Q

(nuclear physics) disintegration energy
(physics) A measure of the ability of a system with periodic behavior to store energy equal to 2π times the average energy stored in the system divided by the energy dissipated per cycle. Also known as Q factor; quality factor; storage factor.
(thermodynamics) A unit of heat energy, equal to 10¹⁸ British thermal units, or approximately 1.055 × 10²¹ joules.

Often called the quality factor of a circuit, Q is defined in various ways, depending upon the particular application. In the simple RL and RC series circuits, Q is the ratio of reactance to resistance, as in Eqs. (1),
1.
where X_L is the inductive reactance, X_C is the capacitive reactance, and R is the resistance. An important application lies in the dissipation factor or loss angle when the constants of a coil or capacitor are measured by means of the alternating-current bridge.

Q has greater practical significance with respect to the resonant circuit, and a basic definition is given by Eq. (2),
2.
where Q₀ means evaluation at resonance. For certain circuits, such as cavity resonators, this is the only meaning Q can have.

For the RLC series resonant circuit with resonant frequency f₀, Eq. (3)
3.
holds, where R is the total circuit resistance, L is the inductance, and C is the capacitance. Q₀ is the Q of the coil if it contains practically the total resistance R. The greater the value of Q₀, the sharper will be the resonance peak.

The practical case of a coil of high Q₀ in parallel with a capacitor also leads to Q₀ = 2πf₀L/R. R is the total series resistance of the loop, although the capacitor branch usually has negligible resistance.

In terms of the resonance curve, Eq. (4) holds,
4.
where f₀ is the frequency at resonance, and f₁ and f₂ are the frequencies at the half-power points. See also Resonance (alternating-current circuits).