Magnet for standard 8 to 12 physics practical

Mechanical method of Magnet making 

Magnet making process

Mechanical method

Take an iron nail or a needle. Keep it horizontally on a table. Take a bar magnet. Rub the nail from any one end to the other (in only one direction), with any one pole of the bar magnet. Repeat this many times without changing the pole or the direction of rubbing. The nail becomes magnetised (fig ). This process of magnetising by rubbing with a magnet is called mechanical method.

Do it yourself

Take a clean test-tube and fill it with iron filings. Close its mouth with a rubber cork. Observe the arrangement of iron fillings. Keep any one pole of a bar magnet near one end of the test tube. Rub that test tube with this pole along its surface in one direction (fig 1.14(a)), Repeat the rubbing without changing the pole and direction of rubbing. Now observe the arrangement of iron fillings in the test tube. Bring one end of the test tube near the north pole of a magnetic needle. What do you observe ? (fig 1.14(b)(. Bring the same end of the test tube near the south pole of the magnetic needle. What do you observe ? Record your observations. Shake the test tube and again bring the ends of the test tube near the poles of the magnetic needle and observer. Explain your observations with reasons.

Electric Magnet

Making a Magnet by Electric method 

Electromagnet making process

Electronagnet

Electrical method

Wind a good length of insulated copper wire around a steel bar which is to be magnetised. Connect the ends of the wire to the poles of a battery. When the electric current passes through the copper wire the steel bar becomes a magnet. (fig). If a soft iron bar is used. It becomes a strong magnet temporarily. It loses its magnetism as soon as the current in the copper wire is switched off. Such temporary magnets are called electromagnets.

Electric Magnet:-
An electromagnet is a device in which magnetism is produced by an electric current.
British electrician, William Sturgeon invented the electromagnet in 1825. The first electromagnet was a horseshoe-shaped piece of iron that was wrapped with a loosely wound coil of several turns. When a current was passed through the coil; the electromagnet became magnetized and when the current was stopped the coil was de-magnetized. Sturgeon displayed its power by lifting nine pounds with a seven-ounce piece of iron wrapped with wires through which the current of a single cell battery was sent.
Sturgeon could regulate his electromagnet; this was the beginning of using electrical energy for making useful and controllable machines and laid the foundations for large-scale electronic communications. Five year later an inventor called Joseph Henry - made a far more powerful version of the electromagnet. American, Joseph Henry(1797-1878), demonstrated the potential of Sturgeon's device for long distance communication by sending an electronic current over one mile of wire to activate an electromagnet which caused a bell to strike. Thus the electric telegraph was born.

Magnet Keeper

Magnet Keeper

Horse shoe Keeper

Magnet Keeper

 

1.9         Protection of magnetiom

Magnetism is lost if a magnet is heated red hot or dropped from a considerable height or beaten with a hammer. Magnets must be kept in a specific manner to ensure retention of magnetism for a long time. Let us now learn the technique of preserving magnets.

First keep two bar magnets with a wooden piece in between them so that their opposite poles point the same direction. Then join the opposite poles at each end with soft iron pieces. These are called magnetic keepers (fig 1.18). Similarly horseshoe magnet or U-shaped magnets are also preserved by using keepers (fig 1.19)

Earth's magnetiom : Why dies a freely suspended bar magnet or a magnetic needle come to rest in N-S direction ? What is the force that keeps them in the particular direction ? Earth itself acts as a permanent magnet. It has its magnetic north pole near its geographic south and its south pole near its geographic north (fig 1.20). Hence any freely suspended magnet or magnetic needle comes to rest in north-south direction.

Since Earth acts as a magnet, it has its own magnetic field. One of the effects of Earth's magnetic field is beautiful aurora formation.

A piece of steel that has been magnetized can lose much of its magnetism by improper handling. If it is jarred or heated, there will be a disalignment of its domains resulting in the loss of some of its effective magnetism. Had this piece of steel formed the horseshoe magnet of a meter, the meter would no longer be operable or would give inaccurate readings. Therefore, care must be exercised when handling instruments containing magnets. Severe jarring or subjecting the instrument to high temperatures will damage the device.

A magnet may also become weakened from loss of flux. Thus when storing magnets, one should always try to avoid excess leakage of magnetic flux. A horseshoe magnet should always be stored with a keeper, a soft iron bar used to join the magnetic poles. By using the keeper while the magnet is being stored, the magnetic flux will continuously circulate through the magnet and not leak off into space.

