Lesson Notes By Weeks and Term v4 - SHS 1
MAGNETOSTATICS
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MAGNETOSTATICS
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Subject: Physics
Class: SHS 1
Term: 2nd Term
Week: 14
Grade code: 1.3.2.LI.1
Strand code: 3
Sub-strand code: 2
Content standard code: 1.3.2.CS.2
Indicator code: 1.3.2.LI.1
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: MAGNETOSTATICS
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Welcome, students! Today, we are exploring the fascinating world of magnets. We see magnets every day—holding notes on the fridge, inside the speakers of the 'trotro' radio, and even in the motors that power our fans. But have you ever wondered how a normal piece of iron or steel becomes a magnet? Or how a magnet can lose its power? In this lesson, we will uncover the secrets of magnetization (how to make magnets) and demagnetization (how to destroy them). Understanding this is crucial for building powerful tools like the huge cranes at Tema Harbour that lift scrap metal, or simple devices like the electric bell that rings in our schools.
A. The Domain Theory of Magnetism Before we can make a magnet, we need to understand what happens inside a magnetic material like iron, steel, or cobalt. Magnetic Materials: These are materials that can be magnetized. They are also called ferromagnetic materials. Magnetic Domains: Imagine that a piece of iron is made up of millions of tiny regions, each behaving like a miniature magnet with its own north and south pole. These regions are called magnetic domains. In an unmagnetized material: The magnetic domains are arranged randomly. They point in all different directions. Because of this jumbled arrangement, their magnetic effects cancel each other out, and the material as a whole does not act as a magnet. *Visualisation:* Think of a crowded market where everyone is shouting in different directions. The result is just noise, not a clear message. In a magnetized material: When the material is placed in a strong magnetic field, the domains rotate and align themselves, all pointing in the same direction. Now, their magnetic effects add up, creating a strong overall North pole at one end and a South pole at the other. The material is now a magnet. *Visualisation:* Now imagine a choir master directs all the singers to sing the same note in the same direction. The result is a powerful, clear sound.
Definitions: Magnetization: The process of aligning the magnetic domains within a ferromagnetic material to make it a magnet. Demagnetization: The process of randomizing the aligned magnetic domains within a magnet, causing it to lose its magnetic properties. B. Methods of Magnetization
There are two main ways to force the domains to align. Stroking Method (Single and Divided Touch)
This method uses an existing permanent magnet to induce magnetism in a piece of iron or steel. Single-Touch Method: Place the steel bar on a non-magnetic surface (like a wooden table). Take a strong bar magnet. Place one pole (e.g., the North pole) at one end of the steel bar. Stroke the steel bar from that end to the other, pressing firmly. At the far end, lift the magnet high up and away from the bar. Bring the magnet back to the starting point and repeat the process about 20-30 times, always in the same direction. Result: The end where you *start* the stroke becomes the same pole as the stroking pole (e.g., North), and the end where you *finish* the stroke becomes the opposite pole (South). Divided-Touch Method: Place the steel bar on the table. Take two strong bar magnets. Place their opposite poles (e.g., North and South) together at the centre of the steel bar. Simultaneously, stroke outwards from the centre to the opposite ends of the bar. Lift the magnets high, bring them back to the centre, and repeat. Result: This creates a stronger magnet with poles at the ends that are opposite to the stroking poles used at those ends. Electrical Method (Making an Electromagnet) This is the most effective and widely used method. It is based on the principle that an electric current produces a magnetic field. Procedure: Take a ferromagnetic core, such as a large iron nail. This is a soft magnetic material. Take a long piece of insulated copper wire. Wrap the wire tightly around the nail to form a coil. This coil is called a solenoid. The more turns you have, the stronger the magnet will be. Connect the two ends of the wire to the terminals of a DC power source, like a 1.5V battery cell. Observation: As soon as the circuit is complete, current flows through the coil. The nail will now behave like a magnet and can attract paper clips, pins, or iron filings. When you disconnect the battery, the nail loses most of its magnetism. Explanation: The current flowing in the solenoid creates a strong, concentrated magnetic field inside the coil. This field forces the magnetic domains inside the iron nail to align, turning it into a magnet. Finding the Polarity (North/South Pole): We use the Right-Hand Grip Rule. Imagine gripping the coil with your right hand such that your fingers curl in the direction of the conventional current (from positive (+) to negative (-)). Your thumb will point towards the North pole of the electromagnet. C. Methods of Demagnetization To demagnetize a magnet, we need to supply energy to the domains to jumble them up again. Hammering: Placing the magnet in an East-West direction and hammering it hard provides mechanical energy. The vibrations shake the domains out of alignment, causing them to become random again. Heating: Heating a magnet strongly (to a specific temperature called the Curie Point) gives the domains enough thermal energy to vibrate violently. This breaks the forces holding them in alignment, and they return to a random arrangement upon cooling. For iron, the Curie Point is about 770°C. Using Alternating Current (AC): Place the magnet inside a solenoid connected to an AC power supply. The AC current rapidly changes direction (in Ghana, 50 times per second). This creates a rapidly reversing magnetic field. This field flips the domains back and forth, causing them to become disoriented. Slowly withdraw the magnet from the solenoid while the AC is still on. As the magnet moves into a weaker and weaker field, the domains are left in a random, unaligned state. This is the most effective method for complete demagnetization. D. Soft vs. Hard Magnetic Materials Soft Magnetic Materials (e.g., Soft Iron): Easy to magnetize. Easy to demagnetize. They lose their magnetism almost immediately after the magnetizing field is removed. Use: Perfect for making temporary magnets or electromagnets (like in electric bells and lifting cranes) because their magnetism can be switched on and off easily. Hard Magnetic Materials (e.g., Steel, Alnico): Difficult to magnetize (require a very strong magnetic field). Difficult to demagnetize. They retain their magnetism for a long time. This property is called retentivity. Use: Perfect for making permanent magnets (like fridge magnets, compass needles, and magnets in loudspeakers).