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--- 184_notes:examples:week9_earth_field [2018/07/03 12:20] – curdemma
+++ 184_notes:examples:week9_earth_field [2021/07/05 21:55] (current) – [Solution] schram45
@@ Line 18: / Line 18: @@
 ===Approximations & Assumptions===
-  * The UFO can be approximated as a moving point charge.
+  *  $q$ ,  $\vec{v}$ ,  $h$ , and  $\vec{B}_{\text{earth}}$  are all constants: All these variable could easily be changing in this problem. However, to simplify down the model to better analyze the situation we will assume all these things are constant. This could be an accurate assumption if the UFO were cruising steadily over us at the given height.
-  *  $q$ ,  $\vec{v}$ ,  $h$ , and  $\vec{B}_{\text{earth}}$  are all constants.
   * Your sense of direction and eyesight can be trusted.
@@ Line 25: / Line 24: @@
   * We represent the Biot-Savart Law for the magnetic field from a moving point charge as
  $\vec{B}= \frac{\mu_0}{4 \pi}\frac{q \vec{v}\times \vec{r}}{r^3}$
+<WRAP TIP>
+===Approximation===
+We will approximate the UFO as a point charge. This may not be the most accurate assumption since the UFO is most likely a large object, but it is an approximation that must be made in order to apply our knowledge to the problem. Since the problem doesn't state anything otherwise and it is a necessary assumption to use the equation above, we will approximate it as a point charge.
+</WRAP>
   * We represent the situation with the following pictures. Coordinate axes and cardinal directions are specified.
@@ Line 57: / Line 60: @@
  $q=\frac{4 \pi h^2 B_{\text{earth}} \tan \theta}{\mu_0 v} = 131 \text{ C}$
+This is a lot of charge. Since magnetic fields are very small in magnitude, and this can be seen in the magnetic constant, it makes sense that we would need such a large amount of charge to have a noticeable effect on our compass. Uf we were to compare the magnetic field of a moving charge to the electric field from that charge, the electric field would be much larger.