When bar magnets are stored, the same principle must be remembered. Therefore, bar magnets should always be stored in pairs with a north pole and a south pole placed together. This provides a complete path for the magnetic flux without any flux leakage.

Magnetic lines of force

Magnetic lines of force

1.4         Magnetic field : Experiments have shownthat a magnet or a current carrying conductor can exert force on the nearby magnetic substances even when they are not in touch with each other. It implies that there is a magnetic force around a magnet or a current carrying conductor. The area in which magnetic force is present and magnetic effect is felt is called magnetic field.

Detection of the presence of a magnetic field.

Keep a magnetic needle in the area to be tested. If the magnetic needle deflects, then it means that there is a magnetic field.

Usually magnetic field is represented by a set of lines. These lines are called magnetic lines of force. Why is magnetic field represented by a set of lines ? Magnetic field or magnetic lines of force are invisible. The fact that the iron filings arrange themselves along a few curved lines (fig 1.12(b)) in a magnetic field might have resulted in the concept of lines of force. The magnetic lines of force can also be traced by a magnetic needle. The pattern of magnetic lines of force depends on the shape of the magnet and its strength. Therefore the pattern of magnetic lines will be different for different magnets. The study of magnetic lines of force around a powerful bar magnet reveals the following properties.

1.      Magnetic lines of force are relatively compact wherever the field strength is strong (near poles) and less in number wherever the field strength is weak.

2.      Magnetic lines of force are spread from pole to pole. The convention is to show them as starting from north pole and ending at south pole.

3.      Magnetic lines of force keep pubshing each other away and never corss each other.

4.      Magnetic lines of force appear to possess tnsion as in a bent cane held pressed in the from of an are.

5.      Magnetic lines of force can penetrate substances like paper, glass etc.

6.      The field strength decreases as one mover away from the source causing it.

Do it yourself

Keep a bar magnet on a table. Keep a rectangular card board horizontally on the magnet. Sprinkle iron fillings uniformly on the card board. Gently tap the cardboard. Gently tap the cardboard. Observe the arrangement of iron fillings on the cardboard. Draw a figure of what you observe.

Magnetic substances

The substances that are attracted by a magnet are called magnetic substances. Example : Iron, nickel, cobalt, steel etc. 

Nonmagnetic substances : The substances that are not attracted by a magnet are called nonmagnetic substances. Example : Paper, glass, water, common salt, plasti etc.

In a sense, all substances have magnetic properties. Howerer the degree of magnetism exhibited by them differs. Based on their properties when brought under the influence of powerful magnetic field, They are classfied into three groups – Diamagnetic, Paramagnetic and Ferromagnetic substances.

Do You Know ?

Experiments have revealed that the ability of paramagnetic substance to get magnetised goes on increasing as we go on cooling it. This abillity goes on decreasing as its temperature increases and at one specific temperature it altogether vanishes, Can you guess why ?

Magnetic field and magnetic lines of force : Space around a magnetic pole or magnet or current carrying wire within which it's effect can be experienced is defined magnetic field. Magnetic field can be represented with the help of a set of lines or curves called magnetic lines of force.

General Properties of Magnetic Lines of Force
Magnetic lines of force have a number of important properties, which include:
They seek the path of least resistance between opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loops from pole to pole.
They never cross one another.
They all have the same strength.
Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability.
Their density decreases with increasing distance from the poles.
They are considered to have direction as if flowing, though no actual movement occurs.
They flow from the south pole to the north pole within a material and north pole to south pole in air.

Electricity and Electric meter

Electricity

Electrical energy can be easily converted into heat, light and other forms of energy. During such conversions the quantity of energy neither increases nor decreases. This phenomenon is called 'conservation of energy'. The effects of electric current are used for different purposes. Make a list of the applications of electricity that you see in your surroundings. Mention the energy transformation involved in each application.

Do you know ?

Gilbart was the first person to pur forward the idea of Earth as a magnet. To test this ieea he prepared a small sphere our of magnetite ore and called it Terrella or little earth. When a magnetic needle was kept on terrella. It always used to come to rest in a particular direction. On this basis he declared that there was only one difference between his Terrella and Earth. “The Earth is bigger, my Terrella is small”. It is a wonder how magnetism exists even at the high temperature in the interior of Earth.

Units of measurement of electriity in the S.I. System is 'Joule'. Multiple unit called kilojoule is also used. One kilojoule is equal to one thousand joules.

