Physics Lab: Voltages, Circuits, and Ohm's Law - Prof. Rose A. Finn, Lab Reports of Physics

Download Physics Lab: Voltages, Circuits, and Ohm's Law - Prof. Rose A. Finn and more Lab Reports Physics in PDF only on Docsity! General Physics 2 Lab 6: Circuits Siena College Voltages in Series Circuits 1) Connect the circuit shown in the figure at the right, which consists of two batteries connected to two bulbs in series. 2) Predict how the potential difference across the battery will compare with the potential difference across the series combination of bulbs. 3) Using your multimeter to measure potential differences, make the following measurements: Potential difference across batteries: __________ Potential difference across both bulbs: __________ 4) How do the two potential differences compare? Did your observations agree with your prediction? 5) Assuming two bulbs are identical, predict how the potential difference across each bulb will compare to the potential difference across the battery. 6) Measure the potential difference across each bulb: Potential difference across bulb 1: __________ Potential difference across bulb 2: __________ 7) Did your measurements agree with your predictions? Discuss any discrepancies. 8) Formulate a rule for how potential differences across individual bulbs in a series connection combine to give the total potential difference across the series combination of the bulbs. How is this related to the potential difference across the battery? 2 Voltages in Parallel Circuits 1) Connect the circuit shown in the figure above, which consists of two batteries connected to two bulbs in parallel. 2) Predict how the potential difference across the battery will compare with the potential difference across the parallel combination of bulbs. 3) Using your multimeter to measure potential differences, make the following measurements: Potential difference across batteries: __________ Potential difference across both bulbs: __________ 4) How do the two potential differences compare? Did your observations agree with your prediction? 5) Predict how the potential difference across each bulb will compare to the potential difference across the battery. 6) Measure the potential difference across each bulb: Potential difference across bulb 1: __________ Potential difference across bulb 2: __________ 7) Did your measurements agree with your predictions? Discuss any discrepancies. 8) Formulate a rule for how potential differences across the different branches of a parallel circuit. How are these related to the potential difference across the battery? 5 5) Turn the current knobs all the way down. Substitute a light bulb in place of the resistor in the circuit. Using both the coarse and fine adjustment current knobs, increase the current through the light bulb to a maximum of 0.30 amps in steps 10 steps of 0.03 amps each. Do not increase the current beyond 0.30 amps or the light bulb will blow. 6) Sketch your graphs for the light bulb below: 7) If a graph of V vs. i is a straight line, what quantity does the slope of the line represent? 8) Did either of the graphs of V vs. i result in a straight line? If so, which one? 9) Did either the light bulb or the resistor obey Ohm’s law? If so, which one(s) and why? 10) What is the resistance of your resistor? R = _________ Voltage (V) Current (A) 6 Internal resistance of a battery A battery is not a source of constant current – the current varies according to the resistance in the circuit. A battery is, however, a nearly constant voltage source, but not perfectly constant as you may have already noticed. If you have measured the voltage across the batteries in other parts of this experiment while a current is being drawn (by a bulb or a resistor), you may have noticed that the potential difference measured across the batteries drops below the value that you measure with no current being drawn. This voltage drop occurs because the chemical reactions in a battery cannot supply charge fast enough to maintain the full emf and there is always some hindrance to completely free flow of charge within the battery. Therefore, a battery itself has some resistance, called its internal resistance, and we can never separate it from the battery. When we attach the leads of a voltmeter to a battery, we are measuring the terminal voltage. When no current is drawn from the battery, the terminal voltage equals the emf. But when current flows from the battery through a circuit, there is an internal drop in voltage due to the internal resistance. We can model a real battery as a perfect emf in series with a resistor, r, as shown in the figure. We can determine the internal resistance of our batteries using the circuit above. For this circuit, use one D cell battery and one resistor with a known resistance (there should be plenty of 10 resistors available). 1. First, measure the potential difference across the battery without the resistor attached. This is the emf of the battery. E = __________ 2. Now connect the 10 resistor in series with the battery and measure the potential difference across the resistor. VR = _________ 3. Calculate the current in the circuit. i = _________ 4. In our model, the battery’s internal resistance is just another resistor that is placed in series with the 10 resistor. Most of the emf of the battery is dropped across the 10 resistor, while a small amount is dropped across the internal resistance, r. Using the values you determined above, calculate the internal resistance of the battery, r. r = __________ E R r