Lab. 5 Ohm's Law & Resistance The fundamental relationship among the three important electrical quantities current, voltage, and resistance was discovered by Georg Simon Ohm. The relationship and the unit of electrical resistance were both named for him to commemorate this contribution to physics. One statement of Ohm's law is that the current through a resistor is proportional to the voltage across the resistor. In this experiment you will test the correctness of this law in several different circuits using a Current & Voltage Probe System and a computer. These electrical quantities can be difficult to understand because they cannot be observed directly. To clarify these terms, we can make the comparison between electrical circuits and water flowing in pipes. Here is a chart of the three electrical units we will study in this experiment. Electrical Quantity Potential Difference or Voltage Current Resistance Description Potential energy differencelunit Volt (V) charge of a charge at two points in a circuit IV-1JC Rate of flow of charge through Ampere (A) a conductor A measure of how difficult it is TA-IC Ohm Water Analogy Water pressure difference between two points in a pipe Rate of flow of water through a pipe Ameasure of how difficult for charged particles to flow through a conductor. through a pipe OBJECTIVES Determine the mathematical relationship between current, potential difference, and resistance in a simple circuit. Compare the potential difference vs. current behavior of a resistor to that of a light bulb and light emitting diode (LED). MATERIALS computer Labquest Mini adjustable 6-volt DC power supply current probe & voltage probe PRELIMINARY QUESTIONS two resistors (about 56 and 82 (2) connecting wires light bulb (6.3 V) LED 1. A TV news reporter once stated that a person was electrocuted "when 20,000 volts of electricity surged through his body." What is wrong with this description? What was it that "surged" through his body? 2. Do you expect the resistance of a light bulb to remain constant as the current through it is increased and the filament goes from red-hot to white-hot? Explain why or why not. 3. The definition of the resistance of a conductor is R-VI, where is the potential difference between its ends and / is the resulting current through it. Ohm's Law, as an equation, states that V-IR. The two equations look the same. What is the difference? PRELIMINARY SETUP The resistance of a resistor is often indicated by a series of colored bands painted on the body of the resistor. The colors of the bands indicate numbers: black 0, brown-1, red-2, orange 3, yellow-4, green 5, blue 6, violet-7, gray-8, white 9. A "tolerance" band is indicated by silver (10%) or gold (15%). If you orient the resistor with the tolerance band on the right, the two leftmost bands give the first and second digits, and the third band gives the number of following zeros. So if the band colors are red, green, black, and silver, the resistance is 25.0 and the tolerance is + 1055