LABORATORY VI ELECTRIC CIRCUITS In Laboratory IV, you studied the behavior of electric fields and their effect on the motion of electrons using a cathode ray tube (CRT). This beam of electrons is one example of an electric current – charges in motion. The current in the CRT was simple in that the electrons moved through a vacuum. The forces on them were completely known. Their behavior could be determined by knowing the electric field and then applying kinematics. In contrast to the CRT, the most familiar electric currents are inside materials such as wires or light bulbs. Even though the interactions of electrons inside ma-terials are quite complicated, the basic principles of physics still apply. Conserva-tion of energy and charge allow us to determine the overall behavior of electric currents without the need to know the details of the electron interactions. This approach to problem solving will give you more experience in applying the prin-ciples of conservation to the very useful realm of electric circuits. OBJECTIVES: After successfully completing this laboratory, you should be able to: • Apply the concept of circuits to any electrical system. • Apply the concept of conservation of charge to determine the behavior of the electrical current through any part of a circuit. • Apply the concept of conservation of energy to determine the behavior of the energy output of any element in a circuit. • Use the concept of electric potential to describe the behavior of a circuit. • Relate the electric charge on a circuit element to the potential difference across that element and the capacitance of that element. • Relate the electric current through a circuit element to the resistance of that element and potential difference across that element. • Measure the current through a circuit element with a digital multimeter (DMM). • Measure the voltage between two points in a circuit with a DMM. • Measure the resistance of a circuit element with a DMM.Lab PREPARATION: Read Tipler: Chapter 24, section 1; Chapter 25, sections 1, 2, 4; Chapter 26, sections 1-5; and Chapter 27. It is likely that you will be doing these laboratory problems before your lecturer addresses this material. The purpose of this laboratory is to give you these experiences as an introduction to the material. So, it is very important that, when you read the text before coming to lab, you remember the objectives of the laboratory. Before coming to lab you should be able to: • Describe the relationship between charge and current. • Describe the relationship between potential and potential energy. • Describe the essential difference between an insulator and a conductor. • Identify what is an electrical circuit and what is not. • Apply conservation of energy and conservation of charge to current flowing around a circuit. • Write down Ohm's law and know when to apply it. • Describe the difference between a capacitor, a resistor, and a battery. • Use a DMM to measure potential difference, current, and resistance.Lab IV - 3 EXPLORATORY PROBLEM #1: SIMPLE CIRCUITS You need more light in your workroom, so you decide to add another light fixture to your track lighting. However, you are concerned that adding another light may dim the lights that are already in the track. When you proceed with the addition of another light, you notice that none of the lights are dimmer than be-fore. You wonder what type of circuit your track lighting uses. You decide to build models of circuits using two bulbs and compare the brightness of the bulbs in these circuits to a circuit with a single bulb. You know that the circuit where the bulbs are as bright as your reference circuit is the same type as the circuit your track lighting uses. ? How does the brightness of the bulbs com-pare in one-bulb and two-bulb circuits? EQUIPMENT You will build the three simple circuits shown below out of wires, bulbs, and batter-ies. Use the accompanying leg-end to help you build the cir-cuits. Legend:light bulbbatterywirePROBLEM #1: SIMPLE CIRCUITS LabPROBLEM #1: SIMPLE CIRCUITS Lab PREDICTION Rank, in order of brightness, the bulbs A, B, C, D, and E from the brightest to the dimmest (use the symbol ‘=’ for "same brightness as" and the symbol ‘ >’ for "brighter than"). Write down your reasoning. EXPLORATION Reference Circuit I: Connect Circuit I to use as a reference. Observe the brightness of bulb A. Replace the bulb with another one and again observe the brightness. Repeat until you have determined the brightness of all your bulbs when they are connected into the same type of circuit. If the bulbs are identical, they should have the same brightness. Circuit II: Connect Circuit II. Compare the brightness of bulbs B and C. What can you conclude from this observation about the amount of current through each bulb? Note: Pay attention to large differences you may observe, rather than minor differ-ences that may occur if two "identical" bulbs are, in fact, not quite identical. How can you test whether minor differences are due to manufacturing irregularities? Is current "used up" in the first bulb, or is the current the same through both bulbs? Try switching bulbs B and C. Based on your observation, what can you infer about the current at points 1, 2, and 3? How does the brightness of bulb A (Circuit I) compare to the brightness of bulbs B and C (Circuit II)? What can you infer about the current at point 1 in each of the two circuits? Circuit III: Connect Circuit III. Compare the brightness of bulbs D and E. What can you conclude from this observation about the amount of current through each bulb? Describe the flow of current around the entire circuit. What do your observa-tions suggest about the way the current through the battery divides and recom-PROBLEM #1: SIMPLE CIRCUITS Lab bines at junctions where the circuit splits into two branches? How does the cur-rent at point 1 compare with the currents at points 2 and 3? How does the brightness of bulb A (Circuit I) compare to the brightness of bulbs D and E (Circuit III)? What can you infer about the current at point 1 in each of the two circuits? Comparing the three circuits, does the amount of current at point 1 appear to remain constant or to depend on the number of bulbs and how they are con-nected? CONCLUSION Rank the actual brightness of the bulbs. How did this compare to