1 kJ = 1000J

Electric power

Electricity can heat a coil of wire, run machines and make bulbs emit light. All these prove that electricity can do work. Electric power is the rate at which electricity is made use of.

Electric Power = Electrical energy used / time

If a device uses E joules of current in 't' seconds, then power (P) is expressed as follows.

P = E / t

Unit of electric power : The S.I. Unit of electric power is watt. If a device uses electricity at the rate of 1 joule per second. Then its power is a watt.

I watt = 1 joule / second

1 w = 1 J/s

Watt is a small unit. Hence a multiple unit, kilowatt is used. One kilowatt is equal to one thousand watts.

1 kW = 1000 W

= 1000 J / s

Example : If a bulb uses 1500 joules of current in 25 seconds, what is its power ?

P= E / t

= 1500 / 25

= 60 watt.

Electric Meter

Practical unit of electricity.

Joule and kilojoule are not conventent units for practical purposes. Therefore kilowatt-hour (kWh) unit is used to measure electricity for commercial purposes. Electricity that we use in factories and our homes is measured with this unit. Let us now understand what one kilowatt hour means.

Let us assume that we are using 1000 Joules of electricity every second. If we continue to use electricity at the same rate for one hour. 


The quantity of electricity used is one kilowatt-hour. One kilowatt-hour is one unit of current.

1 kilowatt-hour = 1 kilowatt x 1 hour.

= 1000 watt x 1 hour

= 000 J/s x 3600 s

= 360000 joules.

Effects of electric current

The flow of current can causes the following effects on substances.

1.   magnetif effect.

2.   chemical effect.

3.   heating effect.

Magnetic effect of electric current

Whenever an electric current flows through a conductor a magnetic field is created around the current-carrying conductor. This was discovered by Henry Christian Oersted. Magnetism and electricity go hand in hand. One does not exist without the other. We now know that magnetic property of some substances is due to the movement of electric charges in their atoms.

Chemical effect of electric current

Whenever an electric current flows through some solutions, they undergo chemical changes. This is called the chemical effect of electric current. Electric current does not flow through some substances. They are insulators. It flows through some substances only when they are in liquied form or in solutions : Sodium chloride. Copper sulphate, suphuric acid etc. Such compounds are called electrolytes. Those compounds through which the electric current does not pass either in aqueous solution or in liquid form are called non-electrolytes : sugar, glucose, castor oil etc.

Heating effect of electic current : 
Whenever an electric current flows through a wire. A small part of electrical energy is converted into heat energy. This is called heating effect of electric current. The wires made of certain substances produce relatively more heat than others of the same dimension. This effect of electric current is utilised in appliances like electric stove, boiler, electric fuse, electric iron, electric bulb, water heater etc.

Electric Fuse

Fuse

1.15       Electric fuse

Fuse is a protective device which works on the heating effect of electric current. It is a wire made up of an alloy of lead and tin. This alloy has high resistance and low melting point. Fuse is kept in a ceramic box at the beginning of an electric circuit. (fig.)

Electric circuits are designed to carry electric current of a definite strength. If the strength of electric current crosses the safe level, the circuit and the instruments in the circuit may get damaged. In such situations the fuse-wire provides the neede protection. Fuse-wire melts and breaks the circuit as soon as the strength of electric current in the circuit crosses the safe level and thus prevents possible damage.

Electric bulb : 
The bulbs that we use function on the basis of heating effect of electric current. A tungsten filament in the form of a coil is mounted on two copper leads in the bulb. When electric current is passed through the coil it heats up and emits light. Nitrogen is filled in the bulb to give it a long life. 

Activity : 

Make a list of electrical gadgets used in your house. Find out the power of each of them.

Dry cell, Source of Electricity

Dry cell

1.16       Sources of electricity : 
We get electricity from a variety of methods. We generate elecricity by running electric generators with the help of water, wind and steam energy. Solar energy is converted into electricity by solar cells. Cells and batteries are also the sources of electricity These batteries convert chemical energy to electrical energy.

Do you know ?

Tube lights use less electrical energy and emit more light than bulbs of same power Less heat is produced in tube lights. Hence the wastage of electricity is less in these lighs. Now-a-days electric lamps based on CFT are in use. They work well even when the voltage is low. Such bulbs are useful in saving electricity. (CFT : Compact Fluoresent Tube)

Electric Cell : 
Electric cell is a device that converts chemical energy into electrical energy. This is called a generating cell. It consists of two elecrodes kept in a suitable electrolytic mixture. One electrode becomes positive whereas another becomes negative. When these electrodes are connected by a conducting wire. Electrons flow from negatives to positive. But the flow of current is indicated from positive to negative.

