Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Electromagnetism
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Electromagnetism
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Subject: Basic Electronics
Class: Senior Secondary 1
Term: 3rd Term
Week: 7
Theme: Basic Electronic Theory
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Explain the terms:- electric field- electromagnet-electromagnetism- in ductance State the applications of electromagnetism, (e. g. electric bell, relays, transformer, etc) Describe the principle of operation of a transformer.
A. Electric Field An electric field is a region around an electrically charged particle or object where its influence can be felt. This influence manifests as a force exerted on other charged particles within that region.
Characteristics: It is a vector quantity, having both magnitude and direction. Its direction is conventionally defined as the direction of the force that a positive test charge would experience if placed in the field. Electric field lines (or lines of force) are used to represent the field visually. They originate from positive charges and terminate on negative charges. The density of field lines indicates the strength of the field: closer lines mean a stronger field. Electric field lines never intersect.
Creation: An electric field is created by stationary electric charges. B. Electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Unlike a permanent magnet, the magnetism of an electromagnet can be turned on and off by controlling the current.
Construction: Typically consists of a coil of wire (solenoid) through which an electric current is passed, often wound around a core of ferromagnetic material (like soft iron).
How it works: When current flows through the coil, it generates a magnetic field. The soft iron core intensifies this magnetic field, concentrating the magnetic flux. When the current is switched off, the magnetic field largely disappears.
Factors affecting electromagnet strength:
1. Current: The greater the current flowing through the coil, the stronger the magnetic field.
2. Number of turns: More turns in the coil lead to a stronger magnetic field (flux density is directly proportional to the number of turns per unit length).
3. Core material: Using a ferromagnetic core (e.g., soft iron) significantly increases the magnetic field strength compared to an air core or non-magnetic core. Soft iron is preferred due to its high magnetic permeability and low retentivity (it easily magnetises and demagnetises).
4. Length of the coil: For a given number of turns, a shorter coil (more concentrated turns) generally produces a stronger field.
Right-Hand Grip Rule (for solenoids): If the fingers of the right hand are curled in the direction of the current flow in the coil, the thumb points in the direction of the magnetic North Pole of the electromagnet. C. Electromagnetism Electromagnetism is the branch of physics that studies the interaction between electric currents (or changing electric fields) and magnetic fields. It describes how electricity produces magnetism and how magnetism can, in turn, produce electricity (electromagnetic induction). This fundamental force is one of the four basic forces of nature. D. Inductance Inductance is the property of an electrical conductor to oppose any change in the electric current flowing through it. This opposition arises from the creation of an opposing voltage (electromotive force, EMF) within the conductor itself or in a nearby conductor, due to the changing magnetic field produced by the current.
Explanation: When the current in a coil changes, the magnetic field around it also changes. This changing magnetic field induces an EMF in the coil, according to Faraday's law of electromagnetic induction. This induced EMF opposes the change in current (Lenz's Law).
Unit: The SI unit of inductance is the Henry (H).
Types: Self-inductance: The property of a coil to induce an EMF in itself due to a change in the current flowing through it.
Mutual inductance: The property where a changing current in one coil induces an EMF in an adjacent coil.
Importance: Inductors (coils) are crucial components in electronic circuits for filtering, energy storage, and tuning circuits.
E. Applications of Electromagnetism
1. Electric Bell: Construction: Consists of an electromagnet, an armature (a soft iron strip) with a hammer attached, a gong, a contact screw, and a switch. * Principle of Operation: When the switch is pressed, current flows through the electromagnet, magnetising it. The electromagnet attracts the armature, causing the hammer to strike the gong. As the armature moves towards the electromagnet, it breaks the circuit at the contact screw. This de-energises the electromagnet, allowing the armature to spring back to for filtering, energy storage, and tuning circuits.
E. Applications of Electromagnetism
1. Electric Bell: Construction: Consists of an electromagnet, an armature (a soft iron strip) with a hammer attached, a gong, a contact screw, and a switch.
Principle of Operation: When the switch is pressed, current flows through the electromagnet, magnetising it. The electromagnet attracts the armature, causing the hammer to strike the gong. As the armature moves towards the electromagnet, it breaks the circuit at the contact screw. This de-energises the electromagnet, allowing the armature to spring back to its original position, re-establishing the circuit. The process repeats rapidly, causing continuous ringing.
