Electric bell
Key idea: Current makes the electromagnet pull the armature, which strikes the gong and breaks the circuit. The spring pulls the armature back, the circuit closes again, and the cycle repeats.
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Guided steps
- Locate the fixed core, moving armature and series contact.
- Trace current, magnetic pull, contact opening and spring return.
- Explain why the bell’s core must release the armature when current stops.
- Predict the sound and current if the return spring fails.
- Decide whether the coil current can start with its series contact open.
- Explain how weaker current changes the magnetic pull and ringing.
- Explain one complete cycle and compare all four mark points.
- Compare the currents and failed steps in the two bells.
- Check what breaks the circuit and what closes it again.
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Press the button of an old-fashioned doorbell and it rattles away for as long as you hold it. There is no motor inside: an electromagnet pulls a hammer against a gong, and the hammer’s own movement switches the electromagnet off again. This lesson explains how that switching keeps the bell ringing.
The parts of an electric bell
- The electromagnet is a coil of wire wound round a soft-iron core.
- The armature is a separate strip of soft iron, held by a spring, with the hammer on its end.
- The make-and-break contact is a screw that touches a springy metal strip on the armature. The current to the coil passes through this contact, so moving the armature opens and closes the circuit.
How the bell keeps ringing
- Pressing the push switch completes the circuit, so a current flows through the coil.
- The current magnetises the soft-iron core, and the electromagnet attracts the armature.
- The armature moves towards the electromagnet, and the hammer strikes the gong.
- As the armature moves, the contact opens. This breaks the circuit, so the current stops.
- The soft-iron core loses its magnetism, so it no longer attracts the armature.
- The spring pulls the armature back. The contact closes again, and the current flows again.
- The cycle repeats many times a second for as long as the push switch is pressed.
The make-and-break contact is the key to the bell. Without it, the electromagnet would pull the armature in once and hold it there. Because the armature’s own movement breaks the circuit, the electromagnet switches itself off each time it pulls, and the spring can reset it.
The coil’s field strengthens as its current increases. The soft-iron core helps it magnetise and release, while the spring opposes the inward movement. If the magnetic pull is too weak to move the armature far enough, closing the push switch alone will not make the bell ring.
Worked example 1
Why the core is soft iron
Problem
Explain why the core of the electromagnet in an electric bell is made of soft iron.
Show full solution
When the current flows
Method
Soft iron is easily magnetised by the coil’s field.
Reason
The core must become a strong magnet quickly, to pull the armature in.
Working
Answer
Current on: core magnetised, armature attracted.
When the current stops
Method
Soft iron loses its magnetism almost at once.
Reason
The armature must be released so that the spring can pull it back and close the contact.
Working
Answer
Current off: core demagnetised, armature released, cycle repeats.
Quick check
A magnetically hard steel core can retain enough magnetism after the contact opens to hold the armature in. That would prevent the return needed for repeated ringing.
Guided practice 1
A failed return spring
Problem
The make-and-break contact of a bell is initially closed. It opens normally on the first strike, but a broken spring leaves the armature in that position instead of returning it. What is heard when the push switch is held down? Explain what happens to the coil current.
Follow the current, then the armature
Add a note (optional)
Hints
Hint 1
the current
What happens to the current when the first strike opens the contact?
Hint 2
the armature
What would normally move the armature back and close the contact?
Show solution step by step
Follow the current
Method
Current flows initially, then stops when the first strike opens the contact.
Reason
The make-and-break contact is in series with the coil.
Working
Answer
First strike → contact opens → coil switches off.
Follow the armature
Method
The armature does not return, even though the electromagnet has switched off.
Reason
The broken spring cannot close the contact again, so the current cannot restart.
Working
Answer
One strike, then silence: the contact stays open and the coil current is zero.
Quick check
Continuous ringing needs the current to switch off and on repeatedly, not just a strong electromagnet.
Spot the mistake 1
A contact stuck open
Learner claim
The make-and-break contact of a bell is stuck open. A learner predicts that pressing the push switch will pull the armature in once and hold it. Find the error and give the correct outcome.
Find the broken link
Show solution step by step
Start at the contact
Method
The open contact breaks the circuit through the coil.
Reason
The contact is in series with the push switch and the coil. Closing the switch cannot complete a circuit that is open somewhere else.
Working
Answer
No current flows in the coil.
Follow the consequences
Working
Answer
With no current, the core is not magnetised and the armature is never attracted. The bell is silent.
Common mistake
The learner assumed the coil gets a current. Every part of a series circuit must be closed for a current to flow.
Check your understanding 1
The battery of an electric bell is running flat, so the current is smaller. How does this affect the ringing? Explain why.
Show answer
A smaller current makes the electromagnet weaker, so it pulls the armature less strongly. The strikes may become weaker and the ringing may stop. If the magnetic pull cannot overcome the return spring, the armature cannot move far enough to strike and operate the contact. Do not infer normal repeated ringing merely from the push switch being held closed.
Exam-style question 1
How the bell rings
Examination question
Describe how an electric bell rings continuously while its push switch is held down. [4 marks]
Describe one full cycle
Explain one complete cycle, including what breaks the circuit and what restores it. Work on paper or here, then compare the complete reasoning with the model below. You need not transcribe your paper work.
Show solution step by step
The electromagnet pulls
1 markMethod
A current in the coil magnetises the soft-iron core, which attracts the armature.
Reason
Start with what the current does.
Working
Answer
Current flows, the core becomes a magnet, and the armature is pulled in so the hammer strikes the gong.
The circuit breaks
1 markMethod
The armature’s movement opens the make-and-break contact.
Reason
This is the step that most answers leave out.
Working
Answer
The contact opens, so the current stops.
The armature returns
1 markMethod
The core loses its magnetism, and the spring pulls the armature back.
Reason
Soft iron does not keep its magnetism.
Working
Answer
The armature springs back and the contact closes.
The cycle repeats
1 markWorking
Answer
With the contact closed, the current flows again and the whole cycle repeats rapidly, so the hammer strikes the gong over and over.
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Mark each stage of the cycle separately.
Common mistake
Stopping after the hammer strikes once loses the last two marks: describe the contact breaking, the armature returning and the cycle repeating.
Check your understanding 2 Same sound, different current
Two bells each make one strike when their push switches are held down. In bell A, the contact opens on the strike but a broken spring prevents return. In bell B, incorrect wiring bypasses the make-and-break contact, so opening it does not interrupt the coil circuit. Compare the coil current, the core’s magnetism and the reason the cycle fails in each bell. Work on paper or here, then compare both causal chains below; the note is optional.
Show answer
In A, the strike opens the contact, so coil current stops and the soft-iron core loses almost all its magnetism. The armature still cannot return because the spring is broken; the contact stays open. In B, the bypass keeps a complete path through the coil even when the contact opens. Current continues and the electromagnet keeps attracting the armature, preventing the return needed for repeated strikes. The same sound does not identify the same electrical state. Check current, magnetic pull and the failed mechanical or electrical step in both cases; a closed push switch alone cannot keep this switching cycle going.
Common mistakes
- Stopping at “the hammer strikes the gong”. Explain how the contact opens and closes to repeat the cycle.
- Calling the armature the core. The core is fixed inside the coil; the armature is the moving piece of iron.
- Saying the electromagnet is a permanent magnet. Its soft-iron core is used so that the pull falls when the current stops.
- Choosing a magnetically hard steel core. Retained magnetism can hold the armature in and prevent its return.
- Saying the push switch makes and breaks the circuit. The push switch stays closed; the make-and-break contact does the switching.
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