Dry Cell  : 
A dry cell  consists of a graphite rod erected at the centre of a zinc can. A layer of a mixture of carbon powder and manganese dioxide surrounds the graphite rod. The space between the layer and the walls of zinc can is filled with a paste of ammonium chloride and zine chloride. Zinc can is sealed with resin. The top visible end of graphite rod is fitted with a brass cap. Graphite rod acts as positive pole and zinc can acts as negative pole. Ammonium chloride is the electrolyte. When the dry cell is connected to a circuit the current flows from the positive pole to the negative pole. The effective life of a dry cell is very short. Now-a-days, long lasting cells are being used. Nickel-Cadmium cells are examples of such cells.

Pole of the earth.

Do you know?

Earth

1.Magnetic south pole of the earth.

2.Geographical north pole of the earth.

  The earth behaves as a magnet. Hence it must also have north and south poles. However the geographical North and South poles as well as  magnetic north and south poles are not at the same place. In 1832 it was discovered that the earth’s magnetic south pole was situated over the north of  Hudson Bay in Canada. Magnetic North pole is situated to the south of Australia. Therefore the axis of rotation of the earth and the axis through the magnetic poles of the earth are not the same.

 

Do it your self

Your are given a piece of iron and a magnet which look identical. With the help of a compass needle identify the magnet. Mark it’s north and south poles is attraction a complete test for magnetism? Justify.

Think it over

What will be the position of the freely suspended magnet in a region over the earth’s magnetic pole?

Properties of a magnet

Important properties of a magnet

In the earth’s magnetic field, a freely suspended magnet comes to rest in North-South direction provided there is no other magnet around it.

There exists a mutual force of attraction between a magnet and magnetic substance.

Every magnet however big or small has two poles, which cannot be isolated.

Unlike poles attract each other and like poles repel each other.

Both the poles of a magnet have equal pole strength.

The strength of a magnet near the poles is more.

 

Magnetic lines of force Practical for standard 8 to 12

 Experiment 1:   
Magnetic lines of  force around a bar magnet. 

Aim :- 
To draw the magnetic lines of force around a bar magnet.

Materials required :- 
Drawing board, brass pins, a sheet of paper, a bar magnet, a magnetic compass, a pencil and a scale. 

Procedure :-  Fix the paper on the drawing board using brass pins so that there are no wrinkles. Place the compass in the middle of the sheet and mark the North-South direction by putting dots against the poles. Remove the compass and draw a straight line through these dots.

Note :- 
Do not disturb the drawing board till the experiment is complete. 

 

Northe South direction

Magnetic lines of force around magnet

Special instruction

The compass may not show any  specific direction at certain places around the magnet and may behave indifferently. Locate such points. (neutral points).

On the basis of your observations in the above experiment state whether the following  statements are
true or false. Correct if necessary.

1.The magnetic lines of force start at one pole and end at the other.

2.The magnetic lines of force do not intersect each other.

3.The pattern of the magnetic lines of force depends on the position of the magnet with respect to the geographical poles of the earth.

Remember

 The magnetic lines of force start at one pole and end in the   other. But conventionally the lines of force are considered   to start at the north pole and end in the south pole.

 The magnetic lines of force do not intersect each other.

Magnetic field : 

Experiments have shownthat a magnet or a current carrying conductor can exert force on the nearby magnetic substances even when they are not in touch with each other. It implies that there is a magnetic force around a magnet or a current carrying conductor. The area in which magnetic force is present and magnetic effect is felt is called magnetic field. 

Detection of the presence of a magnetic field.
Keep a magnetic needle in the area to be tested. If the magnetic needle deflects, then it means that there is a magnetic field. Usually magnetic field is represented by a set of lines. These lines are called magnetic lines of force. Why is magnetic field represented by a set of lines ? Magnetic field or magnetic lines of force are invisible. The fact that the iron filings arrange themselves along a few curved lines (fig 1.12(b)) in a magnetic field might have resulted in the concept of lines of force. The magnetic lines of force can also be traced by a magnetic needle. The pattern of magnetic lines of force depends on the shape of the magnet and its strength. Therefore the pattern of magnetic lines will be different for different magnets. The study of magnetic lines of force around a powerful bar magnet reveals the following properties.