X Physics - Chapter 17 - Magnetism and Electromagnetism

Magnet
Metals like iron, nickel and steel attract each other magnetically. They are called magnets and always point in a particular direction when suspended freely in the air.




Non-Magnetic Substances
Substances that are neither attracted nor repelled by a magnet are called non-magnetic substances. Examples are wood, glass and paper.




Ferromagnetic Substances
A substance which behaves like a magnet in the presence of a strong magnetic field is called a ferromagnetic substance..


1. Hard Ferromagnetic Substances

The ferromagnetic substances which retain their magnetism when removed from the magnetic field are known as hard ferromagnetic substances. Example is steel.


2. Soft Ferromagnetic Substance

The ferromagnetic substances which become magnets in the presence of a magnetic field and lose their magnetism when removed from the magnetic field are known as soft ferromagnetic substance. Example is soft iron.




Magnetic Field
The space surrounding a magnet in which its magnetic effect is felt is called a magnetic field. It is the region within which the magnet can exert its magnetic force.




Methods of Making Magnets
1. Single-Touch Method
Take a hard steel bra and rub it with one end of a magnet in the direction from S to N, keeping the magnet in an inclined position. On reaching the end N of the steel bar, bring the same end of the magnet to the end S of the steel bra and rub it again. Repeat the process several times and the steel bar will be magnetized. The end S will have the same polarity as that of the rubbing pole of the magnet and the end N will have the polarity opposite to that of the rubbing pole.




2. Electrical Method
Take a U-shape steel bar and wound it with an insulated copper wire making sure that the two core arms are wound in the opposite directions. Connect the coil to a battery and pass strong current. The steel bar becomes a magnet as long as current passes through them. In a similar way, a bar can be magnetized by putting it inside a solenoid and passing current through the solenoid. The polarity of the magnet is determined by the direction of the current.


Demagnetization
There are three methods for demagnetizing magnets.

1.Hammering
Magnets can be partially demagnetized by hammering them when they are pointing in the east or west direction.

2. Heating
Magnets loose their magnetism when they are heated strongly.

3. Electrical Method
The most efficient method of demagnetizing a magnet is to use n alternating current. Take a solenoid and place it in the east west direction. Pass an alternating current (about 12 V) through it. Now, put the magnet in the solenoid from one end and pull it out from the other. While the current is still flowing. The magnet will loose its magnetism.

Alternating current reverses its direction at a rate of 100 times per second and hence causes the magnetism of the material to reverse the polarity at the same rate. Due to this rapid reverse in the polarity, the magnet looses its magnetism.




Magnetic Effect of Current
When an electric current passes through a straight wire a magnetic field is created which consists of field lines in concentric in concentric circles with the wire at their center.




Right Hand Rule
The direction of the magnetic field can be determined by the following rule:

"Imagine the wire to be grasped in the right hand with the thumb pointing along the wire. The direction of the fingers will give me direction of the magnetic lines of force."




Solenoid
A coil of insulated copper wire in the form of a long cylinder is called a solenoid.


Magnetic Field of a Solenoid

When an electric current is passed through a solenoid a magnetic field is produced which is very similar to that of a bar magnet. One end of the solenoid acts as the north pole and the other as the south pole. The magnetic field inside a solenoid is very strong because the lines of force are parallel and close to one another. The magnetic field outside the solenoid is very weak.




Electromagnet
If soft iron is inserted in the core of a solenoid, the magnetic field due to the current in the solenoid is multiplied by thousands. When the current is switched off, the magnetic field disappears. Such a magnet which can be energized by an electric current is called an electromagnet.




Applications of Electromagnets
Industry

They are used to transport heavy pieces of iron and steel safely from one place to another. With the help of electromagnets, iron from mixture is separated.

They are used to produce strong magnetic fields for high power motors and generators.




