Tag: conservation

Circuit Basics

Studying voltage and current in circuits can start with two laws of conservation.

Note: Some of the links are dead, but this MIT Opencourse pdf has a detailed explanation. And Kahn Academy has some videos on the current laws as well.

  • KCL: Current flow into a node must equal the flow out of the node. (A node is a point on the wire connecting components in a circuit–usually a junction).
(KCL: Kirchoff's Current Law) Current flowing into any point on a circuit is equal to the current flowing out of it, A simple circuit with a voltage source (like a battery) and a resistor.
(KCL: Kirchoff’s Current Law) Current flowing into any point on a circuit is equal to the current flowing out of it, A simple circuit with a voltage source (like a battery) and a resistor.

  • KVC: The sum of all the voltage differences in a closed loop is zero.

KVL: The voltage difference across the battery (9 Volts) plus the voltage difference across the resistor (-9 Volts) is equal to zero.
KVL: The voltage difference across the battery (9 Volts) plus the voltage difference across the resistor (-9 Volts) is equal to zero.

Things get more interesting when we get away from simple circuits.

Current flow into a node (10 A) equals the flow out of the node (7 A + 3 A).
Current flow into a node (10 A) equals the flow out of the node (7 A + 3 A).

Note that the convention for drawing diagrams is that the current move from positive (+) to negative (-) terminals in a battery. This is opposite the actual flow of electrons in a typical wired circuit because the current is a measure of the movement of negatively charged electrons, but is used for historical reasons.

Based on the MIT OpenCourseWare Introduction to Electrical Engineering and Computer Science I Circuits 6.01SC Introduction to Electrical Engineering and Computer Science Spring 2011.

Conflict over water

One of the articles my students brought to our Environmental Science discussion was about the growing fears about wars over water. Even within the U.S.A. there are significant conflicts, as demonstrated by this NPR article.

Texas has tried to buy Oklahoma water from the state, its cities and towns, and its Native American tribes. But Oklahoma lawmakers have blocked those efforts with a string of laws restricting out-of-state water exports.

The view in Texas is that Oklahoma isn’t even using its full allocation of Red River water. Oklahomans respond that Texas hasn’t gotten serious enough about conservation.

“Our poor, poor thirsty people in Dallas, Texas,” muses state Sen. Jerry Ellis, a Democrat who represents southeastern Oklahoma. “There’s nobody thirsty in Dallas, Texas.”

— Wertz,J., 2013: Thirsty States Take Water Battle To Supreme Court on NPR.

The full article:

P.S. Lauren Markham has an article about environmental “refugees” forced to leave Ethiopia because of the changing rainfall patterns over the last eight years.