From 01db95c900d83aa6f2534b5820132d63bf04556d Mon Sep 17 00:00:00 2001
From: Erik Strand <erik.strand@cba.mit.edu>
Date: Wed, 13 Mar 2019 22:06:38 -0400
Subject: [PATCH] Answer 7.1

---
 _psets/5.md | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/_psets/5.md b/_psets/5.md
index b63f047..a5aa5c0 100644
--- a/_psets/5.md
+++ b/_psets/5.md
@@ -7,6 +7,23 @@ title: Problem Set 5
 Cables designed to carry signals with minimum pickup of interference often consist of a twisted pair
 of conductors surrounded by a grounded shield. Why the twist? Why the shield?
 
+Twisted pairs are used for differential signaling, i.e. where we listen to the difference between
+the voltages on the wires as opposed to either absolute value. Noise can be imparted to our
+conductors by external electric or magnetic fields. In particular, we need to worry about changing
+fields (since static electric fields won't affect our current, and static magnetic fields will
+induce a [Hall effect](https://en.wikipedia.org/wiki/Hall_effect) but not overlay a different
+signal.)
+
+If they were weren't twisted, a close noise source would add more noise to the closer wire, thus
+distorting the voltage difference. Twisting distributes noise on average evenly across the two
+wires, helping preserve the differential signal.
+
+A shield helps prevent noise sources from interacting with the conductors at all. As we've seen,
+oscillating electric fields can only penetrate so far. So the shield effectively provides a low pass
+filter for the sorts of noise that can reach the conducting pair, with the cutoff frequency
+determined by the shield's thickness. Grounding it ensures that built up static charge has a place
+to go, though not all shields are grounded.
+
 ## (7.2)
 
 Salt water has a conductivity ∼4 S/m. What is the skin depth at $$10^4$$ Hz?
-- 
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