Alternating Current Free PDF Download
FAQs on NCERT Books Free Download for Class 12 Physics Chapter 7 - Alternating Current
1. Which topics from Chapter 7, Alternating Current, are most important for 5-mark questions in the CBSE 2025-26 board exam?
For 5-mark questions, you should focus on comprehensive topics that allow for detailed explanation, derivation, and numerical application. The most expected questions are:
- Derivation of the expression for impedance, phase angle, and current in a series LCR circuit using phasor diagrams. This is often paired with a numerical problem.
- The principle, construction, and working of a transformer, including a discussion on energy losses like copper loss, eddy currents, and flux leakage.
- Analysis of resonance in a series LCR circuit. This includes deriving the resonant frequency (ω₀), explaining the sharpness of resonance, and defining the Quality (Q) factor.
2. What types of derivations are frequently asked from the Alternating Current chapter for the board exams?
Derivations are a crucial part of scoring well in this chapter. For the CBSE Class 12 Physics exam 2025-26, prioritise the following:
- Expression for instantaneous current when an AC voltage is applied to a pure inductor and a pure capacitor.
- Derivation of impedance (Z) and phase angle (ϕ) for a series LCR circuit.
- The formula for resonant frequency: f₀ = 1 / (2π√(LC)).
- The transformer equation: Vp/Vs = Np/Ns = Is/Ip for an ideal transformer.
3. Which concepts in Alternating Current are important for 3-mark questions?
For 3-mark questions, focus on specific concepts and their direct application. Important areas include:
- Explaining the significance of RMS (root mean square) value of current and voltage and deriving its relation with the peak value (I_rms = I_₀/√2).
- Calculating the power factor and average power dissipated in an LCR circuit.
- Explaining the principle and working of an AC generator with a labelled diagram.
- Distinguishing between a step-up and a step-down transformer.
4. Why is the concept of RMS value more significant than the peak value for AC circuits?
The RMS value of an alternating current is more significant because it provides a measure of its effective value. The RMS value is the equivalent DC current that would produce the same heating effect in a resistor over a given time. Since the average value of a complete AC cycle is zero, it cannot be used to determine power (P = I²R). Therefore, all AC voltmeters and ammeters are calibrated to measure the RMS value, which is crucial for calculating power dissipation and rating electrical appliances.
5. How can the voltage across the inductor or capacitor in a series LCR circuit be greater than the source voltage? Is this a violation of conservation of energy?
Yes, in a series LCR circuit (especially near resonance), the individual voltages across the inductor (V_L) and capacitor (V_C) can be much larger than the source voltage. This is not a violation of energy conservation or Kirchhoff's laws. The reason is that V_L and V_C are 180° out of phase. The total voltage across the L-C combination is their vector sum (V_L - V_C), which can be very small. The net source voltage is the vector sum of V_R and (V_L - V_C), correctly following Kirchhoff's loop rule. This phenomenon is known as voltage magnification and is a key feature of resonant circuits.
6. What is the significance of the Quality Factor (Q-factor), and what kind of HOTS questions can be framed from it?
The Quality Factor (Q-factor) of a series LCR circuit signifies the sharpness of resonance. A high Q-factor indicates a sharp resonance peak, meaning the circuit is highly selective to a particular frequency. This is vital in tuning circuits for radios and TVs. A Higher-Order Thinking Skills (HOTS) question could ask you to analyse how the sharpness of the resonance curve changes if the resistance (R) in the circuit is doubled while keeping L and C constant, and to explain the practical implication of this change for a radio receiver.
7. What are the common errors students make when solving problems on transformers?
A common error is confusing the equations for step-up and step-down transformers; always remember that for a step-up transformer, Ns > Np and Vs > Vp. Another frequent mistake is incorrectly applying the current relationship, which is inverse: Is/Ip = Np/Ns. Students also often forget to consider the efficiency (η) of the transformer. Unless stated as ideal (100% efficient), the output power will be less than the input power (P_out = η × P_in). Forgetting to list the sources of energy loss when asked conceptually is also a common oversight.
