What is resonance in AC circuits?

Short Answer

Resonance in AC circuits is a condition in which the inductive reactance and capacitive reactance become equal in magnitude but opposite in nature. When this happens, they cancel each other, and the circuit allows maximum current to flow.

Resonance occurs mainly in circuits that contain an inductor (L) and a capacitor (C). At the resonant frequency, the circuit behaves like a purely resistive circuit, and the voltage and current are in phase. This condition is very useful in radio tuning, filters, and communication systems.

Detailed Explanation

Resonance in AC circuits

Resonance in AC circuits is a special and important condition that occurs when an inductor and a capacitor in a circuit exchange energy at the same rate. In such a situation, the inductive reactance (XL) and capacitive reactance (XC) become equal in magnitude. However, because inductive reactance is positive and capacitive reactance is negative, they cancel each other. This cancellation makes the circuit behave like it contains only resistance.

Resonance occurs in AC circuits that include both inductance (L) and capacitance (C). These circuits are called LC circuits or RLC circuits. The point at which resonance occurs is known as the resonant frequency. At this frequency, the current becomes maximum in a series circuit, or voltage becomes maximum in a parallel circuit.

Condition for resonance

Resonance happens when:

Where:

  •  is inductive reactance
  •  is capacitive reactance
  •  is the angular frequency

At resonance:

This gives the resonant frequency:

Resonance in a series RLC circuit

In a series RLC circuit:

  • Resistance (R), inductance (L), and capacitance (C) are connected in series
  • Inductive and capacitive reactances cancel each other at resonance

At resonance:

  • Total reactance becomes zero
  • Impedance equals resistance (Z = R)
  • Current becomes maximum
  • Voltage and current are in phase

This is why resonance is sometimes called the condition of maximum current.

Resonance in a parallel RLC circuit

In a parallel RLC circuit:

  • L and C are connected in parallel
  • At resonance, currents through L and C cancel each other

At resonance:

  • The circuit draws minimum current from the source
  • The impedance becomes maximum
  • Voltage is maximum across the circuit

This is why parallel resonance is also called current resonance.

Physical explanation of resonance

Resonance occurs because:

  • Inductor stores energy in the form of a magnetic field
  • Capacitor stores energy in the form of an electric field

When frequency matches the natural frequency of the LC pair:

  • Energy oscillates smoothly between L and C
  • There is no energy loss in the reactances
  • Only the resistor uses energy
  • Current reaches a maximum or minimum depending on the circuit type

This smooth energy transfer creates resonance.

Characteristics of resonance

  1. At resonance, reactances cancel each other
  1. Impedance is minimum in series circuits and maximum in parallel circuits
  2. Current is maximum in series resonance and minimum in parallel resonance
  3. Voltage and current become in phase
  4. Power factor becomes unity
  5. Energy oscillates between inductor and capacitor

Applications of resonance

Resonance is used in many real-life applications where selecting or generating a particular frequency is required.

  1. Radio and TV tuning

When you select a radio station, you adjust the LC circuit inside your radio to resonate at the frequency of that station.

  1. Filters

Used in band-pass and band-stop filters to allow or block selected frequencies.

  1. Wireless communication

Used in mobile networks, antennas, transmitters, and receivers.

  1. Induction heating

Resonance allows high-frequency currents to heat metals efficiently.

  1. Oscillators

Electronic oscillators use resonance to generate stable AC signals.

Importance of resonance in AC engineering

Resonance is important because:

  • It helps select specific frequencies
  • It improves signal quality
  • It allows designing efficient circuits
  • It is essential for communication systems
  • It helps minimize power losses

Understanding resonance is key to working with alternating current.

Conclusion

Resonance in AC circuits occurs when inductive and capacitive reactances are equal, causing them to cancel each other. This results in either maximum or minimum current depending on the type of circuit. Resonance is crucial for tuning, filtering, and communication systems, as it allows the selection of a specific frequency. It also ensures efficient transfer of energy between inductors and capacitors and improves the performance of AC circuits.