Illustrated Physics
physics, drawn

Physics, drawn.

Physics is taught in equations and understood in pictures, and the pictures are usually the part left out. This is a collection of essays about the second kind — one idea at a time, illustrated to the point where the argument becomes visible.

The field of a dipoleField lines traced from a positive charge toward a negative one. Every line does eventually close on the negative charge, but the outer ones loop far outside any frame, so this picture is a crop rather than the whole field.+−
Fig. 1 The field around a positive and a negative charge. Every line is traced by stepping along the local field direction, so the shape is a consequence of the inverse-square law rather than a drawing of what it ought to look like. The lines running off the edge do close on the negative charge — they simply do it outside the frame.

Start anywhere

2 essays

−π−π/2π/2π-22angleangular velocityat rest, hanging downbalanced upside downclosed: swingingopen: going over the topdashed: the boundary Mechanics

The pendulum, and the small lie that makes it simple

A pendulum's period does not depend on how far it swings. This is one of the most useful false statements in physics, and it is worth knowing exactly how false.

3 figures +− Electromagnetism

Field lines are a choice, not a discovery

Nothing in space is arranged in lines. The lines are a drawing convention — and an unusually good one, because three separate facts about the field survive the translation.

3 figures

Threads running through

themes, not chapters

What stays the same

Conservation laws, and the habit of solving a problem by refusing to look at the middle of it.

1 essays

The same equation again

A pendulum, a circuit, a molecule and a bridge, all obeying one differential equation and not knowing about each other.

0 essays

Approximations that lie

The small-angle assumption, the frictionless plane, the point mass — where each is fine, and precisely where it stops being.

1 essays

Fields, not forces

Replacing action at a distance with something that fills space, and what that buys.

1 essays

The arrow of time

Nearly every law works equally well backwards. Almost nothing else does, and the gap between those facts is thermodynamics.

0 essays

Where the model stops

Every picture in physics has a domain of validity, and the interesting physics usually lives at its edge.

1 essays

Order out of the random

Enormous numbers of unpredictable particles producing quantities you can print on a dial.

0 essays