Magnetic Effect of Electric Current - Class 7, Electric Current and its Effects, Science (2023)

Question 1 What is an electromagnet? How can we make an electromagnet?

Question 2 Mention two advantages of electromagnet over a permanent magnet?

Question 3 Describe the construction and working of electromagnet?

Question 4 What is meant by magnetic effect of current?

Question 5 Why do we use iron for making an electromagnet and not steel?

Question 6 State few uses of electromagnet?

Contents

  • 1 Magnetic effect of Electric Current
    • 1.1 Activity: To demonstrate the magnetic effect of current
  • 2 Electromagnets
    • 2.1 Activity : To make an electromagnet
  • 3 Advantages of Electromagnets over Permanent Magnets
  • 4 Uses of Electromagnets
  • 5 Electric Bell
  • 6 Construction of Electric Bell
  • 7 Working of Electric Bell

Magnetic effect of Electric Current

When electric current passes through a wire, the current-carrying wire behaves likea magnet. This is called the magnetic effect of current.The magnetic effect of current isutilised 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. Oerstedfound that when electric current was passed in a wire, then the needle ofdeflected 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 electriccurrent flowing in a wire gives rise to a magnetic field around it. It is this magnetic field produced by thecurrent-carrying wire which exerts a force on the compass needle and deflects it (or moves it).

Activity: To demonstrate the magnetic effect of current

Magnetic Effect of Electric Current - Class 7, Electric Current and its Effects, Science (1)

(1) We take a thick, insulated copper wire AB and fix it on a table in such a way that this wire is in thenorth-south direction.

(2) A compass is placed under the wire AB. The two ends of thewire AB are connected to a cell through a switch by using connecting wires.

(3)The switch isopen, so no current is flowing in the wire AB. When no current is flowing in the wire AB, then the needle ofcompass is parallel to the wire AB and points in its usual north south direction

(4) Now close the switch so that an electric current passes through the wire AB.We will find thatwhen 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.

(5) A compass needle isdeflected only when it is acted upon by another magnet (or magnetic field). So, the deflection of compassneedle here shows that the current-carrying wire placed above it is behaving like a magnet and producinga magnetic field around it.

(6) It is the magnetism produced by current-carrying wire which is exerting a forceon the compass needle and deflecting it from its usual north-south position.

(7) If we now open the switch so that the current stops flowing in the wire AB, we will find that the compass needlecomes back to its original position. This shows that when electric current flowing in a wire stops, thenthe magnetism produced by it disappears.

Themagnetic effect is increased if we use a long coil of wire (instead of a straight wire). The magnetic effectis even further increased if the coil of wire is wound around an iron rod and then current is passedthrough it.

Electromagnets

Magnetic Effect of Electric Current - Class 7, Electric Current and its Effects, Science (2)

The magnet made by using electric current is calledan electromagnet. An electromagnet works on the magnetic effect of current.

An electromagnet consists of a long coil of insulated copperwire wound around an iron rod. When the twoends of the coil are connected to a cell, a current passes throughthe coil and produces a magnetic effect. This magnetic effectmagnetises the iron rod. So, the iron rodbecomes an electromagnet.

The magnetism of an electromagnet remains as long as thecurrent is flowing in its coil. If we switch off the current in thecoil (by opening the switch), all the magnetism of the iron roddisappears and it no longer behaves as a magnet.

An electromagnet is a magnet consisting of a coil of insulated wire wrapped around apiece of iron that is magnetised only when electric current ispassed through the coil.

Only iron is used for makingelectromagnets. This is because when current is switched off inthe coil of an electromagnet made of iron piece, then the iron piece loses all its magnetism.

Steel is not usedfor making electromagnets because when current is switched off from the coil of an electromagnet made ofa steel piece, the steel piece does not lose all its magnetism. The steel piece retains the magnetism andbecomes a permanent magnet.

An electromagnet can attract and hold only the objects made of magnetic materialssuch as iron, steel, nickel and cobalt. An electromagnet cannot attract objects made of non-magneticmaterials such as wood, plastic, paper, copper and aluminium, etc

Activity : To make an electromagnet

Magnetic Effect of Electric Current - Class 7, Electric Current and its Effects, Science (3)

(1)Take a long insulated copper wire (Theinsulated copper wire should be quite thin).

(2) Also take a largeiron nail.

(3) Wind the insulatedcopper wire round and round closely on the iron nail so thatit may form a coil.

