Lab 8 Simple Electric Circuits INTRODUCTION When we talk about the current in a river, we are referring to the flow of water. Similarly, when we refer to the electric current in a circuit, we are talking about the flow of electric charge. In the circuits in this lab, the movement of negatively charged electrons results in an electric current. In several respects, electric charge travels through wires like water travels through pipes. in electrical circuits (that is, the flow of electrons) behaves similarly to the flow of water in pipes. With some Christmas tree lights, you can remove a bulb from the "string", and all of the bulbs go out. On the other hand, with the lamps and ceiling lights in your house, you can remove a bulb from a fixture and the other bulbs in the house remain lit. The reason for this has to do with the way the lights are wired and how different circuits affect voltage and current. One of the most useful laws when looking at circuits is Ohm s Law. Ohm s Law states current = voltage reistance, measured in Amperes [A], is the amount of charge that flows per second past a specific point in a circuit. Voltage, measured in Volts [V], is the potential difference across two points in an electric circuit. Whenever the ends of an electrical conductor (a light bulb or a wire, for instance) are held at different potentials, electric charge will flow from the end with the higher potential to the one with the lower potential. This is similar to water flowing down a waterslide where water at the top has a higher potential energy and water at the bottom has a lower potential energy. As long as the two ends remain at different potentials, charge will continue to flow. It is a battery s function to maintain a potential difference between two parts of a circuit. Resistance, measured in Ohms [Ω], is a measure of the resistance to the flow of current in a wire. The sum of the potential differences for each component in a single circuit loop, including the battery, is zero! Imagine adding up all the height changes water experiences as it flows down a waterslide, into a pool and is pumped back to the top; the net result is zero! In this lab, you will look at different types of circuits (the way electrical equipment is wired) and investigate how the current and voltage behaves in these circuits. 1
PROCEDURE Use identical light bulbs and make sure the holders are all the same. This will ensure that all bulbs have the same resistance. Part A: Exploring a Lightbulb 1. You will be given one battery, a light bulb, and a single wire. You will not be using a lamp holder. 2. Connect the wires in such a way that it lights the bulb. 3. In the space provided on the worksheet, draw a sketch of the connections you made. Figure 1 filament base CHECKPOINT 1: HAVE YOUR TA CHECK YOUR WORK BEFORE PROCEEDING Part B: Single Bulb 1. Set up the single- bulb circuit shown in Figure 2. 2. Observe the brightness of the bulb. As a reference point for the rest of the lab, we will say that the value of the current through the bulb, based on the brightness of the bulb, is "high". 3. Use the voltmeter to measure the potential difference across the battery, and record this value in the worksheet. Figure 2 - One Bulb 4. Measure and record the potential difference across the light bulb. It is a good idea to keep the single bulb circuit available for comparison for the rest of the lab. CHECKPOINT 2: HAVE YOUR TA CHECK YOUR WORK BEFORE PROCEEDING Part C: Two Bulbs in Series It is important to make sure that you are using two identical bulbs. Connect each bulb to a single bulb circuit. If they do not glow with the same brightness, find two that will. 1. Use two identical bulbs. Connect each bulb to a single bulb circuit. If they glow with the same brightness, then they have the same resistance. 2
2. Set up the two- bulb series circuit shown in figure 3. You may need to use 2-4 batteries. You can gather any extra equipment you need to complete the circuit. In a simple series circuit, there is only one current loop. There are no junctions where the current splits or combines. Therefore, the same current flows through all the elements in the circuit (battery, wire, bulbs). 3. Compare the brightness of the two bulbs in this circuit to each other and record your observations. 4. Measure and record the potential difference across the battery. Figure 3 Series Circuit 5. Measure and record the potential difference across each light bulb individually. CHECKPOINT 3: HAVE YOUR TA CHECK YOUR WORK BEFORE PROCEEDING Part D: Two Bulbs in Parallel It is important to make sure that you are using two identical bulbs. Connect each bulb to a single bulb circuit. If they do not glow with the same brightness, find two that will. 6. Set up the two- bulb parallel circuit shown in figure 4. In a parallel circuit, there are multiple current loops. that goes through the battery is then split into multiple branches. Because of this, the current through the battery must be equal to the combination of the currents in each of the branches. 7. Compare the brightness of the two bulbs in this circuit to each other and record your observation. Figure 4 Parallel Circuit 8. Measure and record the potential difference across the battery. 9. Measure and record the potential difference across each individual light bulb. CHECKPOINT 4: HAVE YOUR TA CHECK YOUR WORK BEFORE PROCEEDING 3
