Grade 7 Chapter 19: Properties of a Magnetic Field

Introduction to Magnetism

Magnets are fascinating objects that have the power to attract certain materials. This property of attraction is due to an invisible force field around them called a magnetic field. In this chapter, we will explore the fundamental properties of magnets, how magnetic fields behave, and their applications in our daily lives.

19.1 What is a Magnet?

A magnet is a material or object that produces a magnetic field. This magnetic field is responsible for the force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, and attracts or repels other magnets.

• Magnetic Materials: Materials that are attracted by a magnet (e.g., iron, nickel, cobalt, steel).
• Non-magnetic Materials: Materials that are not attracted by a magnet (e.g., wood, plastic, copper, aluminum).

Types of Magnets:

• Natural Magnets: Occur naturally in the Earth. The most common natural magnet is Lodestone (magnetite), a naturally magnetized mineral.
• Artificial Magnets: Man-made magnets, which are much stronger and can be shaped as desired.
  • Permanent Magnets: Retain their magnetism for a long time (e.g., bar magnet, horseshoe magnet, disc magnet).
  • Temporary Magnets (Electromagnets): Magnets whose magnetism is temporary and can be controlled (e.g., turned on or off). They are created by passing electric current through a coil of wire.

19.2 Properties of a Magnet

1. Attractive Property

Magnets attract magnetic materials. The attractive force is strongest at the ends of the magnet, which are called its poles.

• Magnetic Poles: Every magnet has two poles: a North-seeking pole (N-pole) and a South-seeking pole (S-pole). These poles always exist in pairs and cannot be isolated.

2. Directive Property

When a magnet is suspended freely, it always aligns itself in a North-South direction. This property is used in compasses for navigation.

• The North-seeking pole of the magnet points towards the Earth's geographic North.
• The South-seeking pole of the magnet points towards the Earth's geographic South.

3. Like Poles Repel, Unlike Poles Attract

This is a fundamental law of magnetism:

• Repulsion: Two like poles (North-North or South-South) repel each other.
• Attraction: Two unlike poles (North-South) attract each other.

Note: Repulsion is the surest test of magnetism, as attraction can also occur between a magnet and a magnetic material (like iron).

4. Magnetic Induction

A magnetic material can acquire magnetic properties temporarily when it is brought near or in contact with a magnet. This phenomenon is called magnetic induction.

• Example: If you touch a steel paperclip with a strong magnet, the paperclip temporarily becomes a magnet and can attract other small paperclips.

19.3 Magnetic Field and Magnetic Lines of Force

The region around a magnet where its magnetic force can be experienced is called a magnetic field. This field is represented by imaginary lines called magnetic lines of force.

• Characteristics of Magnetic Lines of Force:
  • They emerge from the North pole and enter the South pole outside the magnet.
  • They form continuous closed loops (inside the magnet, they travel from South to North pole).
  • They never intersect each other.
  • They are denser (closer together) near the poles, indicating a stronger magnetic field.
  • They are sparser (farther apart) away from the poles, indicating a weaker magnetic field.

19.4 Earth's Magnetism

The Earth itself behaves like a giant magnet. This is why a freely suspended magnet aligns itself in the North-South direction.

• The Earth's magnetic North pole is near the geographic South pole.
• The Earth's magnetic South pole is near the geographic North pole.
• This magnetic field protects Earth from harmful solar radiation.

19.5 Electromagnetism

Electromagnetism is the branch of physics that deals with the relationship between electricity and magnetism. It states that electric currents create magnetic fields.

• Electromagnet: A temporary magnet created by passing an electric current through a coil of wire wound around a soft iron core. The strength of an electromagnet can be controlled by changing the current or the number of turns in the coil.
• Uses of Electromagnets:
  • Electric bells
  • Cranes (for lifting heavy iron objects)
  • Loudspeakers
  • Magnetic Resonance Imaging (MRI) machines in hospitals
  • Maglev trains
  • Security systems (metal detectors)

19.6 Demagnetization of Magnets

Magnets can lose their magnetic properties (become demagnetized) under certain conditions:

• Heating: Heating a magnet to a high temperature (above its Curie temperature) can destroy its magnetism.
• Hammering/Dropping: Repeated hammering or dropping a magnet from a height can disalign its internal magnetic domains, causing it to lose magnetism.
• Improper Storage: Storing magnets improperly (e.g., without keepers for bar magnets) can lead to demagnetization over time.

To prevent demagnetization, magnets should be stored properly, often in pairs with their opposite poles facing each other, separated by a piece of wood, and with soft iron keepers at the ends.