There are all sorts of resonances around us, in the world, in our culture, and in our technology. A tidal resonance causes the 55 foot tides in the Bay of Fundy. Mechanical and acoustical resonances and their control are at the center of practically every musical instrument that ever existed. Even our voices and speech are based on controlling the resonances in our throat and mouth. Technology is also a heavy user of resonance. All clocks, radios, televisions, and gps navigating systems use electronic resonators at their very core. Doctors use magnetic resonance imaging or MRI to sense the resonances in atomic nuclei to map the insides of their patients. In spite of the great diversity of resonators, they all share many common properties. In this blog, we will delve into their various aspects. It is hoped that this will serve both the students and professionals who would like to understand more about resonators. I hope all will enjoy the animations.

For a list of all topics discussed, scroll down to the very bottom of the blog, or click here.

Origins of Newton's laws of motion

Non-mathematical introduction to relativity

Three types of waves: traveling waves, standing waves and rotating waves new

History of mechanical clocks with animations
Understanding a mechanical clock with animations
includes pendulum, balance wheel, and quartz clocks

Water waves, Fourier analysis



Saturday, July 16, 2011

Transformation of electric and magnetic fields

all topics by author introduction to relativity contents-mathematics of relativity contents-transforming electromagnetic fields previous: four vectors next: transforming Maxwell equations-intro and 1st ME

15. Transformation of electric and magnetic fields

Computational strategy

We cannot feel or see electric and magnetic fields. We hypothesize them to account for forces and trajectories of charged particles and currents. Electric and magnetic fields are detected or "felt" with charges and currents. Along these lines, in order to figure out how electric and magnetic fields transform under change in reference frame, we must see how the Lorentz force changes upon change of reference frames:

Lorentz force equation    .    (15.1)


Box 15.1. Writing the force F in terms of forces in the primed reference frame

To start, we need the transformation for force. We define force by the effect it has on the motion of objects using Newton's second law:

      F = dp/dt    .     (15.2)

We need the momentum transform from Chapter 11, which to repeat is:

transform for momentum     which means     differential of momentum ,    (15.3)

where ε is energy as discussed in Chapter 11. Script "E" is used to distinguish energy from electric field. We've also assumed that the velocity V of the reference frame does not change, which also means γ does not change either.

We also need the inverse Lorentz transform for time from Chapter 4:

inverse Lorentz transform for time     which means that     differential of t     .

Dividing dp by dt gives us the force in Equation (15.2):

transform for force, step 1    .    (15.4)

Dividing both the numerator and denominator by dt' and replacing the components of dp'/dt' with the corresponding components of F', we have:

transform for force, step 2    .    (15.5)

We can relate the time derivative of energy in the above equation to force using:

derivative of energy    ,    (15.6)

This makes (15.5) become:

transform for force, step 3    .    (15.7)

Continuing to simply:

transform for force, step 4        (15.8)


Next we need to transform the components of u' into corresponding components of u. This may seem like a step backwards, but in the end we need primed variables times unprimed velocities, u, to match the form of (15.1) above where E and B will be functions of E', B' and V (the relative velocity of the primed and unprimed reference frames).


Box 15.2. Converting the velocities in the primed reference frame to the unprimed frame.

We start by repeating the transform for velocity from Chapter 9:

inverse velocity transform         (15.9)

The messiest factor in this transform will be converting the u'x in the  1 + u'xV/c2  factor occurring in the denominators of (15.8) to ux. We first transform and simplify this factor:

simplifying one factor        

We use this to transform the u' in (15.8) into corresponding components of u as promised:

transform for force, step 4     .    

We cancel factors as appropriate to write:

transform for force, step 5        (15.10)


Box 15.3. Introducing electric and magnetic fields.

We next need to substitute the Lorentz force (containing electric and magnetic fields) in for the forces. Just to remind you, the Lorentz force in the primed reference frame is:

Lorentz force in primed coordinates     .    (15.11)

When we substitute the various force components of (15.11) into (15.10), F becomes:

transform for force, step 6     .    (15.12)

This last substitution has reintroduced some primed velocities. We transform these into velocities in the unprimed reference frame using (15.9):

transform for force, step 7     .    (15.13)

With some rearrangement, this becomes

transform for force, step 8        (15.14)

where a number of terms in the first line have been collected and designated as T given by:

one set of terms in the Lorentz force matrix     .    (15.15)

T can be further simplified by the following steps:

one set of terms in the Lorentz force matrix

one set of terms in the Lorentz force matrix

one set of terms in the Lorentz force matrix

one set of terms in the Lorentz force matrix

Substituting this into (15.14) we have:

simplified Lorentz force     and

simplified Lorentz force     (15.16)

We need to interpret the above in terms of the standard Lorentz force:

Lorentz force in unprimed reference frame    .    (15.17)

Matching up (15.16) with (15.17), looking at the multipliers of the various velocities, we see that (15.16) will become (15.17) provided that

electric field transforms          magnetic field transforms     .    (15.18)


Summary

Below we have repeated (15.18) as well as a vector notation of the results (in terms of the parallel and perpendicular components). These are the transforms of the electric and magnetic fields to be used when changing from one reference frame to another.

transformation equation for electric fields    .    (15.19)

transformation equation for magnetic fields    .    (15.20)


all topics by author introduction to relativity contents-mathematics of relativity contents-transforming electromagnetic fields previous: four vectors next: transforming Maxwell equations-intro and 1st ME