Magnetic field

(A) concept of magnetic field

Magnetic field is a region around a magnet in which the influence of the magnet can be felt. Magnitude of magnetic force: 

F = /q/vBsin¢ where /q/ is the magnitude of the charge, ¢ is the angle measured from the direction of v to the direction of B. The total force F_T is the vector sum of the Electric and magnetic forces:

  /F/ = qE + qvB sin¢

Patterns of magnetic field

Permanent magnet

Source: www.en.wikipedia.org

Perpendicular to a straight current carrying wire

Source: www.en.wikipedia.org

The axis of circular current carrying loop

Source: www.phys.libretexts.org

Centre of a cylindrical coil/solenoid

Source: www.en.wikipedia.org

Note the magnetic flux ¢_B = BAcos¢.

Where A is the cross sectional area where the flux threads.

Angle of dip or the inclination

Source: www.esaral.com 

The angle of dip is the angle between the direction of the Earth’s resultant magnetic field and the horizontal.

        tan¢ = V/H

Where ¢= angle of dip, V= vertical components, H= horizontal components

(B)Theory of magnetism

Wilhelm weber, in the middle of the nineteenth century considered that every molecule of a magnetic material is itself a magnet with two poles. For example if a magnet is broken into two halves, instead of separating the north and the south poles, we obtain two magnets.

Diamagnetism

Using an applied magnetic field, the electron orbits are slightly disturbed by induced electromagnetism which reduces the field strength inside the material giving rise to a little magnetic effect called diamagnetism. Examples are bismuth, Mercury, silver, carbon and lead and these materials are not affected by temperature.

Paramagnetism

In the absence of external magnetic fields, these molecular magnets are randomly arranged giving no net magnetic effect to the material; but with the application of external magnetic fields, the molecular magnets become partially aligned and the magnetic strength increases. Examples are Platinum, aluminum and sodium. They are affected by temperature.

(C) Ferromagnetism

In some magnetic materials of which iron, nickel Cobalt, gadolinium and certain alloys such as alnico are examples, there are strong interactions between atomic magnetic moments so that their magnetic axes line up with each other in regions called magnetic domains.

Ferromagnetic materials can be used in electromagnetics, transformer cores, motors and generators. Steel and alnico can be used in the construction of permanent magnets. The susceptibility is very high.

(D) Earth’s magnetic field

A magnetic needle or bar magnet when freely suspended will always point approximately in a north-south direction as a result of the Earth’s magnetic field.

Terms to note

The magnetic meridian at any place is a vertical plane passing through the magnetic axis of a magnet suspended freely under the influence of the Earth’s magnetic field.

The geographic meridian at any place is a vertical plane containing the geographic north and south poles of the earth.

The angle between the geographic north and magnetic south at a place is called the angle of declination at the place.

Magnetic meridian

Source: www.commons.wikimedia.org 

(E)Bar magnet in Earth’s field

Fields due to a bar magnet in the Earth’s field.

Source: www.commons.wikimedia.org

(F)Electromagnets and the applications of electromagnetic fields

Electromagnetism is a temporary magnet which is constructed by winding solenoids in opposite directions round a soft iron bar bent into a U-shape.

Homemade Electromagnet

Source: www.commons.wikimedia.org 

Electromagnets finds many common applications in industry:

1. To produce strong magnetic fields required in generators and Electric motors.
2. For lifting and transporting heavy equipment made from pieces of iron and steel.
3. To separate iron from mixtures containing non-magnetic elements
4. They are used in construction of devices such as Electric bell and telephone receiver.

Electric bell: the Electric bell consists of two solenoids wound in opposite directions on two of the iron cores connected with a soft iron bar.

Source: www.commons.wikimedia.org 

The telephone receiver

It consists of an Electromagnet formed by placing a short permanent bar magnet across the ends of two iron bars. This is placed so that it exerts a pull on the magnetic diaphragm constructed from a soft alloy. Two solenoids are wound in opposite directions on the soft-iron bars.

Source: www.physicsmax.com 

Past questions

1.In the electrical method of magnetization, the polarity of the magnet depends on the? ( Wassce 1998)

A. Amount of current passed

B. Direction of current

C. Magnetic materials used

D. Size of the magnetic material

E. Orientation of the magnetic material in space.

Answer: B

2. The region around the magnet in which the magnetic influence is experienced is called? (Wassce 2000)

A. Magnetic flux

B. Magnetic field

C. Magnetic meridian

D. Magnetic declination

Answer: B

Solution: magnetic field is a region around a specific magnet where magnetic influence can be felt

3. If a bar magnet is accidentally broken into three pieces as shown in the diagram below the polarities of p and q respectively are? (Wassce 2000)A. S and S

B. N and S

C. N and N

D. S and N

Answer: B

Solution: N and S indicate the north and the south poles of a magnet

4. Which of the following statements about magnetic lines of force is not correct? They. (Wassce 2018)

A. Pass through even non-magnetic materials

B. Run from the north Pole to the south Pole

C. Are in a state of tension and tend to be shorted

D. Intersect one another

Answer: B

5. When a biro pen rubbed on a dry silk cloth is moved very close to a piece of paper on a dry table, the pen is found to pick up the paper. This is because? (Jamb 1994)

A. Both the pen and the cloth are magnetized 

B. The pen is magnetized but the cloth is not 

C. The pen is charged while the cloth is magnetized

D. Both the pen and the cloth are charged

Answer: B

Solution: both carry charges which enable them to be magnetized.

6. Steel is more suitable for permanent magnet than iron because the former? (Jamb 1999)

A. Is easily demagnetized by shaking vigorously

B. Is an alloy of many metals

C. Is easily magnetized by alternating current through one cycle

D. Retains magnetism more than iron 

Answer: D

7.

If current flows in the direction of the arrows in the solenoid above, the north Pole is at? (Jamb 2000)

A. P

B. X

C. Q

D. Y

Answer: Q

8. Defined angle of dip?

Answer: the angle of dip is the angle between the direction of the Earth’s resultant magnetic field and the horizontal.