Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Describe what a transformer is and its purpose.
• Identify the key components of a transformer.
• Explain how electromagnetic induction makes a transformer work.
• Calculate voltage changes using the transformer formula.

Lesson summary

Welcome, students. Today, we are going to study one of the most important devices in our electrical world: the transformer. Look at the electricity poles outside. You will see a large, grey metal box on some of them. That is a transformer. The small, heavy block you plug in to charge your phone is also a transformer. From the mighty Akosombo Dam to the phone in your pocket, transformers are essential for making electricity useful and safe for us. They are the reason we can transport electricity over long distances from power stations and then use it in our homes at a safe voltage.

Lesson notes

A. What is a Transformer? A transformer is a static electrical device (it has no moving parts) that transfers electrical energy from one alternating current (AC) circuit to another through the principle of electromagnetic induction. It is most commonly used to increase ('step-up') or decrease ('step-down') voltage levels.

Key Point: Transformers ONLY work with Alternating Current (AC). They do not work with Direct Current (DC) from a battery because DC does not produce a *changing* magnetic field, which is necessary for induction. B. The Foundation: Laws of Electromagnetic Induction The operation of a transformer is based on two fundamental laws of physics. Faraday's Laws of Electromagnetic Induction: First Law: Whenever the magnetic flux (the amount of magnetic field lines) linked with a coil of wire changes, an electromotive force (e.m.f.), or voltage, is induced in the coil. In simple terms: *A changing magnetic field creates a voltage in a nearby wire.* Second Law: The magnitude (size) of the induced e.m.f. is directly proportional to the rate of change of the magnetic flux. In simple terms: *The faster the magnetic field changes, or the more turns of wire in the coil, the higher the induced voltage.* Lenz's Law: This law gives the direction of the induced current. It states that the direction of the induced e.m.f. and current is always in such a way that it opposes the very change that produced it. Think of it as electrical inertia – it resists the change. This law ensures the conservation of energy.

These laws together explain Mutual Induction, which is the principle behind the transformer. Mutual induction is the process where a changing current in one coil induces an e.m.f. (voltage) in a nearby coil. C. Construction of a Transformer A simple transformer is made of three main parts: The Soft Iron Core: Function: To concentrate the magnetic flux produced by the primary coil and guide it efficiently to the secondary coil. It provides an easy path for the magnetic field lines. Material: Made of soft iron (or silicon steel) because it can be easily magnetised and demagnetised. Construction: The core is not a solid block of iron. It is laminated, meaning it is built up from thin sheets of soft iron, each coated with a layer of insulating varnish (like shellac). Reason for Lamination: To reduce energy losses due to eddy currents. A changing magnetic field induces small, circular currents within the core itself. These eddy currents produce heat (`I²R` loss) and waste energy. By splitting the core into thin, insulated sheets, the paths for these currents are broken, significantly reducing the loss. The Primary Winding (Coil): This is the coil of insulated copper wire that is connected to the input AC power source. The number of turns in this coil is denoted as `Np`. The Secondary Winding (Coil): This is the coil of insulated copper wire where the voltage is induced. It is connected to the load (e.g., the appliance you want to power). The number of turns in this coil is denoted as `Ns`.

The two coils are electrically separate but magnetically linked by the soft iron core. D. Principle of Operation (Step-by-Step) Let's follow the energy from input to output: An alternating voltage (Vp) from the source (like an ECG wall socket) is applied to the primary winding. This causes an alternating current (Ip) to flow through the primary coil. According to Oersted's discovery, this changing current produces a continuously changing magnetic flux in the soft iron core. The soft iron core concentrates and guides this changing magnetic flux to pass through the secondary winding. As the changing magnetic flux lines "cut" through the turns of the secondary coil, a voltage (Vs) is induced in the secondary winding, according to Faraday's Laws of Induction. This is mutual induction. If the secondary circuit is closed by connecting a load, an alternating current (Is) will flow to power the load. E. Types of Transformers and The Transformer Equation

Evaluation guide

• Describe the construction and principle of operation of a transformer.
• Focal Area: Transformer
• Laws of electromagnetic induction
• The construction of a Transformer