2. Relay: Construction: An electromagnet, a movable armature (or switch), and a set of electrical contacts. The contacts are typically "normally open" (NO) or "normally closed" (NC).
Principle of Operation: A relay is an electrically operated switch. A small current applied to the electromagnet coil creates a magnetic field, which attracts the armature. This movement of the armature either closes (for NO contacts) or opens (for NC contacts) a separate, often higher-current circuit. This allows a low-power circuit to control a high-power circuit safely.
3. Transformer: Construction: Two coils (primary and secondary) wound on a common soft iron laminated core. The primary coil is connected to the AC input, and the secondary coil provides the AC output.
Principle of Operation: Operates on the principle of mutual induction. When an alternating current (AC) flows through the primary coil, it produces a continuously changing magnetic flux in the soft iron core. This changing flux links with the secondary coil, inducing an alternating EMF (voltage) across it. The ratio of the number of turns in the primary coil to the secondary coil determines whether the transformer is a step-up (increases voltage, secondary turns > primary turns) or step-down (decreases voltage, secondary turns < primary turns) transformer. Transformers only work with AC because a constant DC current would produce a constant magnetic field, thus no changing flux to induce an EMF in the secondary. Introduction (Engage - 10 minutes)
Teacher Activity: Begin by asking students what they know about magnets and electricity.
Pose questions like: "Can electricity produce magnetism?" "Can magnets be switched on and off?" "How do power lines reduce voltage for home use?" Student Activity: Students share their prior knowledge and ideas, responding to the teacher's questions. Development (Explore & Explain - 45 minutes)
Activity 1: Explaining Electric Fields and Electromagnets Teacher Activity: Define and explain "electric field" using simple diagrams showing field lines for positive and negative charges. Define "electromagnet" and contrast it with a permanent magnet.
Demonstration: Construct a simple electromagnet using a battery (e.g., 9V or D-cells), insulated copper wire, and a large iron nail. Show how it attracts small metallic objects (paper clips, pins) when current flows and releases them when the circuit is broken. Explain the factors affecting electromagnet strength. Introduce and demonstrate the Right-Hand Grip Rule for solenoids.
Student Activity: Students observe the demonstration, noting the temporary nature of the electromagnet. Students draw diagrams of electric field lines and a simple electromagnet. Students practice applying the Right-Hand Grip Rule to determine polarity.
Activity 2: Understanding Electromagnetism and Inductance Teacher Activity: Explain the overarching concept of "electromagnetism" as the interplay between electricity and magnetism. Introduce "inductance" as the property opposing current changes. Use an analogy (e.g., inertia in motion) to simplify the concept. Explain self-inductance and mutual inductance qualitatively.
Student Activity: Students listen, ask clarifying questions, and relate inductance to the idea of opposition or "lag." Activity 3: Exploring Applications (Electric Bell, Relay, Transformer)
Teacher Activity: Electric Bell: Present a diagram or actual model of an electric bell. Describe its construction and explain its operation step-by-step, highlighting how the electromagnet switches on and off.
Relay: Use a diagram or a simple relay module. Explain its construction and how it acts as an electrically controlled switch, connecting a low-power control circuit to a high-power load circuit. Discuss its use in remote control or automation.
Transformer: Display a diagram of a simple transformer (primary coil, secondary coil, laminated core). Explain the principle of mutual induction and why AC is essential for its operation. Distinguish between step-up and step-down transformers. Emphasize their role in power transmission in Nigeria (e.g., from power stations to homes).
Student Activity: Students observe diagrams/models, take notes, and ask questions about the functioning of each device. Students can draw simplified diagrams of the devices and label key parts. Students discuss where they have seen these devices in their daily lives. Consolidation (Elaborate - 15 minutes)
Teacher Activity: Lead a class discussion to summarise the key concepts and applications. Ask students to re-explain concepts in their own words. Correct misconceptions.
Student Activity: Participate in the discussion, offer explanations, and answer questions.
Question 1: Define the following terms: (a) Electric field, (b) Electromagnet, (c) Electromagnetism.
