Activity 8 Ohms Law and Circuits PHYS 010

Transcription

1 Name: Date: Partners: Purpose: To study the relationship between voltage, current and resistance and apply this understanding to simple series and parallel circuits. Materials: 1. 2 identical bulbs, and bulb holders 2. 4 AA batteries. Quad battery holder and single battery holder. 3. Alligator clips (4 or more) 4. ammeter 5. Multimeter 6. A selection of resisters Part 1 Ohms Law. The goal here is to demonstrate Ohms law: V = I R Use the multimeter, set for making resistance measurements, to measure the resistance of each of your resistors, and enter the value in the table that follows. Trial 1 Measured Resistance (Ω) Measured Current (A) Calculated Voltage (V) Measured Voltage (V) Using 4 AA batteries as the power source, build the following circuit. Do not make the final connection until you are ready to make measurements, as having the circuit live will drain down your batteries. Use the ammeter provided to make the current measurements, and the multimeter to measure the voltage dropped across the resister. Note the symbol: refers to a resister Rev /12

2 V A 6V Questions & Activities: 1. Do your calculated voltages agree well with the measured voltage drop? If noyt, do you have any explanation why not? 2. Plot your results on the following graph. Current (A) Resistance Rev /12

3 3. Mystery Resister. You are now given a mystery resister. Without measuring the resistance with the multimeter, put it into your circuit, measure the current, and using the graph predict what the resistance is. 4. Now determine the resistance using the colour code, as follows: What resistance do you predict for the resister based on colour code? 5. Does your measured/calculated value agree with the colour code prediction? Rev /12

4 Part 2 Series circuit with one bulb Consider you have a battery, a bulb in a bulb holder, and a circuit as follows. Caution: Don t connect 2 AA batteries to the circuit if you only have ONE bulb connected. You ll burn out the bulb! Bulb Battery 1. PREDICT: In the diagram above, if you measure current in different parts of the circuit with an ammeter will you -get different or identical measurements? 2. OBSERVE: Have your group connect the circuit so the bulb lights. Use the ammeter to measure the current in the following places: between the positive terminal of the battery and the bulb (1), and then between the bulb and the negative terminal of the battery (2). Note: the ammeter must be connected in series to the circuit to work properly. Measurement 1 (ma): Measurement 2 (ma): Rev /12

5 Were your predictions about current measurement correct? Part 3 Series Circuit with two bulbs 1. PREDICT: You now have two identical bulbs and one battery. You still want the electricity to flow in a continuous loop. a. Sketch how you would build the circuit using two bulbs. b. Predict if one bulb will be brighter than the other, and if so, which one. c. Predict if the current in the circuit will change when you add a second bulb compared to the circuit is Part 2. Explain your reasoning. Rev /12

6 2. OBSERVE: Now you can assemble the circuit. a. Is one bulb brighter than the other? Are the two bulbs each brighter, dimmer, or the same brightness as when you only had one bulb in the circuit? Explain. b. Measure the current on either side of the battery like you did in the onebulb part of this ctivity, and list your results below. Measurement 1 (ma): Measurement 2 (ma): Compare these results to the current measured with only 1 bulb in the circuit. Do these results make sense? Explain. 3. PREDICT: What will happen to the bulb brightness and current in the circuit if you add a second battery? 4. OBSERVE: Now connect the second battery so the positive terminal of the first battery touches the negative terminal of the second battery. How did the brightness of the bulbs change compared to when you were using just one battery? Rev /12

7 5. Use the ammeter to measure current at three points in the circuit: between the batteries and the first bulb (1), between the bulbs (2), and between the second bulb and the batteries (3). Measurement 1 (ma): Measurement 2 (ma): Measurement 3(mA): Are your results from question 4 consistent with the current you measured here? How does the current change when you add the 2 nd battery? Does this make sense? 6. PREDICT: What will happen if you unscrew one of the light bulbs? Will the other two appear brighter? Will something else happen? Explain. 7. OBSERVE: Try it! With the circuit closed (i.e. with both lights shining) carefully unscrew one of the light bulbs. Write down your observations here: 8. You will have noticed the conditions that lead to current increase and decrease in your circuit as you change the number of batteries and the number of light bulbs. From your observations, on what factors does the current in a system depend? Rev /12

8 9. If you saw that the current did not decrease by exactly a factor of two when you added a second bulb, what other factors might have affected the results? 10. As the title of the lab indicates, this is what is called a series circuit, where the electricity flows in a loop from one terminal of the battery, through the circuit, to the other terminal. Strings of Christmas-tree lights used to be configured in a series circuit similar to what you constructed, but with several dozen bulbs instead of just three. Brainstorm some advantages and disadvantages of using a series circuit for a string of lights: Part 4 Parallel Circuits Diagram: battery Light bulbs in holders Procedure: 1. To determine the baseline current, connect a simple series circuit with ONE bulb, and one battery. Measure the current with the ammeter (in series) of this basic series circuit: Rev /12

9 2. PREDICT: If you add a 2 nd bulb in parallel, as shown in the diagram above, will both bulbs be equally bright, or will there be a difference in brightness? Explain. What will happen to the brightness of the first bulb? Also, will the current in the two branches differ, or be the same? 3. OBSERVE: Connect the second bulb to the circuit as shown in the diagram. Record your observations about the brightness of the bulbs. Were your predictions correct? 4. (c) (a) 1 2 (b) This parallel circuit can be thought of as two current loops, as indicated in the above diagram. Measure the current for the following components. Note: to measure current properly, the ammeter must be connected in series. Current in loop 2, between the two bulbs noted by ammeter position (a): Current in loop 1 noted by ammeter position (b): Total current in the circuit - noted by ammeter position (c): Rev /12

10 5. a) Compare the currents across loop with each other. What is the relationship here? b) Compare the currents in each loop with the total current in the circuit. What is the relationship here? c) Compare the total current in parallel with the total current for two bulbs in series, with one battery. (May need to refer back to Series Circuit activity.) Explain your findings. Questions: 1. Remove one of the bulbs. What happens to the brightness of the other one? Why? 2. Using the multimeter, measure the voltage across each bulb in the parallel circuit. Bulb 1: (V) Bulb 2: (V) Rev /12

11 Compare the voltages across each bulb, and compare these voltages with the voltage of the battery: Now, measure the voltage across one of the bulbs when the other one is unscrewed from its holder: Is the voltage across a bulb changed when an identical bulb is placed in parallel with it? Explain how this is different from a series circuit. 3. Are the outlets in your home arranged in parallel or series? Why are they designed this way? 4. If you were to design your own chandelier for your dining room, would you want the bulbs to be in series, or parallel? Explain why. Rev /12

12 Summary and Suggestions for the Future: a. What were the important concepts of physics/science that you learned from this activity? What else did you learn? b. Can you think of alternative hands-on ways in which these concepts could be demonstrated? c. What changes (if any) would you make to teach these activities in a Grade 7-8 classroom Rev /12