NEET Alternating Currents — Set 24

Question 1

What is the effect on the impedance of a series LCR circuit when the frequency is increased beyond the resonant frequency?

Accepted Answer

Beyond the resonant frequency, the inductive reactance ($ X_L = \omega L $) increases with frequency, while the capacitive reactance ($ X_C = \frac{1}{\omega C} $) decreases. Since $ X_L $ grows faster than $ X_C $ decreases, the net reactance ($ X_L - X_C $) becomes positive, increasing the impedance ($ Z = \sqrt{R^2 + (X_L - X_C)^2} $).

Question 2

Why does a step-down transformer increase the current in the secondary coil compared to the primary coil?

Accepted Answer

In a step-down transformer ($ V_s < V_p $, $ N_s < N_p $), power is conserved ($ V_p I_p = V_s I_s $). Since $ V_s $ is lower, $ I_s $ must be higher than $ I_p $ ($ I_s = I_p \times \frac{N_p}{N_s} $, where $ \frac{N_p}{N_s} > 1 $) to maintain the same power output.

Question 3

What is the primary purpose of using laminated cores in transformers?

Accepted Answer

Laminated cores in transformers reduce eddy current losses. By dividing the core into thin, insulated layers, the induced currents are confined to smaller paths, minimizing energy loss as heat and improving efficiency.

NEET Physics: Alternating Currents — Practice Set 24

Q1. What is the effect on the impedance of a series LCR circuit when the frequency is increased beyond the resonant frequency?

Q2. Why does a step-down transformer increase the current in the secondary coil compared to the primary coil?

Q3. What is the primary purpose of using laminated cores in transformers?

Q4. A \( 45 \, \text{mH} \) inductor is connected to a \( 230 \, \text{V} \), \( 50 \, \text{Hz} \) source. What is the peak current?

Q5. A series LCR circuit has \( R = 25 \, \Omega \), \( X_L = 45 \, \Omega \), \( X_C = 20 \, \Omega \). What is the power factor?

Q6. A \( 10 \, \mu\text{F} \) capacitor is connected to a \( 230 \, \text{V} \), \( 50 \, \text{Hz} \) source. What is the rms current?

Q7. A \( 120 \, \Omega \) resistor is connected to a \( 240 \, \text{V} \) (rms) AC source. What is the rms current?

Q8. A \( 40 \, \text{mH} \) inductor is connected to a \( 230 \, \text{V} \), \( 50 \, \text{Hz} \) AC source. What is the rms current?

Q9. A \( 35 \, \mu\text{F} \) capacitor is connected to a \( 230 \, \text{V} \), \( 50 \, \text{Hz} \) AC source. What is the peak current?

Q10. A series LCR circuit has \( R = 70 \, \Omega \), \( X_L = 40 \, \Omega \), \( X_C = 20 \, \Omega \). What is the impedance?