18.0 INTRODUCTION
A magnetic field is a region around a magnetic material or a moving electric charge within which the force of magnetism acts.
CONCEPT OF MAGNETIC FIELD: Region around a magnet where magnetic forces can be detected. Represented by magnetic field lines, which indicate direction and strength.
MAGNETIC FIELD OF A PERMANENT MAGNET: Field lines emerge from the north pole and enter the south pole. Field is strongest at the poles and weakest at the centre.
18.1 EARTH’S MAGNETIC FIELD
Earth’s magnetic field, also known as the geomagnetic field, is a magnetic field that emanates from the Earth’s core, extending into space and protecting the planet from harmful solar radiation and cosmic rays.
PROPERTIES OF THE EARTH’S MAGNETIC FIELD
- Magnetic meridian: Line of magnetic north and south.
- Angle of dip: Angle between magnetic field and horizontal.
- Angle of Declination: Angle between magnetic north and true north.
- Flux and flux density: Measure of magnetic field strength.
VARIATION OF MAGNETIC FIELD INTENSITY: Intensity varies with location on Earth’s surface. Influenced by geological formations and distance from magnetic poles.
APPLICATIONS OF EARTH’S MAGNETIC FIELD
- Navigation: Compasses align with Earth’s magnetic field.
- Mineral Exploration: Magnetic surveys help locate minerals.
18.2 MAGNETS
A magnet is a material or object that produces a magnetic field, capable of attracting certain materials like iron, steel, nickel, and cobalt, and exerting attractive or repulsive forces on other magnets.
TYPES OF MAGNETS
- Natural Magnets: Occur in nature (e.g., lodestone).
- Artificial Magnets: Man-made (e.g., bar magnets, horseshoe magnets).
METHODS OF MAKING MAGNETS
- Stroking with a magnet, electrical methods, and induction.
- Demagnetizing: Heating, hammering, or using alternating current.
MAGNETIC PROPERTIES OF SOFT IRON AND STEEL
- Soft Iron: Easily magnetized and demagnetized; used in electromagnets.
- Steel: Retains magnetism; used in permanent magnets.
18.3 MAGNETIC FORCE ON A CURRENT – CARRYING CONDUCTOR
A current-carrying conductor placed in a magnetic field experiences a force given by:
F = BILsin(θ)
Where F is the force,
B is the magnetic field strength,
I is the current,
L is the length of the conductor in the field, and
θ is the angle between the conductor and the magnetic field.
Magnetic Force on a Charge in a Magnetic Field: F = q(v × B).
18.4 CONCEPT OF MAGNETIC FIELD DEVICES
- D.C. Motor: Converts electrical energy to mechanical energy using magnetic fields.
- Electromagnets: Magnets created by electric current; used in various applications.
- Carbon Microphone: Converts sound waves into electrical signals using carbon granules.
- Moving Coil Ammeter: Measures current using a coil in a magnetic field. Primarily for DC measurements.
- Moving Iron Ammeter: Measures current using a moving iron piece in a coil. Can measure both AC and DC currents.
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