Light Bulbs in Parallel Circuits

Transcription

1 Light Bulbs in Parallel Circuits In the last activity, we analyzed several different series circuits. In a series circuit, there is only one complete pathway for the charge to travel. Here are the basic ideas of a series circuit: one pathway only for charge to travel the total or equivalent resistance is the summation of individual resistances we see voltage drops across resistors and light bulbs these voltage drops = the voltage provided by the batteries power is conserved. Power to resistors and bulbs = power consumed by them ammeters or current probes can be placed at different points in the circuit and will give you the same value. Charge flow is identical throughout the circuit. In a parallel configuration, the current splits into BRANCHES at the NODES. The total current into a NODE equals to the total current leaving the NODE. The amount of current going through a BRANCH (Called BRANCH CURRENT) is inversely related to the resistance in the BRANCH. The battery provides an equal push or pressure (VOLTAGE) to each branch. Consequently, statements we can say about parallel circuits are a bit different than for series circuits. In this activity, we will investigate parallel circuits using several different types of light bulbs. Same Circuit Light bulbs in parallel Lab, p. 1 adg 2/14/2014

2 Part 1: Our basic Ohm s Law circuit. Let s start here to get baseline data and observations. Construct the following circuit using two batteries, a voltmeter, an ammeter, a switch, and a round light bulb. You ll have to move the voltmeter around to get all of the readings. Write down your observations and data here and also calculate the power delivered to the lamp: Bulb brightness: (describe how bright the bulb is) Voltmeteracross battery Reading = Volts Voltmeteracross lamp Reading = Volts Ammeter Reading = Amps Powerfrom battery (P = Vacross battery I) = Watts Powerto lamp (P = Vto lamp I) = Watts Ideally, Power from battery = Power to lamp. It should be close but may not be exactly equal. My data looks like this: Bulb brightness: pretty bright Voltmeter across lamp Reading = 2.44 V Power from battery (P = V across battery I) = W Voltmeter across battery Reading = 2.65 V Ammeter Reading = A Power to lamp (P = V to lamp I) = W Ideally, Power from battery = Power to lamp. It should be close but may not be exactly equal W W Light bulbs in parallel Lab, p. 2 adg 2/14/2014

3 Part 2: Two #14 Round Light Bulbs in Parallel Now, we would like to make a circuit that looks like the following picture below. The total current leaves the battery and splits up at the NODE along two BRANCHES (pathways). Part of the current goes through the first bulb and the rest goes through the second light bulb. The battery provides an equal push (voltage) to both branches. The W stands for wires needed One of the first things we can ascertain is the voltage across the different devices. Theory says that the voltage across parallel devices should be the same. NOTE: I have found that you should leave the switch off until you are ready to make your measurements and then just turn the circuit on, take your readings and then turn it off while you move your meters. The current drains the batteries and can give erroneous readings. Turn on, take readings quickly and turn off. First, put the voltmeter across the batteries, then across both lamps, next across lamp 1, and finally, across lamp 2. Also, write down the current coming from the batteries (next page for the current measurements). Vbatteries = V Vlamp 1 = V Vacross both lamps = V Vlamp 2 = V All of these values should theoretically be the same. One thing you will notice is that it takes more wires in a parallel circuit than in a series circuit! Light bulbs in parallel Lab, p. 3 adg 2/14/2014

4 Now, let s remove the voltmeter and insert an ammeter. First, put the ammeter right after the batteries but before left node (see left picture). Write down your ammeter reading. Then move the ammeter so it is measuring the branch current (middle picture) and record this current reading. Finally, move the ammeter so it is measuring the branch current of the top light bulb. a) Current Reading Coming from Batteries (left pic): A b) Current Reading going into bottom light bulb (middle pic): A c) Current Reading going into bottom light bulb (right pic): A Because we have two identical light bulbs, the current from the batteries should split into equal amounts, with equal currents going to each light bulb. Consequently, the current from line a above = current from line b + current from line c. Is this pretty close for your circuit? Go back to your page 2. You answered questions about the brightness, current, voltage, etc. using one bulb only. Let s try to make some comparisons with one light bulb and two light bulbs. How did the brightness change with two light bulbs in parallel when compared to one light bulb (p. 2) How did the current (out of the batteries) change with two light bulbs in parallel when compared to one light bulb (p. 2) How did the voltage (across the batteries) with two light bulbs in parallel when compared to one light bulb (p. 2) Light bulbs in parallel Lab, p. 4 adg 2/14/2014

