Magnetic Field

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Welcome to class! 

In today’s class, we will be talking about the magnetic field. Enjoy the class!

Magnetic Field

Magnet

A magnet is a material, often in the shape of a bar, which attracts pieces of iron or steel. When suspended or pivoted freely, a magnetic always comes to rest close to the geographical north-south direction. The end which points to the north is called the north-seeking (N) pole while the end pointing to the south is the south – seeking (S) pole.

The force with which a magnet attracts pieces of iron is concentrated at the two poles, i.e. it is stronger at the poles than at the main body of the magnet. The N-pole of a magnet repels the N-pole but attracts thee S-pole of another magnet, i.e. like poles repel, unlike poles attract.

Magnetic-field

A magnetic field is the surrounding a magnet in which it exerts a magnetic force. All force field like a magnetic field, gravitational field and electric field are represented by lines of force. The diagrams below show a bar magnet. A magnet has two poles the North Pole (N) and the South Pole (S).

Magnetic field strength

The magnetic field strength or magnetic flux at a point is a measure of the force on N – pole of a magnet placed at that point.

Magnetic line of force

A magnetic line of force at a point is the path which an imaginary free N-pole will move if placed at that point. Magnetic lines of force will:

(i) Start from N-pole and end at the S-pole

(ii) are closed together where the magnetic field is strongest. The magnetic field pattern near a magnetic can be plotted by sprinkling iron filings on a paper placed on the magnet or by using a compass. When two magnets are placed with like poles facing each other, the point where the magnetic field completely neutralize each other is the neutral point. See diagram below.

Properties of the magnetic field

1. Magnetic lines of force cannot cross each other because the magnetic field moves only in one direction
2. Magnetic line of force does not have two possible directions at the point of intersection.
3. Lines of force do not begin or end in the space surrounding a charge.
4. Lines of force do not intersect.
5. Lines of force normally curve
6. The direction of the field varies from point to point
7. Lines of force are continuous in any region with free charges
8. The magnetic field in a bar magnet is non-uniform.

Magnetic materials

All substances are attracted by magnetic fields and are classified as diamagnetic, paramagnetic or ferromagnetic according to how they are affected.

Diamagnetic and paramagnetic materials exhibit very weak forms of magnetism.

Those materials which exhibit very strong magnetic effects are said to be ferromagnetic. Examples are iron, nickel, cobalt, gadolinium and dysprosium.

Magnetization

A piece of steel can be made into a magnet electrically or by stroking. In the stroking method, one pole of a magnet is used to stroke the steel bar several times in the same direction. The end from which the stroking action is commenced has the same polarity as the stroking pole. See diagram below

In the electrical method, the steel bar is put into a solenoid to which direct electric current is applied for a few minutes. After switching off the current, the steel bar is found to behave as a magnet. If the current in the solenoid is flowing clockwise as seen from one of it ends, that end is a south pole.

Soft iron is more easily magnetized than steel, but it loses its magnetism more readily. Soft iron is therefore used for making temporary magnets (electromagnets) used in electric bells, relays, etc. while steel is used for making permanent magnets.

A magnet can be made to lose its magnetism by:

1. Placing the magnet in the E – W direction in a solenoid, passing an alternating current through the solenoid, and then slowly withdrawing the bar from the solenoid to a long distance.
2. Heating the magnet to reduces and allowing it to cool while lying in the E – W direction
3. Placing the magnet in the E – W direction and hammering hard on it.

Earth’s magnetic field

The earth behaves likes a giant magnet.

The magnetic meridian is the vertical plane through the earth which contains the magnetic N – S axis. It does not coincide exactly with the earth’s geographic meridian

The angle of declination is the angle between the magnetic and the geographic meridian. A freely suspended bar magnet does not lie exactly horizontally, but rather with the North Pole pointing downwards.

The angle of dip is the angle which the axis of the magnet makes with the horizontal.

Presentation

1. Introduce the lesson by giving them definitions of Magnetic field.
2. Explain all the definitions by giving examples on each of them.
3. Pick out the keywords in the definitions and explain to them.
4. Ask them to Magnet by using their own words and correct them if necessary.
5. Define Magnetic flux
6. Define magnetic line of force.

General evaluation

1. Explain what is meant by a magnetic field.

Assignment

1. Define Magnetic lines of force.
2. Describe three methods of magnetizing steel or an iron bar. Which of this method is effective? Discuss the nature of the poles formed by each of the three methods described above.

 

In our next class, we will be talking more about Magnetic Field. We hope you enjoyed the class.

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