Some of the most important Physics Topics include energy, motion, and force.
How does the Magnetic Effect of Electric Current help in the Working of an Electric Bell?
We have just studied that electric current can produce heat. We will now study that electric current can produce magnetism. When electric current passes through a wire, the current-carrying wire behaves like a magnet. This is called the magnetic effect of current.
The magnetic effect of current is used to make a large number of devices which we use in our daily life. For example, the magnetic effect of current is utilised in making electromagnets, electric bells, telephone instruments, electric fans, electric motors, loudspeakers and toys, etc.
The magnetic effect of current was discovered by a scientist named Hans Christian Oersted. Oersted found that when electric current was passed in a wire, then the needle of a compass placed near it got deflected from its usual north-south position.
Now, the needle of a compass is itself a tiny magnet (which can rotate freely on a pivot). A compass needle can be deflected only by another magnet’s magnetic field. Since a current-carrying wire was able to deflect a compass needle, it was concluded that the electric current flowing in a wire gives rise to a magnetic field around it.
It is this magnetic field produced by the current-carrying wire which exerts a force on the compass needle and deflects it (or moves it). We will now describe Oersted’s experiment in detail to understand the magnetic effect of current more clearly.
To Demonstrate the Magnetic Effect of Current
We take a thick, insulated copper wire AB and fix it on a table in such a way that this wire is in the north-south direction as shown in Figure (a). A compass is placed under the wire AB. The two ends of the wire AB are connected to a cell through a switch by using connecting wires.
In Figure (a), the switch is open, so no current is flowing in the wire AB. When no current is flowing in the wire AB, then the needle of compass is parallel to the wire AB and points in its usual north-south direction [see Figure (a)].
Let us now close the switch so that an electric current passes through the wire AB. We will find that when current is passing through the wire AB, then the needle of compass is deflected from its original north-south direction and points in another direction [see Figure (b)]. We know that a compass needle is deflected only when it is acted upon by another magnet (or magnetic field).
So, the deflection of compass needle here shows that the current-carrying wire placed above it is behaving like a magnet and producing a magnetic field around it. It is the magnetism produced by current-carrying wire which is exerting a force on the compass needle and deflecting it from its usual north-south position.
If we now open the switch so that the current stops flowing in the wire AB, we will find that the compass needle comes back to its original position. This shows that when electric current flowing in a wire stops, then the magnetism produced by it disappears.
We have just studied that a straight wire carrying an electric current produces a magnetic effect. The magnetic effect is increased if we use a long coil of wire (instead of a straight wire). The magnetic effect is even further increased if the coil of wire is wound around an iron rod and then current is passed through it. The iron rod then becomes an electromagnet. Let us discuss the electromagnet now.
An electric current can be used to make magnets. The magnet made by using electric current is called an electromagnet. An electromagnet works on the magnetic effect of current. This is described below.
An electromagnet consists of a long coil of insulated copper wire wound around an iron rod (see Figure). When the two ends of the coil are connected to a cell, a current passes through the coil and produces a magnetic effect. This magnetic effect magnetises the iron rod. So, the iron rod becomes an electromagnet. A simple electromagnet is shown in Figure.
The magnetism of an electromagnet remains as long as the current is flowing in its coil. If we switch off the current in the coil (by opening the switch), all the magnetism of the iron rod disappears and it no longer behaves as a magnet. We can now define an electromagnet as follows : An electromagnet is a magnet consisting of a coil of insulated wire wrapped around a piece of iron that is magnetised only when electric current is passed through the coil.
Please note that only iron is used for making electromagnets. This is because when current is switched off in the coil of an electromagnet made of iron piece, then the iron piece loses all its magnetism. Steel is not used for making electromagnets because when current is switched off from the coil of an electromagnet made of a steel piece, the steel piece does not lose all its magnetism.
The steel piece retains the magnetism and becomes a permanent magnet. Another point to be noted is that just like ordinary magnets (called permanent magnets), an electromagnet can attract and hold only the objects made of magnetic materials such as iron, steel, nickel and cobalt. An electromagnet cannot attract objects made of non-magnetic materials such as wood, plastic, paper, copper and aluminium, etc.
To Make an Electromagnets
We can make an electromagnet ourselves as follows :Take about 75 centimetre long insulated copper wire (The insulated copper wire should be quite thin). Also take a large iron nail about 10 centimetre long. Wind the insulated copper wire round and round closely on the iron nail so that it may form a coil (see Figure).
The coil can be held on to the iron nail by putting some cellotape over it. Connect the free ends of the coil of wire to the two terminals of an electric cell through a switch (as shown in Figure). Switch on the current in the coiled wire by closing the gap in the switch.
Now, place some steel pins near one of the ends of iron nail. We will find that the steel pins cling to the iron nail (see Figure). The iron nail attracts the steel pins because the electric current flowing in copper wire coil (wrapped around it) has turned the iron nail into an electromagnet.
