Dr Phill’s Science Made Simple

Mathematics is key to understanding physics. Many physics articles have the unforunate problem that they dive deep into mathematics without establishing a context. This make many such articles incomprehensible.

Another issue is that some mathematical constructs can be written in different ways. Each construct is valid provided that it is used consistently.

Also, the mathematics describing a physical phenomenon can often the derived in several different ways. An equation can sometimes be written in different forms. There are also shorthand notations used to simplify equations. It is also common to get rid of constants by setting their value to 1. These simplifications can make the understanding of the mathematics difficult to understand.

Field Operators

If a scalar field, which is a function of position, is ϕ then the gradient is:

ϕ=(ϕ1x1,ϕ2x2,ϕ3x3)

This is a vector field that points in the direction of greatest change in the scalar field.

The divergence of a vector field a is a scalar field. It is the flux generation per unit volume at each point of the field.

a=a1x1+a2x2+a3x3

The Laplacian is the divergence of a gradient. It is a scalar second deriviative of a scalar field that represents the curvature of the field.

2ϕ=ϕ=2ϕ1x12+2ϕ2x22+2ϕ3x32

There is also a vector Laplacian.

2a=a=(2a1x12,2a2x22,2a3x32)

The curl of a vector field ×a is a vector field. The curl of a vector field at point a measures the tendency of particles at a to rotate about the axis that points in the direction of the curl at P.

×a=(a3x2a2x3,a1x3a3x1,a2x1a1x2)

As a curl is a cross product it is perpendicular to both vectors. The dot product of thwo perpendicular vectors is always zero. Hence, the divergence of a curl is always identically zero.

×a0

The curl of a curl of a vector vector field is another vector field.

××a=a2a

Spacetime Vectors

The spacetime vector has four components, where the time dimension is multiplied by the speed of light c to make it a distance.

In Minkowski spacetime, the vector is:

x=[ctxyz]

The metric tensor ηij is required to calculate the distance between differential offsets in each dimension dxi. Where η00=1, ηii=1i>0, ηij=0ij.

ds2=ηijdxidxj=c2dt2dx2dy2dz2

The ct coordinate is sometimes the fourth element rather than the first. The metric tensor η can also be negated. All of the options are valid as long as they are used consistently.

If ds2>0 then events are time-like and can see each other after a period of time.

If ds2=0 then events are light-like and can see each other if travelling at the speed of light.

If ds2<0 then events are space-like and can’t see each as it would require faster than light travel.

Another, more convenient approach is to make the time dimension complex.

x=[ictxyz]

In this case the metric tensor is not required as it is the identity matrix.

ds2=dxidxj=c2dt2+dx2+dy2+dz2

Note that the sign is negated and ds2<0 is now time-like.

Relativistic Energy Equation

The total energy of a relativistic body is derived from the rest mass and momentum.

E2=p2c2+(mc2)2

Heaviside-Lorentz (HL) Units

Electromagnetic equations in the SI system contain the constants ϵ0 and μ0. The units of related parameters also differ. Oliver Heaviside and Hendrik Lorentz proposed changes to units to simplify equations. These units are in common usage.

The vacuum permittivity ϵ0 has SI units C2s2m3kg1. In HL this constant is merged into other units.

The vacuum permeability μ0 has SI units C2mkg. In HL this constant is merged into other units.

The electric field vector E has SI units ms2C1kg. In HL E=ϵ0ESI has units Cm2.

The polarisation density vector P has SI units Cm2. It is the same in HL.

The electric displacement field vector D has SI units Cm2. It is the same in HL.

The magnetic flux density vector $ has SI units s1C1kg. In HL B=1μ0BSI, 1cB has units Cm2.

The magnetic field strength vector H has SI units Cm1s1. In HL 1cHSI has units Cm2.

The magnetisation vector M has SI units Cm1s1. In HL 1cM has units Cm2.

The current density J has SI units Cm2s1. In HL 1cJ has units Cm3.

The electric potential ϕ, or voltage v has SI units ms2C1kg. In HL ϕ=ϵ0ϕSI has units Cm2.

We can now compare SI equations with HL equations.

SI HL
D=ϵ0E+P D=E+P
B=μ0(H+M) B=H+M

Maxwell’s Equations Revisted

Maxwell’s equations in HL become:

D=ρ B=0 ×E=1cBt ×H=1cDt+1cJ

The Lorentz force defines the effect of electric and magnetic fields on a moving charged particle with charge q, velocity v, and momentum p.

F=dpt=q(E+1cv×B)

Using Potentials

Equations can be simplified using potentials.

The scalar electric potential, or voltage, is ϕ.

The magnetic vector potential A is a vector satisfying:

×A=B

The curl operator doesn’t have an exact inverse. So, any vector A that gives the desired value of B can be used.

The electic field vector can also be defined in terms of potentials.

E=ϕAt