Electric bell - Construction, working principle, Practice problems, FAQs

You might have heard a buzzing noise after the end of every period in school. That buzzing noise is nothing but the electric bell at work. The electric bell was invented by Alexander Graham Bell, an American scientist. He was born in Edinburg, Scotland on March 3rd, 1847. Electric bells are used in houses, schools and factories. They make use of electromagnets, invented by William Sturgeon in 1823. Later developments to the design were made by James Marsh with the discovery of oscillating electric wire in 1824. In order to understand how electric bells work, we have to understand the concept of electromagnetism. A magnetic material can be made to behave like a magnet by passing electric current through it for a short time. Electromagnets find applications in equipment like MRI where a time varying current passes over a huge doughnut shaped steel core which behaves as a magnet. Electromagnetism was studied extensively by James Clerk Maxwell who first gave the proper understanding about the close relationships between electricity and magnetism. For instance, a time varying current can produce a magnetic field and a strong and varying magnetic field can produce an electric current as well. Today, electromagnets find wide applications in equipment like microwaves and generators. In this article, we will explore electric bell in detail.

Table of contents

• What is an electric bell?
• Construction and working of an electric bell
• Types of electric bells
• Practice problems
• FAQs

What is an electric bell?

It is an electromechanical device used for producing clamoring, ringing or buzzing noise when an electric current is passed through it. It consists of an insulated copper wire having many turns which are wound over a core made of ferromagnetic material. When a current is passed through the windings, the magnetic flux linked with them increases, increasing the magnetic field strength of the ferromagnetic core. When the electric current stops, the magnetic flux also stops. This is a classic use of electromagnets. Electromagnets are advantageous in comparison to permanent magnets, since the intensity of the magnetic field can be controlled by changing the electric current passed through it.

Construction and working of an electric bell

Electric bell primarily consists of a round shaped gong (B), a hammer (which strikes against the gong), and an electromagnet (E). The gong and hammer (A) are made of metal in order to produce a long ringing noise when struck with. The soft iron strip and the screw (T) are together known as the armature. When the user activates the switch, the electric current flows through the windings and makes the core behave like an electromagnet. When this electromagnet comes in contact with the armature, it triggers the hammer to strike against the gong. Later, the armature loses contact with the electrical magnet, the circuit is now broken and the ringing stops for a moment. The armature and the hammer return to their original positions. When this happens, the circuit becomes complete again, the current starts flowing. The process keeps repeating until the user releases the switch (K) and the current stops flowing–thereby demagnetizing the magnet. This type of bell is known as an interrupter bell– they are usually operated on 5 to 24 V AC.

Types of electric bells

• Interrupter bells-This type of bells are used in households–their construction is explained above.
• Single stroke bells-They are used in railway stations. They produce a loud sound only once when struck with the hammer.
• Telephones-Telephones make use of a polarized bell, which usually operate on 60-105 Volts.
• Buzzers-Their working principle is similar to that of the interrupter bell.

Practice problems

Q. The emf induced in the electromagnetic core varies with time as =10 sin (100 t). Calculate the maximum value of induced current in the armature, if its resistance is 10 .

Also, calculate the variation of flux with time.

A. Given,

=10 sin (100 t)

So, the max value of emf 0=10 V.

$\epsilon =-\frac{d\varphi }{dt}$

$i.e., d\varphi =-10 sin \left(100\pi t\right) dt\Rightarrow \varphi =\frac{10}{100\pi } cos(100 \pi t)= \frac{0.1}{\pi }cos(100 \pi t)$

Maximum value of current, $i_0= \frac{{\epsilon }_0}{R}=\frac{10 V }{10 \mathrm{\Omega } }=1 A.$

Q. The magnetic flux produced in the armature coil varies with time according to the equation, $\varphi =3 t^3-3 t^2+2$ . Calculate the value of induced emf at time t=1 s.

A.$\epsilon =|-\frac{d\varphi }{dt}|=\frac{d}{dt}(3 t^3-3 t^2+2 )=9{ t}^2-6t.$

The value of emf at t=1 s will be,

$\left|\epsilon \right|= 9{\left(1\right)}^2-6\left(1\right)=3 V.$

Q. A time varying current of 0.2 A/s exists in the armature windings. If the coefficient of self induction of the coil is 3 mH, calculate the emf induced in the coil.

A.

Given, $\frac{di}{dt}=0.2 A/s,L=3 mH,emf\epsilon =?$

$\epsilon =-\frac{d\left(Li\right)}{dt}$

$\left|\epsilon \right|=L\frac{di}{dt}=3\times {10}^{-3 }\times 0.2 =0.6 mV.$

Q. The magnetizing field which is set up in the electromagnet is about 1500 A/m. If the relative permeability of the core is 2000, find the intensity of the magnetic field in the ferromagnet.

A. Given, ${\mu }_r=2000; H= 1500 A/m, I=?$

$$\begin{gathered} I={\chi }_m H;{\chi }_m={\mu }_r-1; \\ {\chi }_m=2000-1=1999. \\ I=1999\left(1500\right)=2.998\times {10}^6 A/m. \end{gathered}$$

FAQs

Q. Write one disadvantage of the electric bell.
A. Their maintenance is expensive and complicated. Sometimes the connection problem occurs because of the continuous hitting in the time of operation.

Q. What are the disadvantages of electromagnets?
A. Electromagnets dissipate a lot of heat and thereby lose a lot of electrical energy.

Q. What is the function of spring in an electric bell?
A. When the coil is magnetized it gets attracted to the gong. The spring brings the armature back to its original position.

Q. Why is a permanent magnet not used in an electric bell?
A. If a permanent magnet is used, it will keep supplying a steady magnetic field, which will make the hammer stick with the bell. On the other hand, the temporary magnet or electromagnet will behave as a magnet for a short time, meaning the hammer will be triggered temporarily.