Pre-Lab for Batteries and Bulbs

Transcription

1 Pre-Lab for Batteries and Bulbs Complex circuits composed of resistors can be simplified by using the concept of equivalent resistors. For example if resistors R 1, R 2, and R 3 are connected in series, their equivalent resistance if just the sum of the resistance. The current in series circuits are equal for every resistor in series. R equivalent = R 1 + R 2 + R 3 R 1 R 2 R 3 If resistors R 1, R 2, and R 3 are connected in parallel, the resistors add as the reciprocals. The voltages are equal across each resistor in parallel. 1 R equivalent = 1 R R R 3 R 1 R 2 R 3 The concept of equivalent resistors allows group of resistors in series or parallel to be replaced with their equivalent resistors. If the equivalent resistor replaces a group of resistors in series, then the equivalent resistor has the same current as the group of resistor that the equivalent resistor replaced. If the equivalent resistor replaces a group of resistors in parallel, the equivalent resistor has the same voltage at the group of resistors that the equivalent resistor replaced. 28

2 1. Simplify the following circuit to find the equivalent resistor. (R = 15.0 Ω) 2. Find the current in the resistor labeled R. 3. Find the potential difference between points a and b. 29

3 Batteries and Bulbs Your friend needs more light in his workroom, so he decides to add another light fixture to his track lighting. However, he is concerned that adding another light may dim the lights that are already in the track. When he proceeds with the addition of another light, he notices that none of the lights are dimmer than before. He wonders what type of circuit the track lighting uses and has asked you for help. You decide to build a model 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 bulbs are as bright as your reference circuit, is equivalent to the circuit that your track lighting uses. Before you actually build your model, you try to predict which bulbs will be brighter. The bulbs you use in your circuits will be identical and so will the batteries.? How does the brightness of the bulb depend on the arrangement of the bulbs in the following circuits? 30

4 Using the digital multimeters to measure electrical quantities. First notice that current is measured at a point in the circuit whereas potential difference is measured between two points in a circuit. To measure current: Symbol for ammeter: You must unhook the circuit at the point where you want to measure the current and insert the multimeter as shown in the example below. The ammeter must be hooked in series so that the current that you want to measure flows through the ammeter. In this example, the current through point 11 is being measured. You must select the ammeter function on the multimeter that is marked A. Since this is a DC circuit, make sure that you selected the DC A. Finally, choose the scale that the meter should read. Some meters will auto select the scale while other meters will make the screen blank or show one digit if you are off-scale. In most cases you will choose 200 ma (milliamps) or 2000 ma (milliamps). To measure potential difference: Symbol for a voltmeter: You must hook one connection from the multimeter to one side of the circuit element (in the example below this is a bulb) and hook the other connection on the other side of the circuit element. In this example the potential difference across bulb G is being measured. You must select the voltmeter function on the multimeter that is marked V. Since this is a DC circuit, make sure the AC/DC button is out. Finally choose the voltage scale to be measured. The batteries that you are using are about 1.5 Volts. 31

5 Procedure: 1. Using two battery holders and three light bulbs. Build circuit 1 as your reference circuit. Build circuit 2 and compare the brightness of bulbs A, B, and C. Use the ammeter to measure the current at points 1 in circuit 1 and 2, 3, and 4 in circuit 2. A common misconception about current is that it is used up as it travels through a bulb. Use the voltmeter to measure the voltage between points 2 and 3, points 3 and 4, and points 2 and 4. Comment on your observations Current (ma) Point 1 Points 2 & 3 Point 2 Points 3 & 4 Point 3 Points 2 & 4 Point 4 2. Replace circuit 2 with circuit 3. Compare the brightness of bulbs A, D, and E. Repeat the current and voltage measurements as was done in procedure 1. Current Point 5 Points 5 & 6 Point 6 Points 6 & 8 Point 7 Points 5 & 7 Point 8 Points 5 & 8 32

6 3. Now replace circuit 3 with circuit 4 and, as you have done with the other circuits, compare the brightness of each bulb and the current and voltage at each point to the bulb and current in the reference circuit (circuit 1). Current Point 9 Points 9 & 10 Point 10 Points 10 & 12 Point 11 Points 9 & 11 Point 12 Points 9 & Do the same comparisons for circuit 1 and circuit 5. Current Point 13 Points 13 & 14 Point 14 Points 14 & 15 Point 15 Points 13 & Challenge Game Light a bulb with one battery and one wire. (Sketch a diagram how you did it.) 33

7 Questions: 1. By applying the conservation of charge, explain what happens to the current at a junction. What happens to the current at a bulb? Describe what happens to the current as it goes through a circuit. 2. Describe the relationship between the potential difference (voltage) across each bulb and battery in a series circuit. 3. Describe the relationship between the potential difference (voltage) across each bulb and battery in a series circuit. 4. Which circuit would you recommend to your friends to solve the track lighting question? Where would you add a third bulb to this circuit so that the resulting circuit had three bulbs of the same brightness as bulb A? 34