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Activity 1.2a – Ohm’s Law and Kirchoff’s Law Purpose: 1. To study the mathematical relationship between voltage, resistance, and current. 2. To learn to use Ohm’s Law to make accurate predictions. 3. To construct electrical circuits and measure voltage, current and resistance using electronic test equipment Equipment: 2 Fischertechnik Bulbs 2 Switches Connecting wires Power supply Control Unit with USB cord Multi-meter Procedure: 1. Complete the table below for the three variables of electricity: Name Symbol Your Definition Unit Voltage Current Resistance 2. Label the Ohm’s Law formula wheel and determine the formulas below: Use values in AMPS, VOLTS, and OHMS with no prefixes (i.e. millivolts, megaohms, etc.) E or V= I= R= 3. Using the Ohm meter (the multi-meter set to read resistance), measure the resistance of one of the Fischertechnik bulbs. Remember, it has to be disconnected. Record the resistance here: Rb1 = 4. Using the volt meter (the multi-meter set to read voltage) measure the voltage of the power supply and record it here: Vs = 5. Using the voltage and resistance measured, calculate the total current, It, your circuit should draw. Start with Ohm’s law, rearrange to derive the formula for current, then substitute values and calculate. Show your work neatly and completely. 6. Hook the bulb to the power supply. Measure the voltage drop across the bulb. Is the power supply voltage the same as what you measured with the bulb disconnected? Vs = The voltage is lower (about 1 volt) because the power supply is small. It heats up when it is being used, which causes the voltage to drop a bit. 7. Disconnect the power. Using the ammeter (the multi-meter set to read current) measure the current. Do this by wiring the ammeter in series with the bulb. Record the current. Is the current what you predicted it would be? Why or why not? It = 8. Disconnect the power to the circuit. Wire two bulbs in series. Draw a schematic of the circuit here. Do it neatly so that it resembles a CAD drawing. Yes, I’m serious. 9. Predict the total resistance (call it RT) using Kirchoff’s Law. Using this RT, calculate current (call it IT) for the circuit. Start with Ohm’s law, derive the formula for current, then substitute values and calculate. Show your work neatly and completely. 10. Disconnect the series circuit from the power supply and measure the resistance. Does it match the prediction? RT = 11. Connect the power to the circuit. Measure the voltage drop across each bulb while it is lit. Record the voltage drop here. What is the relationship of the drop across each bulb to the total voltage? Vb1 = Vb2= Vtotal= 12. Disconnect the circuit and place the ammeter in the circuit. Reconnect the power and measure the current. Does it make any difference where you measure the current? Explain your answer in terms of the properties of current in a series circuit. 13. Disconnect the power. Wire two bulbs in parallel. Draw a schematic of the circuit here. Do it neatly so that it again resembles a CAD drawing. Yes, I’m serious about making decent sketches because it’s important. 14. Predict the total resistance RT for the circuit using Kirchoff’s Law. Then use RT and Ohm’s Law to calculate current, IT. Show your math. RT = It = 15. Measure the resistance across the two bulbs. Does it match the prediction? If not ask Mr. Bayer if your prediction was valid. Or, research “parallel resistors” for the equation needed to improve your estimate in Question 14. Rt = 16. Connect the power to the circuit. Measure the voltage drop across each bulb. Record the voltage drop here. What is the relationship of the drop across each bulb to the total voltage? What if there was three bulbs? Vb1 = Vb2= Vtotal= 17. Disconnect the circuit and place the ammeter in the circuit. Reconnect the power and measure the current. Does it make any difference where you measure the current? Explain your answer in terms of the properties of current in a parallel circuit. Conclusion: 1. Is there a difference in the brightness of the bulbs if they are wired in series or parallel? Explain why or why not. 2. As the electrical energy forces its way through the bulbs and wire, what conversions does the electrical energy go through? Is the energy “used up”? Why or why not? 3. Of the three ways you wired the lights (single, series, and parallel) which gave the most light? Which provided the least? Explain why. 4. DC motors speed up or slow down when the voltage powers them is raised or lowered. If you had two permanent magnet DC motors, and you wanted them to both run at full speed, how should you wire them? 5. Compare and contrast the advantages and disadvantages of parallel and series circuits. Make a 2 row x 3 column table to document advantages, disadvantages, and at least three real-world examples/or ideal situations for each type of circuit.