5 Here are my thoughts over these last three questions My observations: It can be difficult to see changes in brightness. The circuit with one bulb was bright (p. 3 see my observations). Two bulbs in parallel, ideally, should be the same brightness as with one bulb. This is definitely different than when we compare one bulb and then two in series. When you placed two bulbs in series, their brightness levels were much decreased than with one bulb (see pp. 3-4 of the series circuit lab). You should see that current from the battery actually increases in the parallel circuit with two light bulbs. This is most interesting. Again this is much different than with a series circuit. In a series circuit, adding additional resistors or light bulbs in series makes the current decrease. Your voltages should not really change that much. You might see a small change (a few tenths of a volt) depending upon how fresh the batteries are. Now the voltages stay about the same when we have one lamp vs. two lamps in parallel and the current actually increases with two lamps in parallel. How is this possible? What must be happening to the total or equivalent resistance if the current increases given no change in voltage? This shows that the equivalent or effective resistance must decrease. Remember Ohm s Law (V = IR). If V stays constant, there is an inverse relationship between current and resistance. If current goes up, total resistance must decrease. Consequently, R eq in parallel R lamp1 + R lamp2. There must be a different relationship. As we discussed (or will discuss) in class, the following relationship holds for resistors (or lamps) in parallel: 1 R eq 1 1 R R 1 2 As more resistors are added in parallel, the equivalent resistance drops. For the same voltage, more charge flows from the battery with lesser resistance resulting in an increased current. In the previous lab, we said that the round #14 light bulb has an approximate hot resistance of 10 Ω. Using your equation above, what would the equivalent resistance be for two of these light bulbs in parallel? You should get 5 Ω. For two identical resistors or light bulbs the equivalent resistance is just half. 1/10 + 1/10 = 1/Req 2/10 = 1/Req 10/2 = Req = 5 Ohms Light bulbs in parallel Lab, p. 5 adg 2/14/2014

6 Let think about the series circuit again with two light bulbs. Use your knowledge about series circuits to answer the following question: If you unscrew one of the bulbs, what happens to the brightness of the other one? Why is this true? Now try the same thing with your parallel circuit. Unscrew the top bulb. What happens to the bottom bulb? Try it the other way, too. Offer an explanation Why might this property of parallel circuits be advantageous? Think about holiday lights or appliances in your home Light bulbs in parallel Lab, p. 6 adg 2/14/2014

7 Part 3: Three #14 Round Light Bulbs in Parallel NOTE: I have found that you should leave the switch off until you are ready to make your measurements and then just turn the circuit on, take your readings and then turn it off while you move your meters. Leaving the switch on can quickly drain the batteries and can give erroneous readings. Turn on, take readings quickly and turn off. First, build the circuit on the left and move your voltmeter around to the different locations. Then remove the voltmeter and put the ammeter into the circuit at the locations indicated in the right-side picture. See the next page for spaces to put your voltmeter and ammeter readings. Light bulbs in parallel Lab, p. 7 adg 2/14/2014

8 Take the following readings: Vbatteries = V Vlamp1 = V Vlamp2 = V Vlamp2 = V All of these voltages should be similar although the voltage across the batteries may be a bit higher. How did yours turn out? Current coming from batteries = A Current through lamp1 branch = A Current through lamp2 branch = A Current through lamp3 branch = A Because all the bulbs are theoretically identical, all the branch currents should be close. And when you add up these branch currents, it should be close to the current coming out of the batteries. How did yours turn out? My data: V batteries = 1.9 V V across all three bulbs = 1.46 V V lamp1 = 1.44 V V lamp2 = 1.47 V V lamp3 = 1.51 V All are fairly close especially the voltage readings across each lamp. Current coming from batteries = A Current through lamp1 branch = A Current through lamp2 branch = A Current through lamp3 branch = A If I add up my branch currents I get A. This is a bit higher than the A current coming from the batteries. This is within the realm of experimental error. Light bulbs in parallel Lab, p. 8 adg 2/14/2014

