2-PARALLEL AND SERIES CIRCUITS new

2-PARALLEL AND SERIES CIRCUITS new - PARALLEL AND SERIES...

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4 PARALLEL AND SERIES CIRCUITS Purpose: To investigate the properties of parallel and series circuits. Apparatus: Two Multi-meters, three resistors, experimental breadboard, breadboard wires, power supply, 6 wires Background: A battery cannot tell if there are hundreds of resistors in a circuit or if there is just one resistor in the circuit. The battery feels the load or the total resistance of the entire circuit. This leads us to the question of how the current flows in the circuit and what the voltage difference is across each resistor. We can say, and we will verify, that for each resistor in a circuit Ohm’s law is valid. This means that the voltage across each resistor is equal to the product of its resistance and the current moving through it. The total voltage and current that the power supply or battery produces is, in general, different from what each resistor has across it or moves through it. Memorize: The Ammeter measures current through a resistor by connecting in series with it. The Voltmeter measures voltage across a resistor by connecting in parallel with it. Power Supply R R R 1 2 3 Figure 1. Three resistors of values R 1 , R 2 , and R 3 are connected in series with one another. Let’s first take a look at a series circuit. An illustration of this is seen in Figure 1. Here the resistors are lined up so that there is only one path that the current may take through the circuit. We will assume that the total current moving through the circuit is constant and, by conservation of charge, does not depend on which resistor it moves through. The sum total of the voltages across the resistors is equal to that of the power supply. This last sentence is another way of stating Kirchhoff’s loop rule. We can use this to calculate the total resistance that the power supply sees as R T = R 1 + R 2 + R 3 Eq.1) For a parallel circuit, shown in Figure 2, the current has several different paths to take. However, the voltage across each resistor is the same because each resistor is connected across the power supply. Using Ohm’s law we can determine the current through each resistor. The sum total of the currents is that through the power supply. With this, the total resistance that the power supply sees is 1 /R tot = 1 /R 1 + 1 /R 2 + 1 /R 3 Eq.2)
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5 Power Supply R R R 1 2 3 Figure 2. Three resistors of values R
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This note was uploaded on 08/25/2011 for the course PHYS 164 taught by Professor Johnjames during the Fall '09 term at MO St. Louis.

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2-PARALLEL AND SERIES CIRCUITS new - PARALLEL AND SERIES...

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