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Unformatted text preview: PC1222 Fundamentals of Physics II Basic Circuits 1 Objective • Investigate the relationship among three variables (resistance, current and voltage) in direct current circuits. • Investigate the circuit characteristics of resistors in series and parallel arrangements. 2 Equipment • PASCO Circuits Experiment Boards • Digital multimeter (DMM) • D-cell batteries • Resistors • Wire leads 3 Theory 3.1 Resistance and Ohm’s law When a voltage or potential difference ( Δ V ) is applied across a material, the current ( I ) through the material is found to be proportional to the voltage, I ∝ Δ V . The resistance ( R ) of the material is defined as the ratio of the applied voltage and the resulting current – that is R = Δ V I (definition of resistance) (1) For many materials, the resistance is constant, or at least approximately so, over a range of voltages. A resistor that has constant resistance is said to obey Ohm’s law or to be “ohmic”. From equation (1), it can be seen that the unit of resistance is volt/ampere (V/A). However, the combined unit is called the ohm ( Ω ), in honor of Georg Ohm. A plot of Δ V versus I for an ohmic material is a straight line. Materials that do not obey Ohm’s law are said to be “non-ohmic” and have a nonlinear voltage-current relation- ship. Semiconductors and transistors are non-ohmic. Physics level 1 laboratory Page 1 of 8 Semester I, 2011/12 PC1222: Basic Circuits Page 2 of 8 In common practice, Ohm’s law is written as Δ V = IR (Ohm’s law) (2) where it is understood that R is independent of Δ V . Keep in mind that Ohm’s law is not a fundamental law such as Newton’s law of gravitation. It is a special case, there being no law that materials must have constant resistance. In an electrical circuit, a voltage source (e.g., a battery or power supply) supplies the voltage for charge flow and the magnitude of the current is determined by the resistance R in the circuit. For a given voltage, the greater the resistance, the less current through the resistance, as may be seen from Ohm’s law, I = Δ V / R . In an electrical circuit with two or more resistances and a single voltage source, Ohm’s law may be applied to the entire circuit or to any portion of the circuit. When it is applied to the entire circuit, the voltage is the terminal input voltage supplied by the voltage source and the resistance is the total resistance of the circuit. When Ohm’s law is applied to a particular portion of the circuit, the individual voltages, currents and resistances are used for that part of the circuit. Figure 1: Resistor colour code. For resistors routinely used in electronic instrumentation, resistance is coded by a series of coloured bands on the resistor. The key to the resistor colour coding system is given in Figure 1. The four bands are placed with three equally spaced bands close to one end of the resistor followed by a space and then a forth band. The first two bands are the first two digitsresistor followed by a space and then a forth band....
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