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Wagih Ghobriel

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 potential “V” of a charge “q” is related to potential energy “U” by V  U (1) q 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. R  V (2) I is called the resistance of the conductor. The SI unit is ohm (). 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 = R1+ R2+ … ( in series ) (3) and 1  1  1  ... ( in parallel ) (4) R R 1 R2 2 R R R 1 2 V 1 V2 Equivalent I to I +  +  V V V = V1+V 2 IR 1IR 2 I(R 1R )2 IR Series Resistors: R= R 1 R 2 R +3… (a) Two Resistors Connected in Series R 2 R 2 R 1 Equivalent I I 1 to I +  +  V V V V  1 1   1  I  1  2  R  R V R  R  V R  1 2  1 2   1 1 1 1 Parallel Resistors:    ... R R 1 R2 R3 (b) Two Resistors Connected in Parallel Figure 2: Two Resistors Connected in Series or in Parallel Exercise 1: In this part of the experiment, you will practice setting up some simple circuits using light bulbs rather than conventional resistors. The advantage of a light bulb is, of course, that you can “see” current flowing: an electric current passing through the filament heats it and it glows. The larger the current, the brighter the glow. N.B. The low voltage and currents in this lab are no danger to you; the battery and meters, however, are more sensitive. To protect them, try to remember to disconnect the battery or turn the power off when building or making changes to your circuits. Review Sec 2.3 on “Health and Safety in the Lab – Electricity”, page 9 in your Manual “ Introduction to Experimental Physics”. Also, review the section on “Multimeters” in your Manual “ Instrumentations and Measuring Devices” Construct the circuit shown in Fig.1 using a light bulb as the resistor. In order for current to flow, the circuit must be closed, i.e. you must be able to trace a continuous path of wires and resistors 3 from one terminal of the battery to the other. Now turn on the power to the battery. If the circuit is closed, current will flow and the bulb will light. If the light bulb does not come on, you have one of three problems: 1. The circuit is open. Most likely you have a “bad contact”, i.e. there is a break in the current path between two of the elements. You might try jiggling the wires to see if you can locate it. Turn the power off and try to improve the connections. 2. The light bulb is burned out. Turn the power off and replace it. 3. The
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