Magnetism

Magnetism is a mysterious force that affect some material such as iron, steel cobalt and nickel (ferromagnetic materials).

Properties of Magnets

1. Magnetic materials: Magnets attracts strongly certain materials such as iron, steel cobalt and nickel (ferromagnetic materials).

2. Magnetic poles: The poles are the places in magnet to which magnetic material are attracted, e.g. iron filings.  They are near the ends of bar magnet and in pair of equal strength.

3. North and south poles

When a magnet is freely suspended so that it can swing in a horizontal plane, it comes to rest in N-S direction.  N pole (north seeking pole) points towards the Earth's north pole, and S pole (south seeking pole) points towards the Earth's south pole.  A magnet can therefore be used as a compass.

4. Law of magnetic poles (First law of magnetism): If the N pole of a magnet is brought near the N pole of another magnet repulsion occurs.  Two south poles also repel.  N and S poles always attract.  The law of magnetic poles summarizes these facts as follows:

"like poles repel. unlike poles attract"

The force between two magnetic poles decreases as their separation increases.

5. Magnetic substances which can be magnetized or attracted by a magnet are : iron,  steel  cobalt and nickel.  Non-magnetic materials are others like copper, brass, aluminium, wood, glass, etc.  Magnetic material may be classified into two types:

· Hard magnetic materials: such as steel and alcomax (steel like alloy), are difficult to magnetize but do not readily lose their magnetism.  They are used to make permanent magnet.  Permanent magnets used in some medical instruments, measuring instruments, magnetic tapes, etc.

· Soft magnetic materials: such as iron and mumetal (nickel based alloy) are relatively easy to magnetize but their magnetism is only temporary.  They are used in electromagnets.  In this case they remain magnetised only as long as a current is passing through a surrounding coil.  Unlike permanent magnets can be switched on and off.  Electromagnets used in electric motors, electric bell , relays, etc.

6. When a magnet is broken into pieces, each piece is found to be a magnet with two poles.

How to magnetize a steel bar

A. By induction:

When a magnet approaches a magnetic material, it becomes magnetized. The near end of the bar acquires a polarity opposite to that of the magnet.

B. By stroking with a magnet:

The steel bar is stroked by a strong magnet many times in the same direction. The pole produced at the end of the stroke is opposite to the magnetizing pole.

C. By electrical method:

Place the steel bar inside a long solenoid, when a direct current flows in the solenoid, the steel bar becomes magnetized.  The strength of magnetization of the bar increases by:

(i) increasing the current, and

(ii) increasing the number of turns of the solenoid.

The polarity produced can be found by right-hand grip rule: "when the fingers of the right hand turn in the direction of the current, the thumb will point towards the North pole".

Magnetic Properties of Iron and Steel

· The induced magnetism in soft - iron is greater than that. in steel.

· When the magnetizing pole is removed, soft iron loses its magnetization while steel retains its magnetization

Magnetic Field

It is the space surrounding a magnet in which a magnetic force is exerted.  The magnetic field is a vector quantity i.e. it has a magnitude and direction.  The magnetic field is represented by the magnetic flux lines.  The magnetic flux lines or lines of force represent the magnitude and direction of the magnetic field.

Note That :

· The lines are close together where the field is strong.

· The lines go from the north pole to the south pole.

· The lines never cross each other .

The force around a bar magnet may be detected and shown to vary its direction using the apparatus as shown in the figure.  If the floating magnet is released near the N pole of the bar magnet, it is repelled to the S pole and moves along a curved path known as a line of force.

Plotting Magnetic Field Lines(line of force)

A. Using a Plotting Compass

A plotting compass is a small pivoted magnet in a glass case with non magnetic metal walls as shown in the figure

1. Place the magnet on a sheet of paper and trace around it.

2. Place the plotting compass at point such as point "A" near one pole of the magnet as shown in the following figure.

3. Starting at that point mark the position of the N and S poles of the needle of the compass by a pencil dots

4. The compass is then moved until the near end of needle is exactly over the dot furthest from the magnet and another dot is marked at the other end of needle.

5. Repeat this process many times until the compass reaches the other end of the magnet, and draw a line through all points.

