Is Alternating Current part of the EAPCET Physics syllabus?

Yes. Alternating Current is taught in Intermediate second year Physics, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.

How many Physics questions are there in EAPCET?

40 of the 160 questions in the Engineering paper are Physics, spread across 30 chapters.

What mistakes do students make in Alternating Current?

Adding R, X_L and X_C directly — they combine as Z = √[R² + (X_L − X_C)²]. Calculating average power as V_rms × I_rms — multiply by the power factor cos φ, or use I²ᵣₘₛR. Using peak values where rms values are needed — mains voltage ratings and meter readings are rms. Thinking capacitive reactance increases with frequency — X_C = 1/(ωC) decreases as frequency rises, while X_L increases.

EAPCET Physics · Intermediate second year

Alternating Current

Alternating Current covers AC circuits with resistors, inductors and capacitors, resonance, power in AC circuits and transformers.

Subject: Physics
Taught in: Intermediate second year
Physics in the paper: 40 of 160 questions

Key formulas and results

$I_{\text{rms}} = \frac{I_{0}}{\sqrt{2}}$
$X_{L} = \omega L$ and $X_{C} = \frac{1}{\omega C}$
$Z = \sqrt{R^{2} + (X_{L} - X_{C})^{2}}$
Resonant frequency $f_{0} = \frac{1}{2\pi\sqrt{LC}}$
Average power $P = V_{\text{rms}}I_{\text{rms}}\cos\phi$
Transformer: $\frac{V_{s}}{V_{p}} = \frac{N_{s}}{N_{p}}$
Ideal transformer currents: $\frac{I_{s}}{I_{p}} = \frac{N_{p}}{N_{s}}$
Average and rms values: $i_{\text{avg}} = \frac{\int i\,dt}{\int dt}$ ; $i_{\text{rms}} = \sqrt{\frac{\int i^{2}\,dt}{\int dt}}$
Series RL: $Z = \sqrt{R^{2} + X_{L}^{2}}$ , $\tan\phi = X_{L}/R$ ; series RC: $Z = \sqrt{R^{2} + X_{C}^{2}}$ , $\tan\phi = X_{C}/R$
Phase angle of a series LCR circuit: $\tan\phi = \frac{X_{L} - X_{C}}{R}$
Power factor $\cos\phi = R/Z$ ; average power $P = I_{\text{rms}}^{2}R$
Quality factor of a series resonant circuit: $Q = \frac{X_{L}}{R} = \frac{1}{R}\sqrt{\frac{L}{C}}$
Ideal transformer: $\frac{E_{s}}{E_{p}} = \frac{N_{s}}{N_{p}} = \frac{I_{p}}{I_{s}}$

What this chapter covers

RMS values
Reactance and impedance
LCR circuits
Resonance
Power and power factor
Transformers

How it is usually asked

Impedance and current
Resonance
Power factor
Transformers

Understanding Alternating Current

An alternating voltage varies sinusoidally, so circuits are described with rms values: V_rms = V₀/√2, and ordinary AC meters read rms. A resistor keeps current and voltage in phase. In a pure inductor the current lags the voltage by 90°, with reactance X_L = ωL rising with frequency; in a pure capacitor the current leads by 90°, with reactance X_C = 1/(ωC) falling with frequency.

In a series LCR circuit these oppositions combine as phasors, not simple sums, giving the impedance Z = √[R² + (X_L − X_C)²]. At the resonant frequency X_L = X_C, the impedance is just R and the current is maximum. Only the resistor dissipates power, so the average power is V_rms I_rms cos φ, where cos φ = R/Z is the power factor. An ideal transformer changes voltage in the ratio of the turns while keeping power the same.

EAPCET usually tests impedance, resonance, power factor and transformer ratios. A useful check is the limiting cases: at very low frequency a capacitor behaves like an open circuit and an inductor like a plain wire, while at very high frequency the reverse is true. In an ideal transformer V_p I_p = V_s I_s, so stepping the voltage up steps the current down by the same ratio.

Worked example

EAPCET-style question

A series LCR circuit has $R = 30\ \Omega$ , $X_{L} = 80\ \Omega$ and $X_{C} = 40\ \Omega$ , and is connected to a 200 V (rms) AC supply. The average power consumed is

(1)480 W
(2)800 W
(3)640 W
(4)1333 W

Solution

Impedance $Z = \sqrt{30^{2} + (80 - 40)^{2}} = \sqrt{900 + 1600} = 50\ \Omega$ .
$I_{\text{rms}} = 200/50 = 4\ \text{A}$ .
Power factor $\cos\phi = \frac{R}{Z} = \frac{30}{50} = 0.6$ .
$P = V_{\text{rms}}I_{\text{rms}}\cos\phi = 200 \times 4 \times 0.6 = 480\ \text{W}$ (check: $I^{2}R = 16 \times 30 = 480\ \text{W}$ ).

Answer: 480 W

Shortcuts and results to remember

In a purely inductive or purely capacitive circuit the average power over a cycle is zero, so the current is wattless; in a choke coil the current lags the emf by nearly $\pi/2$ .
At resonance $X_{L} = X_{C}$ , so $Z = R$ , the current is maximum and in phase with the voltage ( $\cos\phi = 1$ ).
$X_{L} = \omega L$ rises with frequency and $X_{C} = \frac{1}{\omega C}$ falls with it, so a capacitor blocks DC and an inductor passes it.
Over a full cycle the average of $\sin\omega t$ or $\cos\omega t$ is zero and the average of $\sin^{2}\omega t$ is $\tfrac{1}{2}$ , which is why $I_{\text{rms}} = I_{0}/\sqrt{2}$ .
A step-up transformer ( $N_{s} > N_{p}$ ) raises the voltage and lowers the current in the same ratio.

Common mistakes to avoid

Adding R, X_L and X_C directly — they combine as Z = √[R² + (X_L − X_C)²].
Calculating average power as V_rms × I_rms — multiply by the power factor cos φ, or use I²ᵣₘₛR.
Using peak values where rms values are needed — mains voltage ratings and meter readings are rms.
Thinking capacitive reactance increases with frequency — X_C = 1/(ωC) decreases as frequency rises, while X_L increases.

How to prepare it

At resonance X_L = X_C, so the circuit behaves like a pure resistor — use that to simplify.

Once the formulas feel familiar, test them the way the exam does: practise Alternating Current questions chapter-wise, then attempt a full previous year paper to see whether it holds up under a 3-hour timer.

Related Physics chapters

See every chapter in the EAPCET Physics syllabus.

Practise Alternating Current chapter-wise

Chapter-wise quizzes show exactly which topics in this chapter you still get wrong — before a full paper does.

Frequently asked questions

Is Alternating Current part of the EAPCET Physics syllabus?: Yes. Alternating Current is taught in Intermediate second year Physics, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.
How should I prepare Alternating Current for EAPCET?: At resonance X_L = X_C, so the circuit behaves like a pure resistor — use that to simplify.
How many Physics questions are there in EAPCET?: 40 of the 160 questions in the Engineering paper are Physics, spread across 30 chapters.
What mistakes do students make in Alternating Current?: Adding R, X_L and X_C directly — they combine as Z = √[R² + (X_L − X_C)²]. Calculating average power as V_rms × I_rms — multiply by the power factor cos φ, or use I²ᵣₘₛR. Using peak values where rms values are needed — mains voltage ratings and meter readings are rms. Thinking capacitive reactance increases with frequency — X_C = 1/(ωC) decreases as frequency rises, while X_L increases.