A periodic phenomenon is one that occurs at regular intervals. Periodic phenomena are all around us, from the rising of the sun every morning to the repeating structure of the DNA in our cells, it's clear that nature loves to repeat itself!
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Jetzt kostenlos anmeldenA periodic phenomenon is one that occurs at regular intervals. Periodic phenomena are all around us, from the rising of the sun every morning to the repeating structure of the DNA in our cells, it's clear that nature loves to repeat itself!
Analyzing these phenomena scientifically is particularly elegant as we only have to understand a few characteristic features to get the full picture. In this article, we will apply this analysis to one of the fundamental phenomena in physics, periodic waves. By doing so we can find a full mathematical description of periodic waves from just a couple of key quantities, the wavelength, and the frequency of a wave.
In physics, waves are a type of energy transfer caused by an initial disturbance that is then propagated through space and time. The size of the disturbance defines the amplitude of the wave. If a wave has a continually repeating pattern, made up of cycles, such as particles oscillating around an equilibrium point, we call it a Periodic Wave. Periodic waves have several key quantities by which they can be characterized, depending on the time and distance between cycles. The characteristic time taken, in seconds \(\mathrm{s}\), for each cycle is known as the Time Period \(T\) of the wave. The number of cycles that occur within one second is known as the Frequency \(f\) of the wave, and is measured in Hertz \(\mathrm{Hz}\) equivalent to \(\mathrm{s}^{-1}\). By definition, the frequency and time period of a wave are inverses.
\[f=\frac{1}{T}\]
Similarly, the characteristic distance taken for a cycle is known as the Wave Length \(\lambda\). The wavelength is related to the frequency of the wave by the velocity \(v\) with which the wave propagates through space.
\[v=f\lambda=\frac{\lambda}{T}\]
Sound Waves are a clear example of a periodic wave. An initial disturbance, such as someone clapping their hands, causes particles in the air to oscillate around their equilibrium position. These oscillations are then transferred through the air causing the sound to be heard away from the person's hands. The rate at which the air particles oscillate determines the frequency of the sound, which is how our ears characterize the 'pitch' of a sound.
The speed of sound, which determines the relationship between a sound's frequency and wavelength, varies in different mediums. For example, in air the speed of sound is \(v=343\,\mathrm{m\,s}^{-1}\) so a sound with a frequency of \(f=300\,\mathrm{Hz}\) will have a wavelength of
\[\lambda=\frac{v}{f}=\frac{343}{300}=1.14\,\mathrm{m}\]
Periodic waves can be split into two main types of waves, depending on the direction of the displacement the wave causes. Longitudinal waves cause oscillations that are parallel to the direction of energy transfer. Some key examples of longitudinal are sound waves and stress waves within materials.
On the other hand, many periodic waves are such that the direction of oscillation is perpendicular to the direction of energy transfer. For example, light is a type of electromagnetic radiation caused by electric and magnetic fields which oscillate at right angles to the direction the light travels in.
Whilst the focus of this article is on periodic waves, its worth briefly looking at a-periodic waves known as pulses to emphasize the defining features of a periodic wave. Pulses are a very common type of energy transfer in physics, caused by sudden short disturbances which propagate as a short burst of energy. Whilst a pulse may have cycles like a periodic wave, pulses usually only contain one or two cycles such that we cannot properly define a wavelength or frequency of a pulse.
For example, consider dropping a pebble into a pool. The disturbance caused by the pebble would lead to a ripple of a few water waves traveling outwards, however shortly after the pebble was dropped the water would return to equilibrium and no further waves would be produced. We say that the pebble produced pulses in the water, if instead the water was continuously disturbed, like with a wave machine, then the water waves would be periodic.
Given what we know about periodic waves so far, let's consider how we could best represent periodic waves using mathematical functions. As we have seen, the period of a wave is defined by its characteristic wavelength or time period. Consider a periodic wave, with an amplitude A, and a wavelength of \(\lambda\). This means we are looking for a function that satisfies the following conditions.
\[\begin{align}f(x)&=f(x+\lambda)\\\max |f(x)|&=A\end{align}\]
As you might be able to guess, the trigonometric sine and cosine functions are the functions we are looking for, given that they also satisfy similar periodicity conditions.
\[\sin(x)=\sin(x+2n\pi),\,\cos(x)=\sin(x+2n\pi)\]
As the sine and cosine functions can be made equivalent by adding a phase of \(\frac{\pi}{2}\), we simply choose which function we want, depending on the initial conditions of the wave. As we usually think of waves as starting with maximum displacement, we'll consider the cosine function.
