Short circuits damage equipment by causing larger

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Unformatted text preview: LORATION WARNING: A short circuit is what happens any time a very low-resistance path (like a wire, or other piece of metal) is provided between points in a circuit that are at different potentials, like the terminals of a battery or power supply. Short circuits can destroy equipment and injure people! Always avoid short circuits in other circuits! Short circuits damage equipment by causing larger currents in a circuit than they are designed for. Only apply the short circuit for a small amount of time. Build Circuit II. What happens to the brightness of bulbs B and C when you place a wire across bulb B? How did the current through C change? How did the current through B change? Did the current through point 1 change? If so, in what way? Did the wire across bulb B get warmer? Explain your answers. Build Circuit III. What happens to the brightness of bulbs D and E when you place a wire across bulb E? Did the current through D change? Did the current through E change? Did the wire across bulb E get warm? What would be the brightness of a bulb inserted in the circuit at point 1? Explain your answers. Build Circuit I. Place a wire across the bulb. What happens to the brightness of the bulb? Hold on to the wire that is across the bulb. Is it getting warmer? How did the current through the bulb change? How about the current coming out of the battery? Make sure you disconnect the battery when you are done. Placing the wire across the bulb causes a short circuit and it is called "shorting out" the bulb. 90 SHORT CIRCUITS – 1302Lab4Prob3 CONCLUSIONS Did your predictions match your observed results? What have you learned from experiments, which agreed with your predictions? Draw more conclusions based on the experiments where your predictions were different from what you observed. 91 SHORT CIRCUITS – 1302Lab4Prob3 92 PROBLEM #4: CHARGING A CAPACITOR (PART A) You have designed a circuit using a battery and a capacitor to automatically dim the lights for a theatrical production. However, the lighting consists of many different kinds of bulbs, which have been manufactured differently, and which consequently have very different resistances. You need to be able to precisely control the rate at which the lights dim, so you need to determine how this rate depends on both the capacitance of the capacitor and the resistance of the bulb. You decide to model this situation using a circuit consisting of a battery, a capacitor (initially uncharged), and a resistor, all in series. You will try to ascertain how the current in the circuit changes with time. Instructions: Before lab, read the laboratory in its entirety as well as the required reading in the textbook. In your lab notebook, respond to the warm up questions and derive a specific prediction for the outcome of the lab. During lab, compare your warm up responses and prediction in your group. Then, work through the exploration, measurement, analysis, and conclusion sections in sequence, keeping a record of your findings in your lab notebook. It is often useful to use Excel to perform data analysis, rather than doing it by hand. Read: Tipler & Mosca Section 25.6. EQUIPMENT Build the circuit shown using wires, resistors, capacitors, and batteries. Use the accompanying legend to help you build the circuits. You will also have a stopwatch, a light bulb, and a digital multimeter (DMM). Circuit Read the section The Digital Multimeter (DMM) in the Equipment appendix. Read the appendices Significant Figures, Review of Graphs and Accuracy, Precision and Uncertainty to help you take data effectively. If equipment is missing or broken, submit a problem report by sending an email to labhelp@physics.umn.edu. Include the room number and brief description of the problem. 93 CHARGING A CAPACITOR (PART A) – 1302Lab4Prob4 WARM UP 1. Draw a circuit diagram, similar to the one shown above. Decide on the properties of each of the elements of the circuit that are relevant to the problem, and label them on your diagram. Label the potential difference across each of the elements of the circuit. Label the current in the circuit and the charge on the capacitor. 2. Use energy conservation to write an equation relating the potential differences across all elements of the circuit. Write an equation relating the potential difference across the capacitor plates and the charge stored on its plates. What is the relationship between the current through the resistor and the voltage across it? Are these three equations always true, or only for specific times? 3. Describe qualitatively how each quantity labeled on your diagram changes with time. What is the voltage across each element of the circuit (a) at the instant the circuit is closed; (b) when the capacitor is fully charged? What is the current in the circuit at these two times? What is the charge on the capacitor plates at these two times? 4. From the equations you constructed above, determine an equation relating the voltage of the battery, the capacitance of the capacitor, the resistance of the resistor, the current...
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