Principles of Parallel and Series Circuits using Development of Basic Principles of Parallel and Series Circuits using concepts of Electric Fields.

Potential energy is lost as charge moves through a circuit. This Is measured in Volts. As an electron moves toward the (+) Terminal it will lose potential energy which is usually Converted into heat. The symbol (V) actually means V. V=ED or EL means Energy lost/charge = Force/charge x Length V = ED or EL Work = Force x Distance Volts Potential Energy - -- - - - - - - - - - - -- -- -- -- ∆V = ED D + + + + + + + + + + + + + +

Series Circuits

Voltage Amperes ∆V I = 8.4 watts e ∆V= E L e e e Nichrome

Voltage Current ∆V I =33.6 watts ∆V=EL Copper

Cu wire has less resistance than nichrome wire

Voltage Current - electrons Copper Nichrome

Have students touch and "which one is hotter"?

The currents thru the resistors are the same even when reversed. Go back and check it out!

Ed =∆V Series Circuit Voltage Current ∆V I < ∆VI I Ed=∆V Copper I Nichrome I

EL= Volts VI=(volts)(3a)= HOT Compare Electric Fields in the wires. Nichrome E EL= Volts Copper Length Electrons build up at the Cu/Nichrome junction and reduce the electric field in copper while increasing the electric field in nichrome UNTIL the two currents are the SAME. It takes less force (electric field) to push the current through the copper wire than through the nichrome wire. VI=(volts)(3a)= HOT VI=(volts)(3a)= cool

2a 6v Electric field in the Nichrome wire must be larger because of its larger resistance. El cu +El nic.=V 6v Electric field in a Series Circuit: Cu wire Have students touch the wires. Nichrome wire Cool hot

Conclusion about Series Circuits: 1. All the currents must be the same! 2. The resistor with the most resistance must have the largest electric field in it. 3. Therefore, The resistor with the most volts lost (greatest resistance) must get the hottest. ∆V I = watts ∆V = E d

Parallel Circuits

A parallel circuit Voltage Current close to 15 Nichrome 12 amps 3 amps Copper

Have students touch and "which one is hotter"?

Power= VI Which wire gets Hotter? Copper Wire Nichrome Wire E E ∆V=EL ∆V=EL 2.8 volts 2.8 volts Length Length The resistance of nichrome wire is large so the current should be smaller than Cu. The resistance of copper wire is small so the current should be larger than in the Nichrome. ∆VI= hotter ∆V I= cooler Power = (2.8 v)(12a)=34watts Power=(2.8v)(3a)=8.4watts

Conclusion In a parallel circuit the voltages across the resistances are equal. (Logic tells us this must be true) The currents add up to the total current. The electric fields which push the electrons around are equal in each resistor…since ∆V=Ed and the d’s are =.

Is the following circuit parallel or series???

Voltage Current 3.0 - + Copper Copper Nichrome

Which wire will get the hottest?

Volts lost R Extension Cord Light bulb ∆V I + ∆V I + ∆V I = total power L cu = L nichrome nichrome Volts lost E copper copper Volts lost Volts lost Electric force field in the copper wires is very small; as in the previous example the field in the nichrome must be larger to produce the same current throughout. Therefore, negligible heat is generated in the ext. cord

Ohm's Law

In “Ohmic” devices…..that follow ohm’s law the current is proportional to the voltage. V α I therefore the ratio of V to I is a constant V/I = constant This constant is called the resistance. Therefore V/I = R or V = IR

I V R=4 Variable resistor: Vary R and record V and I. Graph. Graphing Ohm’s Law: A demo on the black- board. V R=4 V http://webphysics.ph.msstate.edu/jc/library/18-2/ohmslaw.htm R=2 8 V 4 2 4 I

In “Ohmic” devices…..that follow ohm’s law the current is proportional to the voltage. V α I therefore the ratio of V to I is a constant V/I = constant This constant is called the resistance. Therefore V/I = R or V = IR What happens to the total resistance of a circuit when you add resistors in series??

. . R=4 What happens when you add another resistance in series? Is the resistance going “up” staying the “same” or “going down” ?? If you compare slides 4,5,and 7 you can see that adding wires in series increases the resistance and lowers the current. Adding two 2 ohm resistors in series will make the current half and therefore must double the total resistance. Look at the graph from point “a” to point “b”. Resistors placed in series have their values added to find the total resistance. Variable resistor: Vary R and record V and I. Graph. R=4 V . . R=2 b 8 a 4 2 4 I

Conclusion about Series Circuits: 1. All the currents must be the same! 2. The resistor with the most resistance must have the largest electric field in it. 3. Therefore, The resistor with the most volts lost (greatest resistance) must get the hottest. ∆V I = watts ∆V = E d 4. R total = R 1 + R 2 + R 3 etc

The End?