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EXPERIMENT DC CIRCUITS Introduction: Electric charges tend to move from one position to another of lower electrostatic potential energy in much the same way that masses tend to move to a position of lower gravitational potential energy. Electrostatic potential or simply potentia l “ V ” of a charge “ q ” is related to potential energy U ” by q U V = (1) A battery is a mechanism such that a constant potential ( voltage ) difference is maintained across its two terminals. If the terminals are connected through a circuit, charge flows ( current ) from one terminal to the other. When a conducting element that resists the flow of charge (resistance) is placed in the circuit, charge emerges from the resistor at a different voltage than when it entered the other side. The ratio of the voltage V across the resistor to the current I through it is a constant (for most metallic conductors), independent of I , i.e. I V R = (2) is called the resistance of the conductor. The SI unit is ohm ( Ω ). Eq.(2) is called Ohm’s law. Note that the resistance of the wiring itself that makes up a circuit, while not zero, is nonetheless negligible in most cases. An example is the car battery connected to a headlight of a car as shown in Fig.1. car battery headlight I V R A + B 12-V battery + Filament Figure 1: A typical example of a car battery connected to a headlight (The circuit must be closed for the current to flow) Resistors can be combined in a variety of ways in circuits. For example, Fig.2a shows two resistors connected in series across a battery, while Fig.2b shows the same resistors connected in parallel . The net resistance of a combination of resistors is: R = R 1 + R 2 + … ( in series ) (3) and ... 1 1 1 2 1 + + = R R R ( in parallel ) (4)
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2 R 1 R 2 R V 1 V 2 I I + + Equivalent to V V V = V 1 +V 2 = IR 1 +IR 2 = I(R 1 +R 2 )= IR Series Resistors: R = R 1 + R 2 + R 3 + … (a) Two Resistors Connected in Series R 2 R I 2 R 1 I 1 I I + + V V Equivalent to = + = + = + = R V R R V R V R V I I I 1 1 1 2 1 2 1 2 1 Parallel Resistors: ...
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This note was uploaded on 04/01/2011 for the course PHY 135 taught by Professor Wagihghobriel during the Spring '11 term at University of Toronto.

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