Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Electro-magnetic Field
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Electro-magnetic Field
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Physics
Class: Senior Secondary 3
Term: 1st Term
Week: 1
Theme: Fields At Rest And In Motion
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
Students should be able to identify the directions of current, magnetic field. Explain the action of a loop wire carrying current in a magnetic field Explain the basic working principle of the galvanometer and the electric motor. State and explain the implications of Lenz's law Explain how the conversation principle is in volved in both laws with regard to:- Charge- energy State and explain Faraday's law of electromagnetic in duction. explain the principle under lying the production of direct and alternating current State the use of in duction coils and transformers. Explain the cores of the in duction coil and the transformer are laminated
When an electric current flows through a conductor placed within an external magnetic field, the conductor experiences a mechanical force. This phenomenon is known as the motor effect.
Direction of Force: The direction of this force is determined by Fleming's Left-Hand Rule: Thumb: Represents the Direction of Force (or Motion)
Forefinger: Represents the Direction of the Magnetic Field (conventionally from North pole to South pole)
Middle Finger: Represents the Direction of the Current (conventionally from positive to negative)
Magnitude of Force: The magnitude of the force (F) acting on a straight conductor of length (L) carrying current (I) in a uniform magnetic field of flux density (B) is given by the formula: `F = BIL sinθ` Where θ is the angle between the direction of the current in the conductor and the direction of the magnetic field. If the conductor is perpendicular to the magnetic field (θ = 90°), `sinθ = 1`, and the force is maximum: `F_max = BIL`. If the conductor is parallel to the magnetic field (θ = 0° or 180°), `sinθ = 0`, and the force is zero: `F = 0`. When a rectangular coil carrying electric current is placed in a uniform magnetic field, the forces acting on its various sides combine to produce a turning effect, or torque, which causes the coil to rotate. Consider a rectangular coil ABCD placed within a magnetic field. If current flows, for example, from A to B, then B to C, C to D, and D to A: The side AB and CD are perpendicular to the magnetic field lines (assuming the field is horizontal and the coil vertical). According to Fleming's Left-Hand Rule, the force on AB will be upwards, and the force on CD will be downwards. The forces on sides BC and DA are equal in magnitude but act in opposite directions along the axis of rotation, thus they cancel out and do not contribute to the net torque. The upward force on side AB and the downward force on side CD form a couple, which creates a net turning effect on the coil, causing it to rotate about its central axis. This continuous rotation is the fundamental principle behind the operation of an electric motor. An electric motor is a device that efficiently converts electrical energy into mechanical energy, typically in the form of rotational motion.
Working Principle: It operates on the motor effect. A current-carrying coil placed within a magnetic field experiences a continuous torque, causing it to rotate.
Essential Components: Stator: This is the stationary part of the motor. It provides the strong magnetic field, usually generated by permanent magnets or powerful electromagnets.
Rotor (or Armature): This is the rotating component, consisting of a coil (often wound around a soft iron core to concentrate the magnetic field) capable of spinning.
Commutator (Split-ring): A crucial component for DC motors, this is a metal ring split into segments. Its primary function is to automatically reverse the direction of the current flowing through the coil every half-rotation. This ensures that the torque always acts in the same direction, leading to continuous, unidirectional rotation.
Brushes: These are typically carbon blocks that maintain sliding electrical contact with the commutator. They connect the external power supply to the rotating coil, feeding current into the armature.
Operation: When electric current is supplied to the coil through the brushes and commutator, the forces on the coil sides generate a torque, initiating rotation. As the coil rotates past the vertical position, the commutator segments switch contact with the brushes, effectively reversing the current direction in the coil. This reversal ensures that the direction of the torque remains constant, enabling continuous rotation of the motor.
Applications in Nigeria: Electric motors are vital for numerous applications, including powering domestic appliances like ceiling fans, food blenders, washing machines, and refrigerator compressors. Industrially, they are used in water pumps (e.g., borehole pumps), manufacturing machinery, and power tools. A galvanometer is a sensitive electromechanical instrument primarily used for detecting and measuring small electric currents.
Working Principle: Similar to the motor, it operates on the motor effect. A coil pivoted within a strong radial magnetic field experiences a torque when current passes through it. The resulting deflection of a pointer is proportional to the current.
Essential Components: A coil of fine insulated wire, typically wound on a non-magnetic frame, is pivoted between the poles of a powerful permanent magnet. A soft iron core is often placed inside the coil to enhance the magnetic field and make it radial. A restoring spring provides a counter-torque, and a pointer indicates the current on a calibrated scale.
