Fundamentals of Circuits I: Current Models, Batteries & Bulbs

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Fundamentals of Circuits I: Current Models, Batteries & Bulbs v 0.1 Question 1.15 How can you determine which model, if any, correctly describes the current through the bulb What would you measure, and where would you measure it Discuss what measurements you might make to eliminate at least three of the four competing models for current. Talk things over with your partners, and explain your reasoning. In Activity 1.5 you will use the computer based laboratory system and two current probes in your circuit to measure current and test the four models above. In addition to the battery, bulb and wires you used above, you will need: • Computer with Logger-Pro software • Lab Pro interface • two current probes The current probe is a device that measures current. Your computer will take measurements from the current probe and display them, as a function of time, on the computer screen. The current proble will allow us to measure the current at different locations and under different conditions in electric circuits. − + CP Figure 7: Measuring current in a circuit with a battery, bulb and current probe. In order to measure a current, the current must pass through the current probe. For example, to measure the current in the bottom wire of the circuit in Fig. 5, the current probe should be connected as shown in Fig. 7. To connect the current probe at this location we must disconnect the circuit at the point where you want to measure the current (the bottom wire), and insert the current probe. That is, disconnect the circuit, put in the current probe, and reconnect with the probe in place. Keep in mind that the current probe measures both the magnitude and direction of the current. Just like we did for velocities, forces, and other vector quantities in Physics I, we distinguish opposite directions for current flow by positive and negative. A current in through the + terminal and out through the - terminal (in the direction of the arrow) will be displayed as a positive current. By contrast, when the current probe measures a negative current, the current is directed into the − terminal and out of the + terminal of the probe. Take a close look at Fig. 8. If the current in the loop is clockwise both ammeters will measure currents which are positive. PHYS-204: Physics II Laboratory 9
Fundamentals of Circuits I: Current Models, Batteries & Bulbs v 0.1 + − CP1 − + CP2 Figure 8: Circuit with two current probes, one measuring current from the positive terminal of the battery, the other measuring current into the negative terminal of the battery. Note how the + and − terminals of the current probes are connected, Prediction 1.2 The table below is a prediction table for each of the four models. Before making any measurements fill in Table 1 to show how the readings of current probe 1 and current probe 2 in the circuit in Fig. 8 would compare with each other for each of the current models described in Fig. 6. Model A Model B Model C Model D Probe CP1 CP2 CP1 CP2 CP1 CP2 CP1 CP2 Current +, − or 0 CP1>CP2, CP1
Page 1 and 2: Name: Lab Partners: Date: Pre-Lab A
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Fundamentals of Circuits I: Current Models, Batteries & Bulbs

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