Voltage Drops Around Closed Loops 470  220  5V   220  living with the lab © 2012 David Hall

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Select Resistors Find the 220  and the 470  resistors from your parts kit. Now, find the 220  resistor. Example: 470  resistor 4 = yellow 7 = violet Add 1 zero to 47 to make 470, so 1 = brown So, 470 = yellow, violet, brown colordigit black0 brown1 red2 orange3 yellow4 green5 blue6 violet7 gray8 white9 first digit second digit number of zeros tolerance gold = ±5% silver = ±20% 3 living with the lab

Build the Series Circuit Below 470  5V   220  4 living with the lab 5V 220  470  5V  470  All of these circuits are the SAME!!

Compute the Voltage Drops Across the Two Resistors Use Ohm’s Law: V = I · R Now, add the voltage rise of the power source (+5V) to the voltage drops across the resistors (negative numbers). 5 living with the lab V 1 =5V +-+- R 1 = 220  R 2 = 470  given find the equivalent resistance find the current find the voltage drop across R 1 find the voltage drop across R 2

Use Multimeter to Measure Voltages Around Loop (1) From 5V pin to Gnd (2) Across the 220Ω resistor (3) Across the 470Ω resistor Remember... a RESISTOR is a voltage DROP and a POWER SOURCE is a voltage RISE  V 1 = _____  V 2 = _____  V 3 = _____  V 1  V 2  V 3  rises must balance drops!!!! 6 living with the lab 5V  470 

Compare Measurements to Theory Pretty close! 7 living with the lab  V = 3.41V  V = 1.59V V 1 =5V +-+- R 1 = 220  R 2 = 470 

Kirchoff’s Voltage Law (KVL) Kirchoff’s Voltage Law says that the algebraic sum of voltages around any closed loop in a circuit is zero – we see that this is true for our circuit. It is also true for very complex circuits. Notice that the 5V is DIVIDED between the two resistors, with the larger voltage drop occurring across the larger resistor. 8 living with the lab 5V V 1 =5V +-+- R 1 = 220  R 2 = 470   V = 3.41V – 3.41V  V = 1.59V – 1.59V = 0

Gustav Kirchoff (1824 – 1887) was a German physicist who made fundamental contributions to the understanding of electrical circuits and to the science of emission spectroscopy. He showed that when elements were heated to incandescence, they produce a characteristic signature allowing them to be identified. He wrote the laws for closed electric circuits in 1845 when he was a 21 year-old student. Photo: Library of Congress 9 living with the lab