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Ohm's Law Manual - Phys211L/Phys205L OHMS LAW Object To...

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Phys211L/Phys205L OHM’S LAW Object: To investigate the relationship between current and voltage in ohmic and non- ohmic devices and to determine the resistivity of different-material rods. Equipment: Computer; Science Workshop (SW) 750 Interface; electric circuits board containing a light bulb, resistors and a silicon diode, metal rods, multimeters, power supply, banana connecting wires and voltage sensors, Theory: I V When a current I passes through a two-terminal device, a potential difference V develops across it. The resistance R of such a device is defined as: R = V/I When the potential difference is measured in volts (V) and the current in amperes, the resistance is measured in ohms ( ). The resistance is generally a complex function of the current and voltage. The V vs. I curve characterizes the device completely . For some devices, R is a constant. In this case, the V vs I curve is a straight line passing through the origin and the device is said to be ohmic (obeys Ohm’s law). We can now talk about a resistor whose value is the slope of this curve. However, if the resistance changes (if the device is “non-Ohmic”), the graph of V vs. I will not be a straight line. Instead, it will be some characteristic curve. For a typical resistor, the value of its resistance does not change appreciably as a function of the current. However, for a light bulb, the resistance of the filament depends on the temperature and will change as it heats up and cools down, depending on the current. Therefore, a plot of the voltage as a function of current for a light bulb filament will not lead to a straight line.
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