Saturday 20 December 2014

Dec. 19 – Circuits

Last time we talked about Ohm's Law, V = I R.  Which one of these graphs best represents Ohm's Law?


The answer is (a).  If you graph voltage vs. current, and you get a linear graph, that means Ohm's law is obeyed.

Ohmic - the material obeys Ohm’s Law (V vs I graph is linear).

Non-Ohmic - the material does not obey Ohm’s Law (V vs. I graph is curved).

We will always assume Ohmic resistors.

Circuits

Circuits are closed paths for charges to flow.

An analogy of circuits as a roller coaster ride.

Here's what circuits look like in real life.  Every line on the board is a wire.
Conventional current flows from + to –.  Electron flow is opposite ( – to + ).




AC - Alternating Current, direction constantly changes, no + or –.
DC - Direct current, charges only move in one direction.

In this course, we will only be working with DC circuits.

Types of Circuits

Series
 - components are connected with one path
 - total resistance is the SUM of each resistance.




  • The current is the same for all resistors connected in series.



Parallel
 - components are connected with multiple paths for charges to flow.
 - total resistance is the INVERSE SUM 




    • The voltage is the same for all resistors connected in parallel.

    Using these rules, we can replace a set of resistors with an equivalent resistor

     Mixed Circuits
     - contains combinations of components in series or parallel
     - total resistance must be calculated one section at a time



    Kirchhoff's Laws


    Kirchhoff's Current Law

    Also known as the junction rule.
     - All current entering a location must exit that location.
     - Current in equals Current out.
                I in   =  I out


    Kirchhoff’s Voltage Law

    Also known as the loop rule.
     - The potential difference around a loop must be zero.
               V1 + V2 + V3 + … = 0 



    Handout

    Here are some challenging circuit questions that are more difficult than the ones in your text book.





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