Viva Voce: RLC Parallel Circuits and Resonance Characteristics

Definition of Rejector Circuit: A parallel combination of a resistor ( $R$ ), inductor ( $L$ ), and capacitor ( $C$ ) is formally classified as a "rejector circuit."
Functionality of Rejection: This circuit configuration is so named because it disallows or restricts the flow of current within a specific, certain frequency range.
Frequency Response: By rejecting specific frequencies within its operational bandwidth, the circuit acts as a filter that prevents those frequencies from passing efficiently.

Definition of Resonance: Within an RLC parallel circuit, resonance is defined as the specific frequency of the applied sinusoidal voltage at which the current flowing through the circuit reaches its minimum value.
Physical Basis for Minimum Current: The occurrence of minimum current at the point of resonance is directly attributed to the impedance of the circuit. At resonance, the impedance ( $Z$ ) of a parallel RLC circuit reaches its maximum possible value, thereby minimizing the current flow according to Ohm's Law ( $I = \frac{V}{Z}$ ).
Resonance Frequency Conditions: The phenomenon is specific to a certain frequency designated for the given combination of components.

Current Behavior Relative to Resonance: While the current is at its minimum at the resonance frequency, it maintains a higher value at all other frequencies (both below and above resonance).
Lower and Upper Limit Frequencies ( $f_1$ and $f_2$ ): - Lower Limit Frequency ( $f_1$ ): This is the specific frequency below the resonance frequency at which the circuit current rises to exactly $\sqrt{2}$ or approximately $1.414$ times its minimum value. - Upper Limit Frequency ( $f_2$ ): This is the specific frequency above the resonance frequency at which the circuit current rises to exactly $\sqrt{2}$ or approximately $1.414$ times its minimum value.
Visual Representation: These frequencies are typically identified on a response curve graph, often labeled as Fig. (b) in experimental documentation.

Response Curve: For the full definition and procedural context regarding the response curve of an RLC parallel circuit, refer to the documentation provided in Experiment No. 37, Question No. 4.
Bandwidth: The detailed explanation and calculation of bandwidth within an RLC parallel circuit are contained within the materials for Experiment No. 37, Question No. 6.
Quality Factor ( $Q$ ): The definition and significance of the Quality Factor ( $Q$ -factor) are detailed in the records for Experiment No. 37, Question No. 7.
Recorded Measurement: A specific numerical value is noted for $L.S.$ as follows: - $L.S. = 4830\,H$

Q.1. Why is a RLC parallel circuit called a rejector circuit? - Ans. A given combination of $R$ , $L$ and $C$ in parallel dis-allows the current to flow in a certain frequency range only. For this reason it is known as a rejector circuit ie. it rejects some specific frequencies.
Q.2. What is meant by resonance in RLC parallel circuit? - Ans. For a certain frequency of the sinusoidal voltage applied to the RLC parallel circuit, the current flowing in the circuit has a minimum value. This phenomenon is known as resonance.
Q.3. Why is the current minimum at resonance in a RLC parallel circuit. - Ans. Because the impedance of the circuit is maximum at resonance.
Q.4. What is meant by response curve of a RLC parallel circuit? - Ans. Same as Q.No.4, experiment No.37.
Q.5. What are lower and upper limit frequencies? - Ans. In a RLC parallel circuit, the current is minimum at resonance frequency. At all other frequencies the current has a higher value. The frequencies $f_1$ and $f_2$ , below and above the resonance frequency at which the current rises to $\sqrt{2}$ or $1.414$ times of its minimum value are known as lower and upper limit frequencies respectively, Fig. (b).
Q.6. What is bandwidth of RLC parallel circuit? - Ans. Same as Q.No.6, experiment No.37.
Q.7. What is quality factor? - Ans. Same as Q.No.7, experiment No.37.