Lesson Notes By Weeks and Term v4 - SHS 3
ALTERNATING CURRENT
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ALTERNATING CURRENT
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Subject: Physics
Class: SHS 3
Term: 2nd Term
Week: 7
Grade code: 3.3.2.LI.2
Strand code: 3
Sub-strand code: 2
Content standard code: 3.3.2.CS.2
Indicator code: 3.3.2.LI.2
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ALTERNATING CURRENT
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Welcome, future engineers and scientists! In our homes, the electricity we get from the Electricity Company of Ghana (ECG) is Alternating Current (AC). Unlike the Direct Current (DC) from a battery, AC constantly changes direction. This has huge advantages for power transmission, but it also means that components like capacitors and inductors behave in new and interesting ways. In a simple DC circuit, the only thing that opposes the flow of current is resistance. In an AC circuit, we have other forms of opposition. Today, we will explore these new types of opposition, called reactance and impedance.
Recap: Resistance in DC Circuits In a DC circuit, the opposition to the flow of current is called resistance (R). It is a property of the material itself and does not depend on the frequency of the current (since DC has a frequency of 0 Hz). Ohm's Law gives us the relationship: `V = IR`. Resistance is measured in ohms (Ω). New Concepts for AC Circuits
In AC circuits, we have two additional components that oppose current flow in unique, frequency-dependent ways: capacitors and inductors. A. Reactance (Symbol: X) Reactance is the opposition to the flow of alternating current offered by either a capacitor or an inductor. Like resistance, it is also measured in ohms (Ω). The key difference is that reactance depends on the frequency (f) of the AC supply.
There are two types of reactance: Inductive Reactance (Xₗ) An inductor (a coil of wire) opposes a *change* in current by inducing a back e.m.f. (Lenz's Law). In an AC circuit, the current is always changing. The faster it changes (i.e., the higher the frequency), the greater the back e.m.f. and the greater the opposition. Therefore, inductive reactance is directly proportional to the frequency. The formula is: > Xₗ = 2πfL > Where: > * Xₗ = Inductive Reactance in ohms (Ω) > * f = Frequency of the AC supply in Hertz (Hz) > * L = Inductance of the inductor in Henries (H) Capacitive Reactance (X꜀) A capacitor opposes current flow by storing charge. In an AC circuit, it is continuously charging and discharging. At low frequencies, the capacitor has more time to charge up, building up more voltage that opposes the current flow. Thus, the opposition is high. At high frequencies, the current changes direction so quickly that the capacitor has very little time to charge up. Therefore, it offers very little opposition. Therefore, capacitive reactance is inversely proportional to the frequency. The formula is: > X꜀ = 1 / (2πfC) > Where: > * X꜀ = Capacitive Reactance in ohms (Ω) > * f = Frequency of the AC supply in Hertz (Hz) > * C = Capacitance of the capacitor in Farads (F)