Operation: When a small current flows through the coil, the magnetic field exerts forces on the coil sides, creating a torque that causes the coil and its attached pointer to rotate. This rotation continues until the magnetic torque is balanced by the opposing restoring torque of the spring. The angle of deflection is directly proportional to the current flowing through the coil.
Conversion to Ammeter: An ammeter measures larger currents and must possess a very low internal resistance to avoid significantly altering the current in the main circuit it is measuring. To convert a galvanometer into an ammeter, a low-resistance resistor, known as a shunt resistor (Rs), is connected in parallel with the galvanometer. The shunt resistor diverts the majority of the current, allowing only a small, safe current (Ig) to pass through the sensitive galvanometer.
Formula: `IgG = (I - Ig)Rs` Where: `G` = Resistance of the galvanometer coil `Ig` = Full-scale deflection current of the galvanometer `I` = Maximum current to be measured by the ammeter `Rs` = Resistance of the shunt resistor Therefore, `Rs = (IgG) / (I - Ig)` Conversion to Voltmeter: A voltmeter measures potential difference (voltage) and must have a very high internal resistance to draw a negligible current from the circuit branch across which it is connected. To convert a galvanometer into a voltmeter, a high-resistance resistor, known as a multiplier resistor (Rm), is connected in series with the galvanometer.
Formula: `V = Ig(G + Rm)` Where: `G` = Resistance of the galvanometer coil `Ig` = Full-scale deflection current of the galvanometer `V` = Maximum voltage to be measured by the voltmeter `Rm` = Resistance of the multiplier resistor Therefore, `Rm = (V / Ig) - G` Worked Example (Galvanometer Conversion): A galvanometer has a resistance of 40 Ω and gives a full-scale deflection for a current of 25 mA. How would you convert it into an ammeter to measure up to 2 A? How would you convert it into a voltmeter to measure up to 120 V?
Solution: Given: G = 40 Ω, Ig = 25 mA = 0.025 A Ammeter conversion (I = 2 A): Using the formula: `Rs = (IgG) / (I - Ig)` `Rs = (0.025 A * 40 Ω) / (2 A - 0.025 A)` `Rs = 1.0 Ω / 1.975 A` `Rs ≈ 0.506 Ω` Therefore, connect a 0.506 Ω resistor in parallel with the galvanometer. Voltmeter conversion (V = 120 V): Using the formula: `Rm = (V / Ig) - G` `Rm = (120 V / 0.025 A) - 40 Ω` `Rm = 4800 Ω - 40 Ω` `Rm = 4760 Ω` Therefore, connect a 4760 Ω resistor in series with the galvanometer.
Nigerian Power Generation and Transmission (PHCN/DISCOs): The entire Nigerian electricity infrastructure is a prime example of applied electromagnetism. Large-scale AC generators (alternators) at power stations like Kainji Dam (hydroelectric) or Egbin (thermal) produce electricity based on Faraday's Law. Step-up transformers then raise the voltage to extremely high levels (e.g., 330 kV, 132 kV) for efficient long-distance transmission across the national grid, minimizing current and thus `I2R` power losses along power lines stretching across states from Lagos to Kano. Subsequently, numerous step-down transformers at substations and distribution points reduce these voltages to manageable levels (e.g., 33 kV, 11 kV, and finally 220 V/415 V) for safe domestic and industrial consumption in Nigerian homes, markets, and factories. Domestic and Commercial Electrical Appliances: Electric motors are indispensable in almost every Nigerian household and business. They power essential appliances such as ceiling fans to combat heat, blenders for food preparation, washing machines, refrigerators to preserve food, and even borehole pumps for water supply in many communities. These devices directly demonstrate the conversion of electrical energy into useful mechanical work, a direct application of the motor effect. Similarly, small step-down transformers are integrated into chargers for mobile phones, laptops, and various other gadgets, converting the 220-240V AC mains to the specific lower DC voltages required by these electronics.
Transportation and Healthcare: Induction coils play a vital role in the ignition systems of many vehicles and motorcycles commonly found on Nigerian roads, generating the high-voltage sparks necessary to ignite the fuel-air mixture in engines. This is a crucial application for internal combustion engines. In healthcare, advanced technologies like Magnetic Resonance Imaging (MRI) scanners, which use powerful superconducting electromagnets, are found in major Nigerian hospitals for diagnostic purposes, demonstrating sophisticated applications of magnetic fields. Renewable energy sources like wind turbines also utilize large generators, based on electromagnetic induction, to convert wind energy into electricity, contributing to Nigeria's efforts towards sustainable energy.