Magnet and Magnetic Field SS3 Physics Lesson Note

CONTENTS

• Magnets and their properties
• Magnetization and demagnetization
• Temporary and permanent magnets
• Theory of magnetization
• Magnetic flux
• Earth magnetic field

MAGNET AND ITS PROPERTIES

A magnet is any material that is capable of attracting other pieces of the same material as well as pieces of iron. A substance is said to be ferromagnetic if it is attracted by a magnet. Examples are iron, cobalt, Nickel, and certain alloys. Substances which cannot be attracted by a magnet are called non-magnetic materials e.g. brass, wood, copper, and glass.

Properties of magnets

1. The ends of a magnet where the attracting power is greatest are called the poles.
2. A bar magnet suspended freely in a vertical plane called a magnetic meridian comes to rest with its axis in the North-South direction. The part which points northwards is called the north-seeking pole or North Pole while the opposite pole is called the South Pole
3. Like poles of a magnet repel each other while unlike poles attract each other.
4. The polarity of a magnet can be tested by bringing both poles in turn nearer to the known pole of a suspended magnet. Repulsion indicates similar polarity. The attraction could be due to two unlike poles or a pole and a piece of unmagnetised material. Hence, repulsion is the only sure test for polarity.

MAGNETIZATION AND DEMAGNETIZATION

Magnetization is a process whereby a material is made to become magnetic. This can be achieved through any of the following methods:

1. Electrical method – A cylindrical coil wound with several turns of insulated copper wire is connected in series with a six or twelve-volt electric battery and switch. A coil of this type is called a solenoid. A steel bar is placed inside the coil and the current is switched on for some time. On removing and testing the steel, it will be found to have been magnetised. It is unnecessary to leave the current for long as the length of time makes no difference but causes overheating. The induced polarity depends on the direction of flow of the current. A clockwise flow at an end indicates the South Pole while an anti-clockwise flow indicates North Pole.
2. Single touch method – A steel bar is struck from end to end several times in the same direction with a known pole of a magnet. Between successive strokes the pole is lifted high above the bar otherwise the magnetism already induced will be weakened. The disadvantage of this method is that it produces magnets in which one pole is nearer the end of the bar than the other.
3. Divided touch method – Here the steel bar is stroked from the centre outward with unlike poles of two magnets simultaneously. The polarity produced at the end of the bar where the stroking finishes is of the opposite kind to that of the stroking pole.
4. Hammering in the earth field – Magnets can be made by hammering a red hot steel bar and allowing it to cool as it lies in the North-South direction.
5. Induced Magnetism – When a piece of unmagnetised steel is placed either near or in contact with a pole of a magnet and then removed, it will be magnetised. This is called induced magnetism. The induced pole is of the opposite sign to that of the inducing pole.

 DEMAGNETIZATION

This is a process whereby a magnet is made to lose its magnetism. Demagnetization can be achieved by:

• Electrical Method – The magnet is placed in a solenoid through which an alternating current is flowing. The solenoid is placed with its axis pointing in the East-West direction. After a few seconds, the magnet is slowly withdrawn out of the solenoid to a long distance away. This is the most efficient way of demagnetizing a magnet.
• Mechanical Method – Another method of demagnetizing magnets is to hammer it hard when it is pointing in the East-West direction.
• Heating Method – When magnets are strongly heated, they lose their magnetism.

TEMPORARY AND PERMANENT MAGNET

Soft iron is pure iron while steel is an alloy of iron and carbon. Steel is a much harder and stronger material than soft iron. Steel and iron have different magnetic properties.

Iron is easily magnetised than steel but it readily loses its magnetism. Steel produces a stronger magnet which is the reason why steel is used for making permanent magnets such as compass needles. In temporary magnets where the magnetism is required for a short

MAGNETIC FIELDS

Magnetic field is the space surrounding the magnets in which magnetic force is exerted. It is a vector quantity and it is represented by magnetic lines. The direction of the magnetic flux at any point is the direction of the force on a north pole placed at that point.

In the neighbourhood of two magnets placed close together, there exists a field in which the direction of the magnetic flux changes rapidly in a confined space. The magnetic flux can be obtained by using iron filings.

A magnetic meridian at any place is a vertical plane containing the magnetic axis of a freely suspended magnet at rest under the action of the earth field.

The geographical meridian at a place is a plane containing the place and the earth’s axis of rotation.

The angle between the magnetic and geographical meridian is called the magnetic declination.

The angle of dip or inclination is the angle between the direction of the earth’s magnetic flux and the horizontal.

CLASSWORK

1. With the aid of a suitable diagram, explain the following: magnetic flux, angle of inclination, and angle of declination.
2. Differentiate between steel and iron concerning magnetism.

ASSIGNMENT

SECTION A

1. Which of the following statements is CORRECT about the Earth’s magnetic field? (a) The angle of dip is the angle which a freely suspended magnet makes with the vertical (b) the angle of declination is the angle between the magnetic meridian and the geographic meridian (c) the angle of inclination is the difference between the angle of dip and the angle of declination (d) the angle of inclination is the angle which a magnetic compass makes with the magnetic meridian
2. If the angle of declination in a place is 00, calculate the true geographic bearing if the compass needle reads N400E (a) N500E (b) N400E (c) N300E (d) N250E
3. A magnetic substance can be demagnetized by? (a) Dropping on the floor (b) hammering while red hot (c) divided touch (d) single touch
4. A freely suspended needle compass needle on Earth’s surface will come to rest in a plane called (a) geographic equator (b) geographic meridian (c) magnetic equator (d) magnetic meridian
5. In order to make a moving electron follow a circular path (a) a magnetic field is applied perpendicular to its path (b) a magnetic field is applied parallel to its path (c) an electric field is applied parallel to its path (d) an electric field is applied perpendicular to its path

SECTION B

1. (a) Explain what is meant by a magnetic field 

• describe an experiment to show that a magnetic field exists around a straight wire carrying current
• draw a diagram, to show the pattern and direction of the magnetic field produced around the wire [neglect the earth’s magnetic field]
• Sketch the form of the flux pattern due to two straight parallel wires carrying current in the same direction. Indicate the neutral point in the field 
• Explain, with the aid of a diagram, how a delicate material could be protected from the Earth’s magnetic field

1. Explain the term “angle of dip”, and describe how it varies over the earth
2. A charge of 1.6×10-19C enters a magnetic field of flux density 2.0T with a velocity of 2.5×10-19m/s at an angle of 300 with the field. Calculate the magnitude of the force exerted on the charge by the field.