By smartly choosing scaling factors for the cosine function, we can satisfy the conditions needed
\[\begin{align}f(x)&=A\cos\left(\frac{2\pi}{\lambda}x\right)\\\max|f(x)|&=A\\f(x+\lambda)&=A\cos\left(\frac{2\pi}{\lambda}\left(x+\lambda\right)\right)\\&=A\cos\left(\frac{2\pi}{\lambda}x+2\pi\right)\\&=A\cos\left(\frac{2\pi}{\lambda}x\right)\\&=f(x)\end{align}\]
So a periodic wave, with amplitude \(A\) and wavelength \(\lambda\), at one fixed instant of time \(t\), is described by the function
\[f(x)=A\cos\left(\frac{2\pi}{\lambda}x\right)\]
In physics the quantity \(\frac{2\pi}{\lambda}\) is called the 'angular wavenumber' usually denoted \(k\).
Applying the same thinking, we can find the function describing the oscillation of a single point on the wave over time. If the wave has a time period of \(T\) and amplitude \(A\), then the oscillations as a function of time are given by
\[\begin{align}h(t)&=A\cos\left(\frac{2\pi}{T}t\right)\\&=A\cos\left(2\pi f t\right)\\&=A\cos\left(\omega t\right)\end{align}\]
The quantity \(\omega=2\pi f\) is known as the angular frequency of the wave.
Consider an oscillating electric field \(E(t)\) at a point \(x\), if the maximum value of the electric field is \(10\,\mathrm{N}\,\mathrm{C}^{-1}\) and the electric field oscillates with a frequency of \(f=124\,\mathrm{Hz}\), what will the value of the electric field be after \(0.3\,\mathrm{s}\)?
Recall the wave equation given in the previous section.
\[f(t)=A\cos\left(2\pi f t\right)\]
From the question we know \(A=10\,\mathrm{N}\,\mathrm{C}^{-1}\) and that \(f=124\,\mathrm{Hz}\), plugging in these values gives
\begin{align}f(t)&=10\,\mathrm{N}\,\mathrm{C}^{-1}\cos\left(2\pi\cdot124\,\mathrm{Hz}\cdot10\,\mathrm{s}\right)\\&=3.09\,\mathrm{N}\,\mathrm{C}^{-1}\end{align}
What is the formula for the periodic electric field oscillations given below in Figure 6?
Looking at the graph, we see that the wavelength of this periodic wave is \(4\,\mathrm{m}\) and its amplitude is \(4\,\mathrm{N}\,\mathrm{C}^{-1}\). Also, note that the wave begins at \(0\) field strength so we need to use the sine function.
\[E(x)=4\sin\left(\frac{2\pi}{4}x\right)\,\mathrm{N}\,\mathrm{C}^{-1}=4\sin\left(\frac{\pi}{2}x\right)\,\mathrm{N}\,\mathrm{C}^{-1} \]
Water waves and all types of Electromagnetic waves are examples of periodic waves.
Periodic waves are types of energy transfer where the disturbance has a continually repeating pattern made up of cycles. Periodic waves can be transverse, meaning the oscillations caused by the wave are perpendicular to the direction of energy transfer, and longitudinal waves where the oscillations are parallel to the direction of energy transfer.
The period of a wave defines how long it takes the wave to make one full cycle and is inversely related to the frequency of the wave.
Periodic waves are caused by continuous repeated disturbances. For example, if you move a spring up and down in a repeated motion it will produce a periodic wave.
A periodic wave function is any function f(t) defining a wave such that there is some period T such that f(t)=f(t+T).
What is the velocity \(v\) of a periodic wave with frequency \(f\) and wavelength \(\lambda\)?
\(v=f\lambda\)
If a periodic water wave travels across a pond with a speed of \(v=2\,\mathrm{m}\,\mathrm{s}^{-1}\) and a time period of \(t=10\,\mathrm{s}\), what is its wavelength?
\(20\,\mathrm{m}\)
What is the frequency of a wave travelling at \(v=200\,\mathrm{m}\,\mathrm{s}^{-1}\) if it has a wavelength of \(\lambda=0.5\,\mathrm{m}\)?
\(f=400\,\mathrm{Hz}\).
If a sound wave has a frequency of \(5\times10^{3}\,\mathrm{Hz}\) and a wavelength of \(0.2\,\mathrm{m}\) what is its velocity?
\(v=1\times10^3\,\mathrm{m}\,\mathrm{s}^{-1}\)
The frequency \(f\) and time period \(T\) of a periodic wave are inversely related to each other. True or False?
True.
What is the frequency of a periodic wave if it makes 2 complete cycles in \(3\,\mathrm{s}\)?
\(0.66\,\mathrm{Hz}\)
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