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Transcript

Ohm’s Law and Electrical Resistance Leader: ___________________ Skeptic: ___________________ Recorder: __________________________ Encourager: ________________________ Equipment: 560, 680, ohm (or similar) resistors, assorted connectors, battery-eliminator, (2) multi-meters. Introduction: Ohm’s Law is a rule stating that the ratio of voltage across a wire to the current flowing in it is constant as a function of applied voltage. The ratio V/I is called the electrical resistance of a material in ohms. Ohm’s Law applies to a broad range of materials and applied voltages, nevertheless, there are materials and voltage ranges where non-ohmic behavior occurs. Part 1. Resistors in Series. 1. Measure two resistors and record their values: R1 = ________ R2 = __________. 2. Draw a schematic of these resistors in series. 3. Calculate the theoretical equivalent resistance Req = ____________ (calc) 4. Measure the actual equivalent resistance Req = _____________(meas) 5. Calculate the percent difference to two sig. figs. _______________ (% difference) Part 2. Resistors in Parallel. 1. Maintain R1 and R2 as above. 2. Draw a schematic of these resistors in parallel. 3. Calculate the theoretical equivalent resistance Req = ____________ (calc) 4. Measure the actual equivalent resistance Req = _____________(meas) 5. Calculate the percent difference to two sig. figs. _______________ (% difference) 1 Part 3. Ohm’s Law. 1. Choose and record a resistor to be called “R” R = __________ ohms. 2. Connect the circuit as shown below. 3. Connect the resistor by itself with the battery eliminator set at 1.5 volts. Repeat for next higher voltage settings until the table is filled. V-setting (volts) Measured V Across Measured I (amps) Measured V/I (V/A) Resistor (volts) 4. Calculate the average V/I resistance values of your resistor and compute the percentage difference of this value from your value from the table above. 2 Part 4. Voltmeter Internal Resistance Voltmeters are designed to be used in parallel with the element being tested. In order not to draw current away from the element, the voltmeter should ideally have an infinite internal resistance. Ammeters are designed to be used in series with the element being tested. In order not to reduce the current in the element the ammeter should ideally have zero internal resistance. We will now measure the voltmeter resistance by measuring the voltage drops in a series circuit. Naming Conventions: V1 = Voltmeter reading V2 = Voltage across R2 R1 = internal resistance of Voltmeter R2 = decade box resistance, set at 4 mega-ohms. 1. Set the battery eliminator to 6V. 2. Set the voltmeter to the 20V DC setting. Measure and record the voltage output of the power supply to 3 significant figures. Vapp = ____________ 3. Now make a series circuit of the battery eliminator (turned off and set at 6V), the voltmeter (set at 20V DC), and R2. Diagram your Circuit. 4. Turn on the battery eliminator and record the voltage shown on the voltmeter to three significant figures. V1 = _________________ (voltmeter reading) 5. We know that Vapp = V1 + V2. Use this relation to calculate V2. V2 = Vapp – V1 = _______________ (voltage across R2, the resistor box) 5. Calculate the internal resistance of the voltmeter, R1, and record it to three significant figures. R1 = (V1/V2)R2 = _______________ (internal resistance of voltmeter) 3