Magnet and Magnetic Field – SS3 Physics Lesson Note (First Term)

A magnet is a material that attracts iron and other magnetic materials, with its pull strongest at its ends, called the poles.

Behavioural objectives

1. Define a magnet and state the properties of magnets.
2. Describe the methods of magnetising and demagnetising a material.
3. State the difference between iron and steel with regard to magnetism.
4. Define a magnetic field and describe its lines.
5. Define magnetic meridian, geographical meridian, magnetic declination and angle of dip.

What is a magnet?

A magnet is a material that can attract pieces of iron and other magnetic materials.

• Magnetic (ferromagnetic) materials: substances attracted by a magnet, such as iron, steel, cobalt, nickel and certain alloys.
• Non-magnetic materials: substances not attracted by a magnet, such as brass, wood, copper and glass.
• Magnetisation: the process of making a material magnetic.
• Demagnetisation: the process of making a magnet lose its magnetism.

What are the properties of magnets?

The properties of a magnet are its poles, its North-South direction when free, its attraction and repulsion, and its polarity test.

1. Poles: the ends of a magnet, where the attracting power is greatest.
2. Directive property: a freely suspended bar magnet comes to rest with its axis in the magnetic meridian, roughly North-South.
3. North-seeking pole: the pole that points northwards, also called the North Pole. The opposite pole is the South Pole.
4. Attraction and repulsion: like poles repel each other, while unlike poles attract each other.
5. Test for polarity: bring each pole in turn near the known pole of a suspended magnet.

• Repulsion shows that the two poles are alike.
• Attraction may be between unlike poles, or between a pole and unmagnetised magnetic material.
• Hence, repulsion is the only sure test for polarity.

How can a material be magnetised?

Magnetisation is the process of making a material become magnetic, and there are five methods: electrical, single touch, divided touch, hammering in the earth’s field and induction.

What is the electrical method of magnetising?

The electrical method uses a current in a solenoid to magnetise a steel bar.

1. Wind insulated copper wire into a cylindrical coil of many turns. This coil is a solenoid.
2. Connect the solenoid in series with a six or twelve-volt battery and a switch.
3. Place a steel bar inside the coil.
4. Switch on the current for a short time.
5. Remove the bar and test it. It is now magnetised.

• Time: leaving the current on longer does not add magnetism. It only causes overheating.
• Polarity: it depends on the direction of the current.
• Rule: a clockwise flow at an end indicates the South Pole, while an anti-clockwise flow indicates the North Pole.

What is the single touch method?

The single touch method magnetises a steel bar by stroking it with one pole of a magnet.

1. Stroke the steel bar from end to end with a known pole of a magnet.
2. Always stroke in the same direction.
3. Lift the pole high above the bar between strokes, otherwise the magnetism already induced is weakened.
4. Repeat the strokes several times.

• Disadvantage: one pole is produced nearer the end of the bar than the other.

What is the divided touch method?

The divided touch method magnetises a steel bar by stroking it with unlike poles of two magnets at once.

1. Place the unlike poles of two magnets at the centre of the bar.
2. Stroke outward from the centre towards the two ends at the same time.
3. Lift the magnets away at the ends and repeat.

• Polarity: the end where stroking finishes has the opposite polarity to the stroking pole.

What is magnetising by hammering in the earth’s field?

• Method: hammer a red-hot steel bar and let it cool while it lies in the North-South direction.
• Result: the earth’s magnetic field magnetises the bar.

What is induced magnetism?

Induced magnetism is the magnetism produced in an unmagnetised piece of steel placed near or in contact with a pole of a magnet.

• Polarity: the induced pole is of the opposite sign to the inducing pole.
• Result: the steel stays magnetised after it is removed.

How can a magnet be demagnetised?

Demagnetisation is the process of making a magnet lose its magnetism, and there are three methods: electrical, mechanical and heating.

• Electrical method: place the magnet in a solenoid carrying alternating current, with the solenoid’s axis pointing East-West.
• Withdrawal: after a few seconds, slowly withdraw the magnet to a long distance away.
• Efficiency: this is the most efficient way of demagnetising a magnet.
• Mechanical method: hammer the magnet hard while it points in the East-West direction.
• Heating method: strong heating makes a magnet lose its magnetism.

What is the difference between iron and steel in magnetism?

Soft iron is almost pure iron. It is easily magnetised but quickly loses its magnetism, so it makes temporary magnets.

Steel is an alloy of iron and carbon. It is harder and stronger than soft iron and keeps its magnetism, so it makes permanent magnets.

What is a magnetic field?

A magnetic field is the space around a magnet in which magnetic force acts.

• Quantity: a magnetic field is a vector quantity, so it has size and direction.
• Representation: it is drawn as magnetic field lines, also called lines of flux.
• Direction: at any point, the flux direction is the direction of the force on a north pole placed there.
• Pattern: outside the magnet, the lines run from the North pole to the South pole.
• Strength: the field is strongest where the lines are closest together, near the poles.
• Two magnets: near two magnets placed close together, the flux direction changes rapidly in a small space.
• Neutral point: a point where the fields cancel and the resultant field is zero.

How can the magnetic field pattern be shown?

1. Place a sheet of paper or card over a bar magnet.
2. Sprinkle iron filings evenly on the paper.
3. Tap the paper gently.
4. The filings arrange themselves along the field lines.

What are the terms used for the earth’s magnetic field?

Four terms describe the earth’s magnetic field at a place: magnetic meridian, geographical meridian, magnetic declination and angle of dip.

• Magnetic meridian: at any place, the vertical plane containing the magnetic axis of a freely suspended magnet at rest under the action of the earth’s field.
• Geographical meridian: at a place, the plane containing that place and the earth’s axis of rotation.
• Magnetic declination: the angle between the magnetic and geographical meridians.
• Angle of dip or inclination: the angle between the direction of the earth’s magnetic flux and the horizontal.

Evaluation

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.
3. State why repulsion is the only sure test for the polarity of a magnet.
4. List the three methods of demagnetising a magnet.

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 0°, calculate the true geographic bearing if the compass needle reads N40°E.
   • (a) N50°E
   • (b) N40°E
   • (c) N30°E
   • (d) N25°E
3. A magnetic substance can be demagnetised by?
   • (a) Dropping on the floor
   • (b) Hammering while red hot
   • (c) Divided touch
   • (d) Single touch
4. A freely suspended compass needle on the 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. Explain what is meant by a magnetic field, and then:
   • 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, and 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.
2. Explain the term “angle of dip”, and describe how it varies over the earth.
3. A charge of 1.6 × 10^-19 C enters a magnetic field of flux density 2.0 T with a velocity of 2.5 × 10^-19 m/s at an angle of 30° with the field. Calculate the magnitude of the force exerted on the charge by the field.