9 One more circuit: Part 4: Two different Light Bulbs in Parallel In the last section of the previous lab on light bulbs in series, you wired up two different bulbs (one round and one long). In this part, you will take these same bulbs and wire them in parallel. Let s go back to our observations of this part on the previous light bulb lab. Do you remember what we saw for the brightness of each light bulb when placed in series? a) In this part you will use two different light bulbs, one of the #14 round bulbs and then one of the long (#48) light bulbs. Here are the specs on these two light bulbs. Resistances are approximate: Bulb Type Name Approximate Resistance (Hot) Approximate Resistance (Cold) #14 Round 10 Ω 1.2 Ω #48 Long 40 Ω 4.5 Ω b) Build the circuit c) When you have completed the wiring, turn on the switch. d) Observe the light bulbs and you will see something very different than when we used the two different bulbs in series). First off, what are your observations of the round bulb? e) Observations of the long bulb? Light bulbs in parallel Lab, p. 9 adg 2/14/2014

10 f) From your observations, which bulb is emitting more power? Well, let s take some data. Put the voltmeter across the batteries, the round light bulb, and the long light bulb separately and record the values. Vbatteries = V Vround light bulb = V Vlong light bulb = V All of these voltages should be similar although the voltage across the batteries may be a bit higher. Here are my voltages: V batteries = 2.6 V V round light bulb = 2.2 V V long light bulb = 2.3 V Now find your currents. First put your ammeter in the circuit before the node to get the current from the battery. Record the current. Then put your ammeter in each branch to get the current going to each light bulb. Record these two currents, too. Current coming from batteries = A Current through round light bulb branch = A Current through long light bulb branch = A Here are my currents: Current coming from batteries = 0.35 A Current through round light bulb branch = A Current through long light bulb branch = A The branch currents sum = A which is pretty close to the current from the batteries of Add your branch currents and see how close they were to your current from the batteries. Were you close? Light bulbs in parallel Lab, p. 10 adg 2/14/2014

11 The branch current through the long light bulb is much smaller than the branch current through the round bulb. This was not the case when we had two round bulbs in parallel. In that case, the battery current split fairly equally between both branches. Explain why the current from the batteries doesn t split equally given the light bulb resistances as found on page 8 of this lab The long bulb has about four times the resistance so its current should be about ¼ the current of the round bulb. If I take the ratio: longbulbcurrentbranch roundbulbcurrentbranch 0.062A A Let s go back to your observations of the two light bulbs on the previous page. Didn t the round bulb look brighter than the long bulb? And isn t this much different than we saw with the series circuit where we couldn t even see the round bulb light up? Can you explain this in terms of the power to both bulbs in the parallel configuration vs. in the series configuration? You may need to go back to your data in the series activity (pp. 6-8) Power parallel-round bulb = V round light bulb*i round light bulb = * = Watts Power parallel-long bulb = V long light bulb*i long light bulb = * = Watts And from p. 7 of previous activity on series light bulbs: Power series-round bulb = V round light bulb*i round light bulb = * = Watts Power series-long bulb = V long light bulb*i long light bulb = * = Watts Here s all my data: In parallel, I get Watts for the round light bulb and Watts for the long light bulb. This helps explain why the round light bulb is much brighter in parallel. In series, if I go back to the previous lab, the round light bulb had a power output of Watts (do you remember that it didn t light up) and the long light bulb s power was Watts. OK, that s about it for parallel circuits. For a pure parallel circuit, voltage is the same across each resistor (or light bulb) and theoretically equals the voltage provide by the battery(ies). Current splits along the different paths at the node and recombines later. Equivalent resistance actually increases and follows the following relationship. R R R eq 1 2 Power is still conserved power from the battery = power delivered to the resistors (light bulbs). Light bulbs in parallel Lab, p. 11 adg 2/14/2014