1. Electric Bell
Construction

An electric bell consists of an electromagnet. One end of the winding is connected to a terminal (T1). The other end is connected to a spring, which is mounted on a soft iron strip called "Armature." A rod is attached to the armature with its free end having a small hammer that can strike against the bell. a very light spring is attached to a contract adjusting screw which is joined to the second terminal (T2) by a wire. The electric circuit is completed by connecting the terminals to a batter and a switch.

(Diagram)


Working

When the push button switch is pressed, the circuit gets closed and the armature is attracted towards the electromagnet. The spring also gets detatched from the screw. This results in opening the circuit and the electromagnet gets demagnetized. The attraction disappears bringing back the spring to its original position. As soon as the spring touches the screw, the circuit gets closed and the magnet starts to work. It again attracts the armature and this process is repeated as long as the switch is turned on. As a result, the armature vibrates and hammer attached to it strikes the gong. Hence, the bell rings.




2. Telephone Receiver
Introduction

A telephone receiver is a device that converts electrical energy into sound energy.


Construction

The ear piece consists of a permanent magnet in contrast with two electromagnets. A diaphragm of magnetic alloy is positioned in front of the electromagnets.


Working

When the message is transmitted from the other apparatus, sound energy is converted into electric current and is transported to the ear piece through the line. This electric current varies in magnitude depending upon the frequency of the sound waves. In the telephone receiver, the current passes through the electromagnet and energizes the magnet. In this way, the magnetic field strength varies as the current changes. The magnetic force that pulls the diaphragm also varies accordingly. The diaphragm therefore vibrates and gives rise to sound of the same frequency as spoken at the other end.




Fleming's Left Hand Rule
"Place the fore finger and the second finger of the left hand at right angles. Then, if the fore finger points in the direction of the magnetic field and the second finger in the direction of the current, then the thumb will point in the direction of the motion."


Galvanometer
Introduction

A galvanometer is a sensitive and delicate device used to measure the magnitude and direction of small currents.


Principle of Galvanometer

The principle of Galvanometer is based on the interaction of the magnetic field produced by a current forcing in a conductor and the magnetic field of permanent magnet. In this instrument, electrical energy is converted into mechanical energy.


Construction

A rectangular coil of wire is wound on a light frame with a pointer attached on the top. The coil frame is pivoted between the jaws of a large horseshoe magnet. At both ends of the coil, hairsprings are attached. These springs help in keeping the coil at zero potential and also provide the path for entry and exit to the current. A soft iron cylinder is fixed in the core of the coil to enhance the force of conductor. The concave shape of the poles of the horseshoe magnet combined with the cylindrical shape of the core creates the radial field to ensure that the field lines are always perpendicular to the coil.


Working

When current passes through the coil a couple of opposite forces are produced and causes the coil to rotate. By the motion of the coil, pointer moves on the scale and galvanometer is used to determine the magnitude and direction of current.




Ammeter
Introduction

A galvanometer having a low resistance in parallel is called as ammeter. It is used to measure current. The low resistance connected in parallel is called shunt.


Working

When current is passed through a Galvanometer, its coil is deflected and pointer attached with the coil moves over a scale. The range for the measurement of current in a galvanometer is very small. Therefore, a low resistance in parallel is used with a galvanometer. This resistance by passes a great part of the current. Only a small amount of current passes through the galvanometer coil, which is within the range of the galvanometer. This resistance acts as a shunt. An ammeter is always placed in series with other circuit components through which current is to be measured.




Voltmeter
Introduction

A galvanometer having high resistance in series is called a voltmeter. It is used to measure potential difference.


Working

The potential difference across a resistance is directly proportional to the current passing through it. As the deflection of the pointer is directly proportional to the current, therefore the deflection of the pointer is directly proportional to the potential difference. A small potential difference produces a full-scale deflection in a galvanometer. In order to measure high potential difference, a high resistance is connected in series with the galvanometer. Most of the potential difference drops across the high resistance. The value of resistor connected in series depends upon the range of the voltmeter. In order to measure the potential difference, a voltmeter is always connected in parallel to the circuit components.