# Is natural frequency of an LC circuit equal to angular frequency? Why don't the units match?

When I was reading the LC circuit in my textbook I came across the derivation of equations of instantaneous charge and current.

7.8 LC Oscillations

We know that a capacitor and an inductor can store electrical and magnetic energy, respectively. When a capacitor (initially charged) is connected to an inductor, the charge on the capacitor and the current in the circuit exhibit the phenomenon of electrical oscillations similar to oscillations in mechanical systems (Chapter 14, Class XI).

Let a capacitor be charged $$q_m$$ (at $$t=0$$) and connected [to] an inductor as shown in Fig. 7.18.

The moment the circuit is completed, the charge on the capacitor starts decreasing, giving rise to current in the circuit. Let $$q$$ and $$i$$ be the charge and current in the circuit at time $$t$$. Since $$\mathrm{d} i/\mathrm{d}t$$ is positive, the induced emf in $$L$$ will have polarity as shown, i.e. $$v_b. According to Kirchhoff's loop rule, $$\frac{q}{C} - L \frac{\mathrm{d}i}{\mathrm{d}t} = 0 \tag{7.39}$$ $$i = -(\mathrm{d}q/\mathrm{d}t)$$ in the present case (as $$q$$ decreases, $$i$$ increases). Therefore, Eq. (7.39) becomes: $$\frac{\mathrm{d}^2 q}{\mathrm{d}t^2}- \frac{1}{LC}q = 0 \tag{7.40}$$ This equation has the form $$\frac{\mathrm{d}^2 x}{\mathrm{d}t^2}- \omega_0^2 x = 0$$ for a simple harmonic oscillator. The charge, therefore, oscillates with a natural frequency $$\omega_0 = \frac{1}{\sqrt{LC}} \tag{7.41}$$ and varies sinusoidally with time as $$q = q_m \cos(\omega_0 t + \phi) \tag{7.42}$$ where $$q_m$$ is the maximum value of $$q$$ and $$\phi$$ is a phase constant. Since $$q=q_m$$ at $$t=0$$, whe have $$\cos(\phi)=1$$ or $$\phi=0$$.

Which is no problem, but when I got to the derivation to current, it said something like

The current $$i\left(= -\frac{\mathrm{d}q}{\mathrm{d}t}\right)$$ is given by $$i = i_m \sin(\omega_0t)$$ where $$i_m = \omega_0 q_m$$

Which I don't quite understand.

It says in the third line, $$i_m = \omega_0 q_m,$$ where $$\omega_0$$ is the natural frequency of the LC circuit, $$i_m$$ is the maximum current and $$q_m$$ is the maximum charge.

So, in other words, it's trying to say that $$\omega_0 =1/T$$, which is certainly not correct, as $$\omega$$ is an angular frequency and not just a frequency, i.e. it has the unit of radians per second and not just $$\mathrm{s}^{-1}$$.

However, we know that $$\omega_0 = \frac{1}{\sqrt{LC}},$$ so this could help us, as by substituting the units of $$L$$ (coefficient of self-inductance) and $$C$$ (coefficient of capacitance), we can get the unit of $$\omega_0$$. But, surprisingly, we would get $$[\omega_0] = 1/(\mathrm{s}^2)^{1/2}$$ which ultimately is $$[\omega_0] = \mathrm{s}^{-1}.$$

So, basically, it's saying that angular frequency is equal to just "frequency", which contradicts the fact their units are different. How can this be true?

Then again, one might ask, how current can oscillate "angularly"?

So my question is, are both natural frequency and angular frequency the same thing? And if they are, then can someone please explain the relationship $$f_0 = \frac{\omega_0}{2\pi} = \frac{1}{2\pi\sqrt{LC}}$$ to me?

To summarize, I just wanted to know few things:

1. How is angular frequency (radians per second) (has kind-of "physical meaning") be equal to just frequency (cycles per second) [has all meanings {physical and electrical (oscillation of electrons)} but here, In our case, just focusing to electrical]?

2. If they are both equal, then, how can electrons oscillate angularly?

3. If angular frequency is not equal to frequency, then, is my textbook wrong?

Also, I think I'm missing something, probably my basics are not so clear.

• Radians aren't a real dimension. "rad/s" is the same thing as "1/s". Angular frequency is just defined as $2 \pi$ times frequency. Jan 10, 2019 at 22:15
• Jan 10, 2019 at 22:20
• If Wn = 1/T (i.e. = Fo ) , then how can it be, at the same time,. Wn = 2π Fo ? Jan 11, 2019 at 11:49
• Why do you think it's trying to say that $\omega_0 = 1/T$? I don't see that anywhere. Jan 11, 2019 at 14:54
• Can anyone tell me that, is my textbook justified by doing : Im = Wo Qm ? Wouldn't it be Im = Wo Qm/ 2π ? Jan 11, 2019 at 15:16

So my question is , are both natural frequency and angular frequency the same thing ?

No. A resonant system has a natural frequency which is characteristic of the system. From the Wikipedia article Natural frequency:

Natural frequency, also known as eigenfrequency, is the frequency at which a system tends to oscillate in the absence of any driving or damping force.

The natural frequency of system may be expressed in units of ordinary frequency $$(\mathrm{Hz})$$ or angular frequency $$(\mathrm{\frac{rad}{s}})$$

Both ordinary frequency and angular frequency have the same dimension of inverse time $$[\mathrm{T^{-1}}]$$ but ordinary frequency express the number of cycles per second while angular frequency expresses the number of radians per second. Since there are $$2\pi$$ radians in one cycle, it follows that angular frequency is just the ordinary frequency multiplied by $$2\pi\mathrm{\frac{rad}{cycle}}$$.

Typically, the symbol for ordinary frequency is $$f$$ or $$\nu$$ while the symbol angular frequency is $$\omega$$ thus

$$\omega = 2\pi\nu$$

You can see this clearly in the expression for the energy of a photon of frequency $$\nu$$ which can be expressed as

$$E_\gamma = h\nu = \frac{h}{2\pi}2\pi\nu = \hbar\omega$$

This is an angular frequency not a frequency. This is common practice in all areas of physics/math/engineering etc. The frequency $$f_0 = \omega_0/(2\pi)$$ and this is equal to $$1/T$$, where $$T$$ is the period of oscillation.

When dealing with oscillating or periodic equations governed by a term like $$\sin(2\pi f t)$$, the scale factor of $$2\pi$$ ensures that the function complete one cycle when $$t$$ completes one frequency. $$f$$, not $$2\pi f$$ is the quantity that most naturally relates to a physical measure of something (exception = rotational motion where $$\omega = d\theta/dt$$).

Radians are defined as the arc length over the radius: $$\theta=\text{ arc length}/\text{radius}$$. But both can be kept in any units and you would have said that this is dimensionless. Essentially it is.

People could measure $$\theta$$ this way, or by degrees, which are two completely different methods, explaining the same logic. In physics, angles are generally measured in radians.

So when you say $$f=\omega/2\pi$$, $$\omega$$ is describing the same logic $$f$$ does, except in an angular context.

In this way, you could relate $$\theta$$ measured in radians and degrees as follows:

$$\theta_r$$ = $$\theta_0$$ $$\pi/180$$. These are not fundamentally different, it is just a form of definition.

This is the comparison between mechanic system ( spring - mass system) and LC - oscillator.

• Firstly, your image is not clear. Secondly, images should be used to upload diagrams, graphs, etc. Image of texts will usually draw downvotes from passersby.
– lee
Jan 25, 2021 at 17:11