(4) Connectthe free ends of the coil of wire to the two terminals of anelectric cell through a switch.

(5) Switchon the current in the coiled wire by closing the gap in theswitch. Now, place some steel pins near one of the ends of ironnail. We will find that the steel pins cling to the iron nail.

(6) The iron nail attracts the steel pins becausethe electric current flowing in copper wire coil (wrappedaround it) has turned the iron nail into an electromagnet.

Switch off the current through the coiled wire by opening the gap in the switch. We will find thaton switching off current through the coil, the steel pins get detached from the iron nail, they no longer clingto the iron nail. When current is stopped from flowing in the coil of wire, the iron nail nolonger remains a magnet. Since the iron nail loses its magnetism, therefore, the steel pins do not cling to itanymore.

Thus, the iron nail behaves as a magnet only when electric current flows through the coil ofinsulated wire wrapped around it. When the electric current flowing in the coil is stopped, the iron nailloses its magnetism.

Advantages of Electromagnets over Permanent Magnets

An electromagnet is a temporary magnet because its magnetism is only for the duration of currentflowing in its coil.

Advantages of the electromagnets

(1) The magnetism of an electromagnet can be switched on or switched off as desired. This is notpossible with a permanent magnet.

(2) An electromagnet can be made very strong

(a) by increasing the number of turns in the coil

(b) by increasing the current passing through the coil.

Uses of Electromagnets

(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 forloading and unloading purposes. The electromagnets which are used for loading and unloading heavy ironand steel objects are fitted on cranes.

(3) Electromagnets are used to separate magnetic materials like iron and steel objects from a heap ofmetal scrap (or junk).

(4) Electromagnets are used by doctors to remove tiny iron particles from the eyes of a person.

Electromagnets are made indifferent shapes and sizes depending upon the purpose for which they are to be used.

For example : Anelectric bell uses a U-shaped electromagnet for its working. The U-shaped electromagnet is made by usingU-shaped iron piece. In the U-shaped electromagnet, we have two coils of the same insulated copper wirewound on each side of the U-shaped iron piece.

Electric Bell

The electric bell works on the magnetic effect of current. It has an electromagnet in it. The electric belluses an electromagnet to pull the clapper on to a metal bell (called gong). The clapper of electric bell is alsoknown as hammer.

Construction of Electric Bell

Magnetic Effect of Electric Current - Class 7, Electric Current and its Effects, Science (4)

(1) The electric bell has a U-shaped electromagnet.

(2) A smalliron bar called armature is held in front of the poles of the electromagnet.

(3) The lower end of iron bar isattached to a flat spring and the flat spring is itself fixed to a metal bracket.

(4) The upperend of iron bar has a clapper attached to it. Initially, when the bell is not working, the clapper or hammer is at ashort distance away from the gong.

(5) There is a contact screw which just touches the iron bar at point. The contact screw is also called an interrupter because it interrupts (or breaks) the circuit of electricbell periodically.

(6) A metal gong (which is hollow like a bicycle bell) is fixed near the clapper. The bellworks with a battery and a push-button switch is provided to ring the bell.

(7) The end of the electromagnet coil is connected to one terminal of the batteryand the other end of the coil is connected to bracket carrying the iron bar(or armature).

(8) The other terminal of battery is connected to the contact screw through the push-buttonswitch .

Working of Electric Bell

In order to ring the bell, we press the push-button switch. When we pressthe switch, the electric circuit of the bell is completed and a current passes through the coil of theelectromagnet and it gets magnetised. The electromagnet attracts the iron armature towards itself. As thearmature moves towards the poles of the electromagnet, the clapper attached to it strikes the gong andproduces a ringing sound. We say that the bell rings.

When the armature moves towards the magnet, its contact with the contact screw is broken atpoint . Due to this the electric circuit breaks and no current flows in the electromagnet coil. Theelectromagnet loses its magnetism for a moment and the armature is no longer attracted by it. The flatspring 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, the circuit is completed and currentstarts flowing in the electromagnet coil again. The electromagnet attracts the iron armature onceagain and the clapper strikes the gong again producing a ringing sound.

This process of make and breakof the electric circuit continues as long as we are pressing the switch. Due to this, the armature vibratesforwards and backwards rapidly each time making the clapper strike the gong. Thus, the clapper strikes thegongrapidly producing almost continuous sound. And we say that the bell is ringing.

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