MediumPhysicsHigh School (9–10)Electricity & Magnetism › Magnetic Effects of CurrentConceptualBoard Exam
An electric bell has an electromagnet, an iron armature with a hammer, a spring, and a contact that…
Contributed by StemPy LabTry 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 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: , where is the number of turns in each metre and . 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.
Step 1. (a) The cycle.
- The button closes the circuit, and a current flows through the coil of the electromagnet.
- The electromagnet pulls the iron armature, and the hammer strikes the gong.
- The armature moves away from the contact, so the circuit is broken and the current stops.
- The electromagnet loses its magnetism, and the spring pulls the armature back to the contact.
- The circuit is closed again, and it all repeats.
Step 2. (b) The time. .
Answer: the pull of the magnet breaks its own circuit, the spring brings the hammer back, and this repeats; one strike takes .
Check. .
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