Ch 13AC Circuits — Class 12 Physics, concept-first.

Electrical supply comes in two basic forms. Direct current (DC) has a fixed polarity: the positive and negative terminals of the source never swap, so current flows in one constant direction (a batter…

An AC generator works by rotating a coil inside a magnetic field (or, equivalently, rotating a magnet near a stationary coil), so that the magnetic flux linked with the coil changes continuously and s…

Because an alternating voltage or current continuously changes both its magnitude and its sign throughout every cycle, it is often convenient to describe it using a single representative number rather…

The average (or mean) value of an alternating current or voltage over a FULL cycle is always exactly zero: for every instant where takes some positive value, there is a corresponding instant exactly h…

Ordinary moving-coil meters (ammeters and voltmeters) work by responding to the average value of whatever current or voltage is applied to them -- but since the true (full-cycle) average of AC is alwa…

The mathematics of adding several alternating quantities that are out of phase with each other (for example, the separate voltage drops across a resistor, an inductor and a capacitor all carrying the…

Having established the phasor idea, this section works out, one element at a time, exactly how a resistor, an inductor and a capacitor each individually respond when connected to a sinusoidal AC sourc…

Consider a resistor of resistance R connected directly across an AC source of instantaneous emf . Since the potential drop across the resistance must, at every instant, equal the applied emf (, from O…

Now consider a circuit containing only a pure inductor of inductance L (its own resistance assumed negligible), connected to a source of alternating emf .

Now consider a circuit containing only a capacitor of capacitance C, connected to a source of alternating emf .

Now combine a resistor R, an ideal inductor L and an ideal capacitor C all in SERIES with a source of alternating emf, as in Fig. 13.12.

In a DC circuit, electrical power is simply the product of the (constant) voltage and current: . In an AC circuit, however, both the instantaneous voltage e and instantaneous current i are continuousl…

For a pure resistor, current and voltage are exactly in phase: and . The instantaneous power is therefore .

For a purely inductive circuit, the current lags the applied emf by a phase angle of : and . The instantaneous power is .

For a purely capacitive circuit, the current leads the applied emf by a phase angle of : and . The instantaneous power is -- by exactly the same reasoning as for the inductor (section 13.6.2), just wi…

In the general case, when a series LCR circuit carries a current driven by an emf that leads or lags the current by some phase angle (as derived in section 13.5.4), the instantaneous power is .

So far every circuit considered has included an external AC source continuously driving the current. This section instead asks: what happens if a capacitor, already charged to some potential, is disco…

Resonance is a general phenomenon that occurs in any system possessing a natural tendency to oscillate at some particular characteristic frequency, called its natural (or resonant) frequency.

In a series LCR circuit (Fig. 13.17), the impedance is . At very low frequencies, is small (negligible) while is very large, so Z is large and dominated by the capacitor.

Now consider an inductor L and a capacitor C connected instead in PARALLEL with each other (Fig. 13.19), this combination connected across the AC source .

Even away from a series LCR circuit's precise resonant frequency , the current amplitude does not fall to zero immediately -- for a given resistance R, it decreases only gradually as moves away from o…

If the goal is simply to REDUCE the amount of AC current flowing through some part of a circuit (for example, to safely limit the current through a fluorescent tube or a fan), an obvious approach woul…

Sample papers and previous-year board questions for this subject.