6. Other lines of magnetic force are drawn in the same way.

7. Mark the line with arrows indicating the direction of the North pole of the compass.

This method is sensitive and suitable for weak fields, but it is not suitable for fields rapidly changing in a short distance.   A typical field pattern is shown in the following figures

Magnetic field lines around a bar magnet

Fields due to two neighbouring magnets

B. Using a Iron Fillings

1. Place the bar magnet beneath a stiff sheet of white paper.

2.  Sprinkle a thin layer of iron filings on the paper

3. Tap the paper gently with a pencil and notice what happens to filling

4. The filings become magnetized by induction, when the paper is tapped gently, they move and turn in the direction in the magnetic field

Field lines round two bar magnet

This method does not show the direction of the field; also it is not sensitive it is used with strong fields only.  The directions of magnetic field lines are determined by using a plotting compass.

Earths magnetic field

If the lines of force are plotted on a sheet of paper with no magnets near, a set of parallel straight lines is obtained.  They run roughly from S to N geographically as shown in the following figure.  They represent a small part of the earth's magnetic field in a horizontal plane.

North

Magnetic Field lines of the earth

At most places on the Earth's surface a magnetic compass points slightly east or west of true north, . the Earth's geographical and magnetic north poles not coincide. The angle between magnetic north true north is called the declination as shown in the following figure.  In London at present (2000) it is 6° W of N and, decreasing.  By about the year 2140 it should be 0^o.

Bar magnets in the Earth' s magnetic field

In practice, two magnetic fields are present around most bar mag nets -one due to the magnet itself, and one due to the Earth.  Close to a small bar magnet, the field due to the magnet is usually so strong that the effect of the Earth's magnetic field can be ignored.  Half a metre or so away from the magnet however, this isn't the case, and a small plotting compass is as strongly influenced by the Earth's magnetic field as it is by the field due to the magnet.

The following figures show the combined magnetic fields produced in two cases; one with the N pole end of a bar magnet pointing north, and one with it pointing south.  At each of the neutral points, X, the Earth's magnetic field exactly cancels the field due to the magnet.

Demagnetization of a magnet

Magnet may be demagnetized by one of the following methods:

1. Place the magnet in a solenoid carrying A.C. current and directed in East-West direction, then withdraw the magnet slowly from the solenoid to a far distance.

2. Hammer the magnet strongly (in E- W direction also )

3. Heat the magnet excessively to redness

Questions

1. A magnet attracts:

A) plastics                         B) any metal        C) iron and steel

D) aluminium                   E) carbon.

2. Copy the following figure which shows a plotting compass and a magnet. Label the N pole of the magnet and draw the field line on which the compass lies.

3. The three diagrams in the following figure show the lines of force (field lines) between the poles of two magnets.  Identify the poles A, B, C, D, E, F.

4. a) The following figure shows the magnetic field between two magnets.

(i) Copy the diagram and label the other poles of the magnets.

(ii) Which is the weaker magnet?

b) A child has accidentally swallowed some small metal objects. These objects have stuck in the child's throat. A doctor uses the tool in Figure 51.12 to remove them.

(i) What happens to the fixed iron tip when the permanent magnet is moved towards it?

(ii) Why is it an advantage to make the tip out of iron?

(iii) The tool is pushed down the child's throat and guided to the metal object. Explain why the sheath needs to be flexible.

(iv) The doctor tries to use the tool to remove a steel paper clip and an aluminium washer from the child's throat. Which object can the doctor remove? Explain your answer.

5. What is meant by the 'N pole' of a magnet?

6. The following figure shows three metal bars. When the ends of different bars are brought together, it is found that A and B attract. A and C attract, but C and D repel Use this information to deduce whether each of the bars is a magnet or not.

7. Name three ferromagnetic materials.

8. Give two ways in which a ferromagnetic material can be magnetized.

9. What is meant by:

a) a soft                       b) a hard ferromagnetic material?

Give an example of each type. Which type would be used to make a permanent magnet?

10. Give two way' in which a magnet can be demagnetized

11.  Copy and complete the following figure to show

a) the N and S pole, of the magnet

b) which way the N pole end of the compass, needle would point

c) which way a free S pole at point O would move

12. Give two ways in which it would be possible to find the field pattern round a strong bar magnet.  Which of these methods is suitable for a weak magnetic field, and why?

13. Copy and complete the following figure to show magnetic field line, and any neutral point. What is meant by a neutral point?

14. In London the angle of declination is about 7^o. What does this mean?

Dr. Adel A. El-samahy