# Gas station without pumps

## 2011 December 28

### Fields from nowhere

Filed under: home school — gasstationwithoutpumps @ 18:16
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Greg Jacobs, in his post Just the basics, not the sources, of electric, magnetic fields, suggests that when teaching about electrical and magnetic fields, it is best to teach the effect of a uniform field on particle before teaching how the field is created.  He supports this with his claim that gravitational attraction is best taught with $|F|\approx mg$ before teaching $F=-G \frac{m_1 m_2}{|\vec{r}|^2} \vec{r}$Matter and Interactions does this, with the first equation on page 66 in Chapter 2, and the second on page 96 in Chapter 3.

I won’t be doing electrostatics or electromagnetics this year (we’re only aiming to finish the Mechanics part of Matter and Interactions), so I thought I’d leave myself a note here in the blog, so that I can revisit this question next year, when it matters.  Oops, that’s not quite true.  We already had $F= \frac{q_1 q_1}{4\pi \epsilon_0 |\vec{r}|^2}\vec{r}$ back in Chapter 3, so it looks like M&I does precisely what Greg Jacobs was objecting to: introducing the creation of the field prior to talking about the effect of the field on a particle.  Since we’ve already crossed that bridge, I don’t think there is much point to back-tracking.  Especially since the energy chapters will be looking at potential energy in fields (gravitational or electrical).    Next year, when we finally get to Chapter 14, Electric Fields, we’ll be starting with the key concept “A charged particle makes an electric field at every point in space (except its own location),” so there won’t be any opportunity to look at the effect of fields on a particle without looking at how the field is formed.  Perhaps I’ll need to look for opportunities this year to present a simplified view.