Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define magnetism and electromagnetism.
• Describe the properties of magnetic fields.
• Explain how an electric current creates a magnetic field.
• Apply the right-hand rule to find the magnetic field direction.
• Calculate magnetic field strength for a straight wire and a solenoid.

Lesson summary

Magnetism and electromagnetism are fundamental concepts in electrical technology, forming the basis for many devices we use every day. From electric motors in washing machines to transformers that step down voltage for our homes, understanding these principles is crucial. In South Africa, reliable electricity infrastructure is vital for economic development and improving living standards. Technicians and engineers who understand magnetism and electromagnetism are essential for building, maintaining, and repairing this infrastructure.

Lesson notes

Magnetism: Magnetism is a physical phenomenon produced by the motion of electric charge, resulting in attractive and repulsive forces between objects. Materials exhibiting strong magnetic effects are called magnets. Magnets have two poles, a North pole, and a South pole. Unlike poles attract each other, while like poles repel each other.

Magnetic Fields: A magnetic field is a region around a magnet or a current-carrying conductor where magnetic forces are exerted. Magnetic fields are represented by magnetic field lines, which show the direction of the magnetic force. These lines always form closed loops, exiting from the North pole and entering at the South pole. The closer the field lines are to each other, the stronger the magnetic field. Magnetic Flux (Φ): Magnetic flux is a measure of the quantity of magnetism, being the number of magnetic field lines passing through a given area. It is measured in Webers (Wb).

Magnetic Flux Density (B): Magnetic flux density is the magnetic flux per unit area and is a measure of the strength of the magnetic field. It is measured in Teslas (T), where 1 Tesla = 1 Wb/m².

Electromagnetism: Electromagnetism is the interaction between electric currents or fields and magnetic fields. The fundamental principle is that a moving electric charge generates a magnetic field, and a changing magnetic field induces an electric current.

Electromagnetic Induction: When a conductor is placed in a changing magnetic field, a voltage is induced in the conductor. This is the principle behind generators and transformers. Magnetic Field around a Current-Carrying Conductor: A current-carrying conductor produces a magnetic field around it. The shape and strength of the magnetic field depend on the shape of the conductor and the magnitude of the current.

Right-Hand Rule: The right-hand rule is a convenient way to determine the direction of the magnetic field around a current-carrying conductor.

For a straight conductor: Point your right thumb in the direction of the current. Your fingers will curl in the direction of the magnetic field.

For a solenoid: Curl your fingers in the direction of the current in the coil. Your thumb will point in the direction of the magnetic field inside the solenoid.

Magnetic Field Strength (H): Magnetic field strength is the measure of the intensity of the magnetic field produced by a current. It is measured in Amperes per meter (A/m). Relationship between Magnetic Flux Density (B) and Magnetic Field Strength (H): B = μH, where μ is the permeability of the medium. Permeability is a measure of how easily a material can be magnetized. For free space (vacuum), μ = μ₀ = 4π × 10⁻⁷ H/m. Calculating Magnetic Field Strength (B) - Worked

Examples: Straight Wire: The magnetic flux density (B) at a distance r from a long, straight wire carrying a current I is given by: B = (μ₀I) / (2πr)

Example 1: A straight wire carries a current of 5

A. What is the magnetic flux density at a distance of 10 cm from the wire?

Solution: I = 5A r = 0.1 m μ₀ = 4π × 10⁻⁷ H/m B = (4π × 10⁻⁷ 5) / (2π * 0.1) = (20π × 10⁻⁷) / (0.2π) = 100 × 10⁻⁷ = 1 × 10⁻⁵ T Answer: The magnetic flux density is 1 x 10⁻⁵ Tesla.

Solenoid: The magnetic flux density (B) inside a long solenoid with n turns per unit length carrying a current I is given by: B = μ₀nI Example 2: A solenoid has 500 turns per meter and carries a current of 2

A. What is the magnetic flux density inside the solenoid?

Solution: n = 500 turns/m I = 2A μ₀ = 4π × 10⁻⁷ H/m B = (4π × 10⁻⁷) 500 * 2 = 4000π × 10⁻⁷ = 1.257 × 10⁻³ T Answer: The magnetic flux density is 1.257 x 10⁻³ Tesla.

Practical Application for South Africa: Solenoids are used in starter motors in cars. Car ownership is an important factor in socio-economic mobility in South Africa, particularly in rural areas where public transport is limited. Guided Practice (With Solutions)

Question 1: A bar magnet is brought near a compass. Describe what happens and explain why.

Solution: The compass needle will align itself with the magnetic field of the bar magnet. The North pole of the compass needle will point towards the South pole of the bar magnet, and the South pole of the compass needle will point towards the North pole of the bar magnet. This is because unlike poles attract. The compass works because Earth itself has a magnetic field.

Commentary: This question tests the fundamental understanding of magnetic poles and their interactions.

Question 2: A wire carrying a current of 8A is placed near another wire carrying a current of 2

A. Both currents are flowing in the same direction. What will happen?

Solution: The wires will attract each other. This is because the magnetic fields created by each wire interact. Since the currents are in the same direction, the magnetic fields between the wires are in opposite directions, resulting in an attractive force.

Commentary: This question tests understanding of the interaction between magnetic fields produced by current-carrying conductors.