Solution 1: (a)
Electric field: This is the region surrounding an electrically charged object where another charged object would experience a force. It's conventionally represented by lines of force originating from positive charges and terminating on negative charges. (b)
Electromagnet: This is a temporary magnet whose magnetic field is produced by the flow of an electric current through a coil of wire, often wound around a ferromagnetic core like soft iron. Its magnetism can be turned on or off. (c)
Electromagnetism: This is the branch of physics that studies the relationship between electric currents (or changing electric fields) and magnetic fields. It describes how electricity produces magnetism and vice versa.
Question 2: List three factors that determine the strength of an electromagnet and explain how each factor influences it.
Solution 2: Current (magnitude of current): The strength of the electromagnet is directly proportional to the amount of current flowing through the coil. A larger current produces a stronger magnetic field.
Number of turns in the coil: The magnetic field strength is directly proportional to the number of turns of wire in the coil. More turns per unit length result in a more concentrated and stronger magnetic field.
Core material: Using a ferromagnetic material (like soft iron) as the core greatly increases the strength of the electromagnet compared to an air core or non-magnetic core. Ferromagnetic materials concentrate the magnetic flux lines.
Question 3: Describe the basic principle of operation of an electric bell.
Solution 3: The electric bell works on the principle of an electromagnet and make-and-break circuit. When the bell push (switch) is pressed, current flows from the battery through the electromagnet coil. This magnetises the electromagnet, which then attracts a soft iron armature. As the armature is attracted, a hammer attached to it strikes the gong, producing sound. Simultaneously, the movement of the armature causes it to break contact with a contact screw, interrupting the circuit. This de-energises the electromagnet, allowing a spring to pull the armature back to its original position, re-establishing contact with the screw. The circuit is completed again, and the process repeats rapidly, causing continuous ringing.
Question 4: Explain how a relay works as an electrically operated switch.
Solution 4: A relay consists of an electromagnet, an armature, and a set of electrical contacts. When a small control current is passed through the coil of the electromagnet, it becomes magnetised. This magnetic field attracts the movable armature. The movement of the armature causes it to either close or open a separate set of electrical contacts, thereby completing or breaking a secondary, often higher-power, circuit. Thus, a low-power control signal can switch on or off a high-power load circuit remotely and safely.
Question 5: State the principle of operation of a transformer and explain why it must operate with an alternating current (AC) supply.
Solution 5: The principle of operation of a transformer is mutual induction. When an alternating current (AC) flows through the primary coil of the transformer, it generates a continuously changing magnetic flux in the soft iron core. This changing magnetic flux then links with the secondary coil, inducing an alternating electromotive force (EMF) across it, according to Faraday's law of electromagnetic induction. A transformer must operate with an AC supply because its operation relies on a changing magnetic flux to induce an EMF in the secondary coil. If a direct current (DC) were supplied to the primary coil, it would produce a constant magnetic field, resulting in no change in magnetic flux. Without a changing magnetic flux, no EMF would be induced in the secondary coil, and the transformer would fail to operate.
Electricity Transmission and Distribution in Nigeria: Transformers are ubiquitous in Nigeria's power sector (e.g., PHCN/discos). Step-up transformers at power stations (e.g., Kainji Hydroelectric Power Station) increase voltage for efficient long-distance transmission, while numerous step-down transformers in communities reduce the voltage to usable levels (e.g., 230V for residential use) before it reaches homes and businesses. Students see these transformers daily on poles or in substations.
Security and Automation Systems: Electromagnets are used in various security systems, such as magnetic door locks found in modern offices and some homes in Nigerian cities. Relays are integral to automated gate systems, remote control switches for lighting or fans, and alarm systems, enabling low-power signals to control heavier loads. Scrap Metal Industry and Industrial Lifting: Large electromagnets are used in Nigerian scrap yards or manufacturing plants to lift and transport heavy iron and steel materials, making the handling of metallic waste or raw materials more efficient and safer than manual labour.
Electric Motors and Generators: While not explicitly detailed in the performance objectives for this week, electromagnetism is the core principle behind all electric motors (used in fans, blenders, pumps common in Nigerian homes) and generators (used for backup power during outages, or in large-scale power generation), converting electrical energy to mechanical or vice versa.