Now, switch off the current through the coiled wire by opening the gap in the switch. We will find that on switching off current through the coil, the steel pins get detached from the iron nail, they no longer cling to the iron nail. This means that when current is stopped from flowing in the coil of wire, the iron nail no longer remains a magnet. Since the iron nail loses its magnetism, therefore, the steel pins do not cling to it anymore.
Thus, the iron nail behaves as a magnet only when electric current flows through the coil of insulated wire wrapped around it. When the electric current flowing in the coil is stopped, the iron nail loses its magnetism. And if we switch on the electric current in the coil again, the iron nail will again become an electromagnet, and so on.
Please note that while testing the working of an electromagnet, we should switch on the current through the coil only for a few seconds at a time. If the current is kept flowing for a longer time at a stretch, then it may damage the cell quickly. Another point to be noted is that the electromagnets used in various devices and in industry work with current drawn from mains electricity (and not with electric cells).
Advantages of Electromagnets Over Permanent Magnets
An electromagnet is a temporary magnet because its magnetism is only for the duration of current flowing in its coil. Actually, an electromagnet is better than a permanent magnet in many respects. Some of the advantages of the electromagnets over the permanent magnets are as follows :
- The magnetism of an electromagnet can be switched on or switched off as desired. This is not possible with a permanent magnet.
- An electromagnet can be made very strong (i) by increasing the number of turns in the coil, and (ii) by increasing the current passing through the coil. On the other hand, a permanent magnet cannot be made so strong.
Uses of Electromagnets
Some of the important uses of electromagnets are given below :
1. Electromagnets are used in the construction of a large number of devices like electric bells, loudspeakers, electric motors, electric fans, toys and telephone instruments, etc.
2. Electromagnets are used to lift heavy loads like big machines, steel girders and scrap iron objects for loading and unloading purposes. The electromagnets which are used for loading and unloading heavy iron and steel objects are fitted on cranes.
3. Electromagnets are used to separate magnetic materials like iron and steel objects from a heap of metal scrap (or junk). Electromagnets, however, cannot be used to separate non-magnetic materials such as wood, plastic, copper and aluminium objects. For example, we cannot use an electromagnet for separating plastic bags from a garbage heap. This is because plastic bags do not stick to the electromagnet.
4. Electromagnets are used by doctors to remove tiny iron particles from the eyes of a person (which may have fallen into the eyes accidently).
All the electromagnets are not straight, bar type electromagnets. Actually, electromagnets are made in different shapes and sizes depending upon the purpose for which they are to be used. For example, an electric bell uses a U-shaped electromagnet for its working.
The U-shaped electromagnet is made by using U-shaped iron piece. In the U-shaped electromagnet , we have two coils of the same insulated copper wire wound on each side of the U-shaped iron piece (see Figure). We will now discuss an electric bell.
The electric bell works on the magnetic effect of current. It has an electromagnet in it. The electric bell uses an electromagnet to pull the clapper on to a metal bell (called gong). The dapper of electric bell is also known as hammer. We will now describe the construction and working of an electric bell in detail.
Construction of Electric Bell
The electric bell has a U-shaped electromagnet M (see Figure). A small iron bar A called armature is held in front of the poles of the electromagnet. The lower end of iron bar is attached to a flat spring S and the flat spring S is itself fixed to a metal bracket B (see Figure).
The upper end of iron bar has a clapper C attached to it. Initially, when the bell is not working, the clapper C is at a short distance away from the gong G. There is a contact screw D which just touches the iron bar A at point E. The contact screw D is also called an interrupter because it interrupts (or breaks) the circuit of electric bell periodically. A metal gong G (which is hollow like a bicycle bell) is fixed near the clapper.
The bell works with a battery T and a push-button switch P is provided to ring the bell. We will now describe the circuit of the bell. The end N of the electromagnet coil is connected to one terminal of the battery as shown in Figure, and the other end S of the coil is connected to bracket B carrying the iron bar (or armature). The other terminal of battery is connected to the contact screw D through the push-button switch P.
Working of Electric Bell
In order to ring the bell, we press the push-button switch P. When we press the switch, the electric circuit of the bell is completed and a current passes through the coil of the electromagnet and it gets magnetised. The electromagnet attracts the iron armature A towards itself. As the armature moves towards the poles of the electromagnet, the clapper C attached to it strikes the gong and produces a ringing sound. We say that the bell rings.
When the armature A moves towards the magnet, its contact with the contact screw D is broken at point E. Due to this the electric circuit breaks and no current flows in the electromagnet coil. The electromagnet loses its magnetism for a moment and the armature is no longer attracted by it. The flat spring S brings back the iron armature to its original position and the clapper also moves away from the gong.
As soon as the armature comes back and touches the contact screw at E, the circuit is completed and current starts flowing in the electromagnet coil again. The electromagnet attracts the iron armature once again and the clapper strikes the gong again producing a ringing sound.
This process of ‘make and break’ of the electric circuit continues as long as we are pressing the switch. Due to this, the armature vibrates forwards and backwards rapidly each time making the clapper strike the gong. Thus, the clapper strikes the gong rapidly producing almost continuous sound. And we say that the bell is ringing.