Fundamentals of Circuits I: Current Models, Batteries & Bulbs

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Fundamentals of Circuits I: Current Models, Batteries & Bulbs v 0.1 Investigation 3: Current and Electric Potential Difference Since a battery is a device that has an electric potential difference across its terminals, it is capable of giving energy to charges, which can then flow as a current through a circuit. Exploring the relationship between the potential differences in a circuit and the currents that flow in that circuit is an essential part of developing an understanding of electrical circuits. Since potential differences are measured in volts, a potential difference is informally referred to as a voltage, and usually denoted by the symbol V . Voltage is an informal term for potential difference. If you want to talk to physicists, you should refer to potential difference. Communicating with a sales person at the local Radio Shack store or buying batteries at a hardware store is another story. There you should probably refer to voltage. We will use these two terms interchangeably. You will use voltage probes to measure electric potential difference. The voltage probe has two clips. When the clips at the ends of the wires are connected at two points in a circuit the computer can measure the potential difference. Fig. 10 shows our simple circuit from Fig. 5 with a voltage probe connected to measure the voltage across the battery. Note that the word across describes how the voltage probes are connected. + VP1 − Figure 10: Voltage probe connected to measure potential difference of the battery Let’s measure the potential difference across different elements of this circuit. You have available the following equipment. • Computer with Logger-Pro software • Lab Pro interface • D-cell battery and holder • light bulb and socket • SPST switch • two voltage probes PHYS-204: Physics II Laboratory 13
Fundamentals of Circuits I: Current Models, Batteries & Bulbs v 0.1 Activity 3.1: Electric Potential (Voltage) Step 1: Unplug the current probes from the interface, and connect the voltage probes to channels 1 and 2. Step 2: Open the experiment file L1A3-1(Two Voltages) to display the axes that will allow you to compare two voltages. Step 3: Connect the circuit shown in Fig. 9, but do not connect the voltage probes yet. Step 4: Zero the voltage probes, being sure that the clips of the voltage probes are not connected to anything. Step 5: Connect both clips to the negative end of the battery holder in the circuit. Click on the collect button to start graphing. Collect data with the switch open, and with the switch closed. Record the potential difference with the switch open, and with the switch closed. V open = volts V closed = volts Step 6: Move the red clip and connect it at the other end of the wire connecting the negative terminal of the battery to the light bulb. Connected this way the voltage probe will measure the potential difference between the ends of the wire. Step 7: Collect data to determine the potential difference with the switch open, and with the switch closed. V open = volts V closed = volts Question 3.1 What do you conclude about the potential difference when the two leads from the voltage probe are connected to the same point What do you conclude about the potential difference between the two ends of the same wire Prediction 3.1 In the circuit in Fig. 11, how would you expect the voltage difference across the battery to compare to the voltage across the bulb with the switch open and with the switch closed Explain. Step 8: Now test your prediction. Connect voltage probe 1 across the battery, and voltage probe 2 across the light bulb, as in Fig. 11. Step 9: Collect data and make graphs of the two potential differences. Make sure the switch is closed during part of the time that the computer is collecting data, and open during part of the time. PHYS-204: Physics II Laboratory 14
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Fundamentals of Circuits I: Current Models, Batteries & Bulbs

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