliseconds?

Need a reminder? The key ideaShow

Inside a long solenoid the field is strong and uniform: $B = \mu_0\,n\,I$ , where $n$ is the number of turns in each metre and $\mu_0 = 4\pi \times 10^{-7}\ \text{T m/A}$ . A soft iron core makes it an electromagnet, many times stronger, that loses its magnetism when the current stops.

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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