Part E: Combination Circuit 1 It is important to make sure that you are using three identical bulbs. Connect each bulb to a single bulb circuit. If they do not glow with the same brightness, find two that will. 10. Set up the combination circuit shown in figure 5. 11. Compare the brightness of the three bulbs in this circuit to each other and record your observation. 12. Measure and record the potential difference across the battery. Figure 5 Combination Circuit 1 13. Measure and record the potential difference across each individual light bulb. CHECKPOINT 5: HAVE YOUR TA CHECK YOUR WORK BEFORE PROCEEDING Part F: Combination Circuit 2 It is important to make sure that you are using three identical bulbs. Connect each bulb to a single bulb circuit. If they do not glow with the same brightness, find two that will. 14. Set up the combination circuit shown in figure 6. 15. Compare the brightness of the two bulbs in this circuit to each other and record your observation. 16. Measure and record the potential difference across the battery. Figure 6 Combination Circuit 1 17. Measure and record the potential difference across each light bulb individually CHECKPOINT 6: HAVE YOUR TA CHECK YOUR WORK BEFORE PROCEEDING 4
SIMPLE ELECTRIC CIRCUITS - WORKSHEET Part A: Exploring a Lightbulb 1. Provide a sketch of your circuit in the space below. Include enough detail in your sketch to indicate the orientation of the battery (plus & minus) and which parts of the bulb are being touched. CHECKPOINT 1 Part B: Single Bulb Difference across Battery (V) Difference across Bulb (V) 2. How does the potential difference across the bulb compare to the potential difference across the battery? CHECKPOINT 2 5
Part C: Two Bulbs in Series Difference (V) Battery Bulb 1 Bulb 2 1. Add- up the potential differences for the two light bulbs in the series combination. How does that sum compare to the potential difference across the battery? 2. From your observation, what can you conclude about the relative amount of current through each bulb? 3. Are the bulbs brighter in the one bulb circuit or the two- bulb circuit? What does this imply about the current going through the bulbs in the two- bulb series circuit versus the one- bulb circuit? 4. Would adding more light bulbs in series increase, decrease, or not change the current going through the bulbs already in the circuit? CHECKPOINT 3 6
Part D: Two Bulbs in Parallel Difference (V) Battery Bulb 1 Bulb 2 1. How does the potential difference across each of the two light bulbs in the parallel combination compare to the potential difference across the battery? 2. From your observation, what can you conclude about the relative amount of current through each bulb? 3. Are the bulbs brighter in the one bulb circuit or the two- bulb parallel circuit? What does this imply about the current going through the bulbs in the two- bulb parallel circuit versus the one- bulb circuit? 4. Would adding more light bulbs in parallel increase, decrease or not change the brightness of the other bulbs in the circuit? CHECKPOINT 4 7
Part E: Combination Circuit 1 Difference (V) Battery Bulb 1 Bulb 2 Bulb 3 1. How does the potential difference across bulb 2 compare to the potential difference across bulb 3? 2. How does the potential difference across bulbs 2 and 3 compare to the potential difference across bulb 1? 3. Based on your observations of brightness, how does the current through bulb 2 compare to the current through bulb 3? 4. Based on your observations of brightness, how does the current through bulb 2 compare to the current through bulb 1? CHECKPOINT 5 8
Part F: Combination Circuit 2 Difference (V) Battery Bulb 1 Bulb 2 Bulb 3 1. How does the potential difference across bulb 2 compare to the potential difference across bulb 3? 2. How does the potential difference across bulbs 2 and 3 compare to the potential difference across bulb 1? 3. Based on your observations of brightness, how does the current through bulb 2 compare to the current through bulb 3? 4. Based on your observations of brightness, how does the current through bulb 2 compare to the current through bulb 1? CHECKPOINT 6 9