Lesson 1: Magnets and Magnetic Field

Video Lesson:

Competencies of the Lesson:

At the end of this section, you will be able to

• Describe magnet and magnetic field.
• Describe the sources of magnetic field.
• Describe the difference between electric field and magnetic field.
• Describe magnetic field lines around permanent magnets and electromagnet.
• Describe the properties of magnetic field lines.
• Compare magnetic and electric field lines.

Key Terms and Concepts

Key Terms and Concepts

Brainstorming Questions:

• How does a magnetic field originate?
• What happens if you cut a bar magnet in half?
• Do you get one magnet with two south poles and one magnet with two north poles?
• Like electric field lines, do magnetic field lines have starting and ending point?

Introduction to Electromagnetism

Electromagnetism is one of the four fundamental forces in nature, integrating the elements of electricity and magnetism. It involves the study of the electromagnetic force, which is carried by electromagnetic fields composed of electric and magnetic fields. On a subatomic level, electromagnetism governs the interactions between electrically charged particles, leading to phenomena such as attraction, repulsion, and the generation of magnetic fields when these particles are in motion. The oscillation of charged particles, such as electrons, produces electromagnetic radiation, including radio waves.

The relationship between electricity and magnetism was first discovered in 1819 by Hans Christian Oersted, who observed that an electric current in a wire could deflect a nearby compass needle, thus revealing a connection between the two forces. Further exploration by Michael Faraday and Joseph Henry in the 1820s demonstrated that a magnetic field could induce an electric current in a circuit, either by moving a magnet near the circuit or by altering the current in an adjacent circuit.

This discovery of electromagnetism marked the advent of modern science and technology. Today, we understand that all magnetic phenomena arise from the forces associated with moving electric charges. This understanding has been crucial in the development of various devices, including radios, televisions, computers, electric motors, and generators, which are foundational to modern technology.

Magnets and Magnetic Fields

A magnet creates a magnetic field, which represents the invisible force surrounding the magnet that can attract or repel certain materials like iron, nickel, and cobalt. Magnets have two poles: a north pole (N) and a south pole (S). The strongest magnetic force is exerted at these poles. When two magnets are near each other, like poles (N-N or S-S) will repel, while opposite poles (N-S) will attract.

A key difference between magnetic poles and electric charges is that electric charges can exist independently (as positive or negative charges), but magnetic poles cannot be isolated. Even if you cut a magnet in half, each piece will still have both a north and a south pole. Magnetic poles always exist in pairs.

• There are two main types of magnets: permanent magnets and electromagnets.
  • Permanent Magnets: These magnets generate their magnetic field from the internal structure of the material itself. Once magnetized, they retain their magnetic properties for a long time. A common bar magnet is an example of a permanent magnet.
  • Electromagnets: These are created by winding a wire into a coil and passing an electric current through it. The electric current generates a magnetic field, which can be turned on or off depending on whether the current is flowing. An iron core is often used within the coil to enhance the magnetic field’s strength.

Figure 4.1 Electromagnet

• The Earth itself acts like a giant magnet, with a magnetic field that resembles a bar magnet buried inside the planet. This magnetic field causes a compass needle to align in a north-south direction, pointing towards the magnetic poles.

Figure 4.3 Earth’s magnetic field is like a bar magnet that resides in the center of the Earth.

Magnetic Fields vs. Electric Fields

Table 4.1 Differences between a magnetic field and an electric field:

Property	Magnetic Field	Electric Field
Region Definition	The region around a magnet where the force of magnetism acts.	The region around an electric charge where the electric force exists.
SI Unit	Tesla (T)	Newton per Coulomb (N/C)
Field Creation	Caused by a dipole (both north and south poles of a magnet).	Produced by a unit charge, either positive or negative.
Field Lines	Form closed loops, indicating a continuous nature.	Start on a positive charge and end on a negative charge.
Independence of Poles/Charges	Magnetic poles cannot be isolated; they always come in pairs (north and south).	Electric charges can exist independently (positive or negative).
Behavior of Field Lines	Do not have distinct starting and ending points.	Do not form loops; have distinct starting and ending points.

• Note: Both electric and magnetic fields are vector quantities, meaning they have both magnitude and direction.

Magnetic Field Lines

• Magnetic field lines are imaginary lines used as a visual tool to represent magnetic fields.
• The density of these lines indicates the strength (magnitude) of the magnetic field. Similar to electric fields, the pictorial representation of magnetic field lines is useful for visualizing both the strength and direction of the magnetic field.
• For example, when iron filings are sprinkled around a bar magnet, they align along the magnetic field lines, revealing the magnetic field pattern around the magnet.

Figure 4.3 a) Magnetic field pattern surrounding a bar magnet b) Magnetic field pattern between opposite poles (N–S) of two bar magnets and c) Magnetic field pattern between like poles (N–N) of two bar magnets

Properties of Magnetic Field Lines:

• Direction and Magnitude: Magnetic field lines have both direction and magnitude at any given point in the field. The direction of the magnetic field at any point is tangent to the field line at that point. If you place a small compass in the magnetic field, it will align itself with the direction of the magnetic field line.
• Field Strength: The strength of the magnetic field is proportional to the closeness of the field lines. The closer the lines, the stronger the magnetic field in that region.
• Non-Intersecting Lines: Magnetic field lines never cross each other. This implies that the magnetic
• field has a unique direction at any point in space.
• Closed Loops: Unlike electric field lines, magnetic field lines are continuous and form closed loops without a beginning or end. The lines emerge from the north pole of a magnet and merge at the south pole. Inside the magnet, the field lines run from the south pole back to the north pole, thus forming closed curves.

Figure 4.4 Magnetic field lines of electromagnet.

Figure 4.5 Comparison of Magnetic and electric field lines.

In summary, magnetic field lines are a useful tool for visualizing the characteristics of magnetic fields, including their direction, strength, and continuity.