Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Explain the behavior of inductors in AC and DC circuits.
• Describe the operation of a bicycle dynamo.
• Illustrate the workings of a simple AC generator and its conversion to a DC generator.
• Solve problems using Faraday's Law.

Lesson summary

This lesson explores the fascinating principle of electromagnetic induction, which is the foundation of how we generate most of the electricity we use in Ghana today. From the massive turbines at the Akosombo Dam to the small generator your family might use during a power outage ("dumsor"), the physics is the same. We will investigate how a simple coil of wire, called an inductor, behaves differently when connected to a battery (DC) versus the mains supply from ECG/VRA (AC). We will then apply these principles to understand practical devices like bicycle dynamos and electric generators.

Lesson notes

PART A: The Inductor in DC and AC Circuits

An inductor is a passive electrical component that consists of a coil of wire. It stores energy in a magnetic field when electric current flows through it. The key principle governing inductors is Lenz's Law, which states that an induced electromotive force (e.m.f.) will always give rise to a current whose magnetic field opposes the original change in magnetic flux that produced it. In simple terms, inductors resist *changes* in current. Behaviour of an Inductor in a DC Circuit

Imagine connecting an inductor in series with a bulb to a battery (a DC source). At the instant the switch is closed: Current begins to flow from the battery. This creates a *growing* magnetic field in the inductor. According to Faraday's and Lenz's laws, this *change* in magnetic flux induces a "back e.m.f." in the coil. This back e.m.f. opposes the battery's voltage, thus opposing the initial flow of current. Result: The current does not jump to its maximum value instantly. It grows gradually. The bulb will brighten slowly, not immediately. The inductor acts as if it has inertia, resisting the change from zero current. A long time after the switch is closed: The current has reached its maximum, steady value (determined by the battery voltage and the total resistance of the circuit, I = V/R). The magnetic field in the inductor is now constant and no longer changing. Since there is no change in magnetic flux (dΦ/dt = 0), there is no induced back e.m.f. Result: The inductor behaves simply as a piece of wire with low resistance. It no longer opposes the current flow. The bulb glows at its normal, steady brightness. When the switch is opened: The circuit is broken, and the current tries to fall to zero rapidly. This causes the magnetic field in the inductor to collapse quickly. This rapid *change* in flux induces a very large e.m.f. in the *same* direction as the original current (Lenz's law again - opposing the decrease). This large induced e.m.f. can be strong enough to cause the current to arc or spark across the switch contacts as it is opened. This is the principle behind spark plugs in engines. Behaviour of an Inductor in an AC Circuit

In an AC circuit, the voltage and current are constantly changing direction and magnitude. In Ghana, our mains supply has a frequency of 50 Hz, meaning it changes direction completely 100 times per second. Because the current is *always changing*, the magnetic field in the inductor is also *always changing*. This means the inductor is *continuously* inducing a back e.m.f. that opposes the flow of current. This continuous opposition to the flow of AC current is called Inductive Reactance (Xₗ). Inductive reactance is measured in Ohms (Ω), just like resistance.

Evaluation guide

• Discuss the behaviour of the inductor in a.c and d.c circuits.
• Talk for learning:
• Discuss the operations of a bicycle dynamo.
• Discuss the op erations of a simple a.c. generator .